Flight instruments chapter 07

Chapter 7

Flight
Instruments
Introduction
In order to safely ﬂy any aircraft, a pilot must understand
how to interpret and operate the ﬂight instruments. The
pilot also needs to be able to recognize associated errors and
malfunctions of these instruments. This chapter addresses the
pitot-static system and associated instruments, the vacuum
system and related instruments, gyroscopic instruments, and
the magnetic compass. When a pilot understands how each
instrument works and recognizes when an instrument is
malfunctioning, he or she can safely utilize the instruments
to their fullest potential.

Pitot-Static Flight Instruments
The pitot-static system is a combined system that utilizes the
static air pressure, and the dynamic pressure due to the motion
of the aircraft through the air. These combined pressures are
utilized for the operation of the airspeed indicator (ASI),
altimeter, and vertical speed indicator (VSI). [Figure 7-1]

7-1

Pitot-static system and instruments

Airspeed indicator (ASI) Vertical speed indicator (VSI) Altimeter

29.8
29.9
30.0

Pressure chamber

Static chamber Static port

Baffle plate

Pitot tube

Drain hole

Ram air Static hole

Heater (35 watts)

Heater (100 watts) Pitot heater switch Alternate static source

Figure 7-1. Pitot-static system and instruments.

Impact Pressure Chamber and Lines be checked prior to flight to insure that neither is blocked.
The pitot tube is utilized to measure the total combined Many aircraft have pitot tube covers installed when they sit
pressures that are present when an aircraft moves through for extended periods of time. This helps to keep bugs and
the air. Static pressure, also known as ambient pressure, is other objects from becoming lodged in the opening of the
always present whether an aircraft is moving or at rest. It is pitot tube.
simply the barometric pressure in the local area. Dynamic
pressure is present only when an aircraft is in motion; The one instrument that utilizes the pitot tube is the ASI. The
therefore, it can be thought of as a pressure due to motion. total pressure is transmitted to the ASI from the pitot tube’s
Wind also generates dynamic pressure. It does not matter if pressure chamber via a small tube. The static pressure is
the aircraft is moving through still air at 70 knots or if the also delivered to the opposite side of the ASI which serves
aircraft is facing a wind with a speed of 70 knots, the same to cancel out the two static pressures, thereby leaving the
dynamic pressure is generated. dynamic pressure to be indicated on the instrument. When the
dynamic pressure changes, the ASI shows either increase or
When the wind blows from an angle less than 90° off the decrease, corresponding to the direction of change. The two
nose of the aircraft, dynamic pressure can be depicted on the remaining instruments (altimeter and VSI) utilize only the
ASI. The wind moving across the airfoil at 20 knots is the static pressure which is derived from the static port.
same as the aircraft moving through calm air at 20 knots.
The pitot tube captures the dynamic pressure, as well as the Static Pressure Chamber and Lines
static pressure that is always present. The static chamber is vented through small holes to the
free undisturbed air on the side(s) of the aircraft. As the
The pitot tube has a small opening at the front which allows atmospheric pressure changes, the pressure is able to move
the total pressure to enter the pressure chamber. The total freely in and out of the instruments through the small lines
pressure is made up of dynamic pressure plus static pressure. which connect the instruments into the static system. An
In addition to the larger hole in the front of the pitot tube, alternate static source is provided in some aircraft to provide
there is a small hole in the back of the chamber which static pressure should the primary static source become
allows moisture to drain from the system should the aircraft blocked. The alternate static source is normally found inside
enter precipitation. Both openings in the pitot tube need to of the flight deck. Due to the venturi effect of the air flowing

7-2

Altimeter
around the fuselage, the air pressure inside the ﬂight deck is
lower than the exterior pressure. 1,000 ft. pointer

100 ft. pointer
When the alternate static source pressure is used, the
following instrument indications are observed: Aneroid wafers 10,000 ft. pointer
1. The altimeter indicates a slightly higher altitude than
actual.
2. The ASI indicates an airspeed greater than the actual
airspeed.
3. The VSI shows a momentary climb and then stabilizes
if the altitude is held constant.

Each pilot is responsible for consulting the Aircraft Flight
Manual (AFM) or the Pilot’s Operating Handbook (POH)
to determine the amount of error that is introduced into the Crosshatch flag
Static port
system when utilizing the alternate static source. In an aircraft A crosshatched area appears on
some altimeters when displaying
not equipped with an alternate static source, an alternate an altitude below 10,000 feet MSL.

method of introducing static pressure into the system should
a blockage occur is to break the glass face of the VSI. This Barometric scale adjustment knob
most likely renders the VSI inoperative. The reason for
Altimeter setting window
choosing the VSI as the instrument to break is that it is the
least important static source instrument for ﬂight.
Figure 7-2. Altimeter.
Altimeter
static pressure. Conversely, if the static pressure is less than
The altimeter is an instrument that measures the height of the pressure inside of the wafers, the wafers are able to expand
an aircraft above a given pressure level. Pressure levels which increases the volume. The expansion and contraction
are discussed later in detail. Since the altimeter is the only of the wafers moves the mechanical linkage, which drives the
instrument that is capable of indicating altitude, this is one of needles on the face of the ASI.
the most vital instruments installed in the aircraft. To use the
altimeter effectively, the pilot must understand the operation Principle of Operation
of the instrument, as well as the errors associated with the
The pressure altimeter is an aneroid barometer that measures
altimeter and how each effect the indication.
the pressure of the atmosphere at the level where the altimeter
is located, and presents an altitude indication in feet. The
A stack of sealed aneroid wafers comprise the main
altimeter uses static pressure as its source of operation.
component of the altimeter. An aneroid wafer is a sealed
Air is denser at sea level than aloft—as altitude increases,
wafer that is evacuated to an internal pressure of 29.92 inches
atmospheric pressure decreases. This difference in pressure
of mercury (29.92 "Hg). These wafers are free to expand
at various levels causes the altimeter to indicate changes in
and contract with changes to the static pressure. A higher
altitude.
static pressure presses down on the wafers and causes them
to collapse. A lower static pressure (less than 29.92 "Hg)
The presentation of altitude varies considerably between
allows the wafers to expand. A mechanical linkage connects
different types of altimeters. Some have one pointer while
the wafer movement to the needles on the indicator face,
others have two or more. Only the multipointer type is
which translates compression of the wafers into a decrease
discussed in this handbook. The dial of a typical altimeter
in altitude and translates an expansion of the wafers into an
is graduated with numerals arranged clockwise from zero
increase in altitude. [Figure 7-2]
to nine. Movement of the aneroid element is transmitted
through gears to the three hands that indicate altitude. The
Notice how the static pressure is introduced into the rear of
shortest hand indicates altitude in tens of thousands of feet,
the sealed altimeter case. The altimeter’s outer chamber is
the intermediate hand in thousands of feet, and the longest
sealed, which allows the static pressure to surround the aneroid
hand in hundreds of feet.
wafers. If the static pressure is higher than the pressure in the
aneroid wafers (29.92 "Hg), then the wafers are compressed
until the pressure inside the wafers is equal to the surrounding

7-3

This indicated altitude is correct, however, only when the sea BELOW.” Conversely, if an aircraft is flown from a low
level barometric pressure is standard (29.92 "Hg), the sea level pressure area to a high pressure area without an adjustment
free air temperature is standard (+15 degrees Celsius (°C) or of the altimeter, the actual altitude of the aircraft is higher
59 degrees Fahrenheit (°F)), and the pressure and temperature than the indicated altitude. Once in flight, it is important to
decrease at a standard rate with an increase in altitude. frequently obtain current altimeter settings en route to ensure
Adjustments for nonstandard pressures are accomplished by terrain and obstruction clearance.
setting the corrected pressure into a barometric scale located
on the face of the altimeter. The barometric pressure window is Many altimeters do not have an accurate means of being
sometimes referred to as the Kollsman window; only after the adjusted for barometric pressures in excess of 31.00 inches
altimeter is set does it indicate the correct altitude. The word of mercury ("Hg). When the altimeter cannot be set to the
“correct” will need to be better explained when referring to higher pressure setting, the aircraft actual altitude will be
types of altitudes, but is commonly used in this case to denote higher than the altimeter indicates. When low barometric
the approximate altitude above sea level. In other words, the pressure conditions occur (below 28.00), flight operations
indicated altitude refers to the altitude read off of the altitude by aircraft unable to set the actual altimeter setting are not
which is uncorrected, after the barometric pressure setting recommended.
is dialed into the Kollsman window. The additional types of
altitudes are further explained later. Adjustments to compensate for nonstandard pressure do not
compensate for nonstandard temperature. Since cold air is
Effect of Nonstandard Pressure and Temperature denser than warm air, when operating in temperatures that are
It is easy to maintain a consistent height above ground if the colder than standard, the altitude is lower than the altimeter
barometric pressure and temperature remain constant, but indication. [Figure 7-3] It is the magnitude of this “difference”
this is rarely the case. The pressure temperature can change that determines the magnitude of the error. It is the difference
between takeoff and landing even on a local flight. If these due to colder temperatures that concerns the pilot. When flying
changes are not taken into consideration, flight becomes into a cooler air mass while maintaining a constant indicated
dangerous. altitude, true altitude is lower. If terrain or obstacle clearance
is a factor in selecting a cruising altitude, particularly in
If altimeters could not be adjusted for nonstandard pressure, a mountainous terrain, remember to anticipate that a colder-
hazardous situation could occur. For example, if an aircraft is than-standard temperature places the aircraft lower than the
flown from a high pressure area to a low pressure area without altimeter indicates. Therefore, a higher indicated altitude may
adjusting the altimeter, a constant altitude will be displayed, be required to provide adequate terrain clearance. A variation
but the actual height of the aircraft above the ground would of the memory aid used for pressure can be employed:
be lower then the indicated altitude. There is an old aviation “FROM HOT TO COLD, LOOK OUT BELOW.” When the
axiom: “GOING FROM A HIGH TO A LOW, LOOK OUT air is warmer than standard, the aircraft is higher than the

5,00
0 foo
t pre
ssur
e lev
el

4,000
foot p
ressu
re lev
el

3,000 fo
ot pres
sure le
vel

2,000 foot
pressure le
vel

1,000 foot pressure
level

Sea level
30°C 15°C 0°C

Figure 7-3. Effects of nonstandard temperature on an altimeter.

7-4

altimeter indicates. Altitude corrections for temperature can If each pilot in a given area is using the same altimeter setting,
be computed on the navigation computer. each altimeter should be equally affected by temperature and
pressure variation errors, making it possible to maintain the
Extremely cold temperatures will also affect altimeter desired vertical separation between aircraft. This does not
indications. Figure 7-4, which was derived from ICAO guarantee vertical separation though. It is still imperative to
formulas, indicates how much error can exist when the maintain a regimented visual scan for intruding air traffic.
temperature is extremely cold.
When flying over high, mountainous terrain, certain atmospheric
conditions cause the altimeter to indicate an altitude of 1,000
Height Above Airport in Feet
Temp 0 °C

feet or more higher than the actual altitude. For this reason, a
Reported

generous margin of altitude should be allowed—not only for

00

00

00

00

00

00
0

0

0

0

0

0

0

0
20

30

40

50

60

70

80

90
10

15

20

30

40

50
possible altimeter error, but also for possible downdrafts that
+10 10 10 10 10 20 20 20 20 20 30 40 60 80 90 might be associated with high winds.
0 20 20 30 30 40 40 50 50 60 90 120 170 230 280
-10 20 30 40 50 60 70 80 90 100 150 200 290 390 490
To illustrate the use of the altimeter setting system, follow a
-20 30 50 60 70 90 100 120 130 140 210 280 420 570 710
flight from Dallas Love Field, Texas, to Abilene Municipal
-30 40 60 80 100 120 140 150 170 190 280 380 570 760 950
Airport, Texas, via Mineral Wells. Before taking off from
-40 50 80 100 120 150 170 190 220 240 360 480 720 970 1210
Love Field, the pilot receives a current altimeter setting of
-50 60 90 120 150 180 210 240 270 300 450 590 890 1190 1500
29.85 "Hg from the control tower or ATIS, and sets this value
in the altimeter setting window. The altimeter indication
Figure 7-4. Look at the chart using a temperature of –10 °C and
should then be compared with the known airport elevation of
the aircraft altitude is 1,000 feet above the airport elevation. The
487 feet. Since most altimeters are not perfectly calibrated,
chart shows that the reported current altimeter setting may place
an error may exist.
the aircraft as much as 100 feet below the altitude indicated by
the altimeter.
When over Mineral Wells, assume the pilot receives a current
altimeter setting of 29.94 "Hg and sets this in the altimeter
Setting the Altimeter window. Before entering the traffic pattern at Abilene
Most altimeters are equipped with a barometric pressure Municipal Airport, a new altimeter setting of 29.69 "Hg
setting window (or Kollsman window) providing a means to is received from the Abilene Control Tower, and set in the
adjust the altimeter. A knob is located at the bottom of the altimeter setting window. If the pilot desires to fly the traffic
instrument for this adjustment. pattern at approximately 800 feet above the terrain, and the
field elevation of Abilene is 1,791 feet, an indicated altitude of
To adjust the altimeter for variation in atmospheric pressure, 2,600 feet should be maintained (1,791 feet + 800 feet = 2,591
the pressure scale in the altimeter setting window, calibrated feet, rounded to 2,600 feet).
in inches of mercury ("Hg) and/or millibars (mb), is adjusted
to match the given altimeter setting. Altimeter setting is The importance of properly setting the altimeter cannot be
defined as station pressure reduced to sea level, but, an overemphasized. Assume the pilot did not adjust the altimeter
altimeter setting is accurate only in the vicinity of the at Abilene to the current setting and continued using the
reporting station. Therefore, the altimeter must be adjusted as Mineral Wells setting of 29.94 "Hg. When entering the Abilene
the flight progresses from one station to the next. Air traffic traffic pattern at an indicated altitude of 2,600 feet, the aircraft
control (ATC) will advise when updated altimeter settings would be approximately 250 feet below the proper traffic
are available. If a pilot is not utilizing ATC assistance, pattern altitude. Upon landing, the altimeter would indicate
local altimeter settings can be obtained by monitoring local approximately 250 feet higher than the field elevation.
automated weather observing system/automated surface
Mineral Wells altimeter setting 29.94
observation system (AWOS/ASOS) or automatic terminal
information service (ATIS) broadcasts. Abilene altimeter setting 29.69
Difference 0.25
Many pilots confidently expect the current altimeter setting
will compensate for irregularities in atmospheric pressure at (Since 1 inch of pressure is equal to approximately 1,000 feet
all altitudes, but this is not always true. The altimeter setting of altitude, 0.25 x 1,000 feet = 250 feet.)
broadcast by ground stations is the station pressure corrected
to mean sea level. It does not account for the irregularities at
higher levels, particularly the effect of nonstandard temperature.

7-5

When determining whether to add or subtract the amount This movement is transmitted through mechanical linkage
of altimeter error, remember that, when the actual pressure to rotate the pointers.
is lower than what is set in the altimeter window, the actual
altitude of the aircraft is lower than what is indicated on the A decrease in pressure causes the altimeter to indicate an
altimeter. increase in altitude, and an increase in pressure causes the
altimeter to indicate a decrease in altitude. Accordingly, if
The following is another method of computing the altitude the aircraft is sitting on the ground with a pressure level of
deviation. Start by subtracting the current altimeter setting from 29.98 "Hg and the pressure level changes to 29.68 "Hg, the
29.94 "Hg. Always remember to place the original setting as altimeter would show an increase of approximately 300 feet
the top number. Then subtract the current altimeter setting. in altitude. This pressure change is most noticeable when the
Mineral Wells altimeter setting 29.94 aircraft is left parked over night. As the pressure falls, the
altimeter interprets this as a climb. The altimeter indicates
Abilene altimeter setting 29.69 an altitude above the actual field elevation. If the barometric
29.94 – 29.69 = Difference 0.25 pressure setting is reset to the current altimeter setting of
29.68 "Hg, then the field elevation is again indicated on the
(Since 1 inch of pressure is equal to approximately 1,000 feet altimeter.
of altitude, 0.25 x 1,000 feet = 250 feet.) Always subtract the
number from the indicated altitude. This pressure change is not as easily noticed in flight since
aircraft fly specific altitudes. The aircraft steadily decreases
2,600 – 250 = 2,350
true altitude while the altimeter is held constant through pilot
action as discussed in the previous section.
Now, try a lower pressure setting. Adjust from altimeter
setting 29.94 to 30.56 "Hg.
Knowing the aircraft’s altitude is vitally important to a
Mineral Wells altimeter setting 29.94 pilot. The pilot must be sure that the aircraft is flying high
Altimeter setting 30.56 enough to clear the highest terrain or obstruction along the
intended route. It is especially important to have accurate
29.94 – 30.56 = Difference –0.62
altitude information when visibility is restricted. To clear
obstructions, the pilot must constantly be aware of the altitude
(Since 1 inch of pressure is equal to approximately 1,000 feet
of the aircraft and the elevation of the surrounding terrain. To
of altitude, 0.62 x 1,000 feet = 620 feet.) Always subtract
reduce the possibility of a midair collision, it is essential to
the number from the indicated altitude.
maintain altitude in accordance with air traffic rules.
2,600 – (–620) = 3,220
Types of Altitude
The pilot will be 620 feet high. Altitude in itself is a relevant term only when it is specifically
stated to which type of altitude a pilot is referring to.
Notice the difference is a negative number. Starting with the Normally when the term altitude is used, it is referring to
current indicated altitude of 2,600 feet, subtracting a negative altitude above sea level since this is the altitude which is
number is the same as adding the two numbers. By utilizing used to depict obstacles and airspace, as well as to separate
this method, a pilot should be able to better understand what air traffic.
is happening with the aircraft’s altitude. This method always
yields the correct result and tells a pilot what the altitude is Altitude is vertical distance above some point or level used as
and the direction. (The implications of not understanding a reference. There are as many kinds of altitude as there are
where the errors lie and in what direction are important to a reference levels from which altitude is measured, and each
safe flight.) If the altitude was lower than actually indicated, may be used for specific reasons. Pilots are mainly concerned
an aircraft could be in danger of colliding with an obstacle. with five types of altitudes:

Altimeter Operation 1. Indicated altitude—read directly from the altimeter
(uncorrected) when it is set to the current altimeter
There are two means by which the altimeter pointers can
setting.
be moved. The first is a change in air pressure, while the
other is an adjustment to the barometric scale. When the 2. True altitude—the vertical distance of the aircraft above
aircraft climbs or descends, changing pressure within the sea level—the actual altitude. It is often expressed as
altimeter case expands or contracts the aneroid barometer. feet above mean sea level (MSL). Airport, terrain,

7-6

and obstacle elevations on aeronautical charts are true surveyed field elevation, the instrument should be referred to
altitudes. a certificated instrument repair station for recalibration.
3. Absolute altitude—the vertical distance of an aircraft
Vertical Speed Indicator (VSI)
above the terrain, or above ground level (AGL).
The VSI, which is sometimes called a vertical velocity
4. Pressure altitude—the altitude indicated when indicator (VVI), indicates whether the aircraft is climbing,
the altimeter setting window (barometric scale) is descending, or in level flight. The rate of climb or descent
adjusted to 29.92 "Hg. This is the altitude above the is indicated in feet per minute (fpm). If properly calibrated,
standard datum plane, which is a theoretical plane the VSI indicates zero in level flight. [Figure 7-5]
where air pressure (corrected to 15 °C) equals 29.92"
Hg. Pressure altitude is used to compute density
altitude, true altitude, true airspeed (TAS), and other
performance data.
5. Density altitude—pressure altitude corrected
for variations from standard temperature. When
conditions are standard, pressure altitude and density
2 3
VERTICAL SPEED
altitude are the same. If the temperature is above THOUSAND FT PER MIN

standard, the density altitude is higher than pressure 4
altitude. If the temperature is below standard, the OWN

density altitude is lower than pressure altitude. This 3
2
is an important altitude because it is directly related
to the aircraft’s performance.

A pilot must understand how the performance of the aircraft
is directly related to the density of the air. The density of
the air affects how much power a naturally aspirated engine
produces, as well as how efficient the airfoils are. If there are
Figure 7-5. Vertical speed indicator (VSI).
fewer air molecules (lower pressure) to accelerate through
the propeller, the acceleration to rotation speed is longer
and thus produces a longer takeoff roll, which translates to Principle of Operation
a decrease in performance. Although the VSI operates solely from static pressure, it is a
differential pressure instrument. It contains a diaphragm with
As an example, consider an airport with a field elevation connecting linkage and gearing to the indicator pointer inside
of 5,048 feet MSL where the standard temperature is 5 °C. an airtight case. The inside of the diaphragm is connected
Under these conditions, pressure altitude and density altitude directly to the static line of the pitot-static system. The area
are the same—5,048 feet. If the temperature changes to outside the diaphragm, which is inside the instrument case,
30 °C, the density altitude increases to 7,855 feet. This is also connected to the static line, but through a restricted
means an aircraft would perform on takeoff as though the orifice (calibrated leak).
field elevation were 7,855 feet at standard temperature.
Conversely, a temperature of –25 °C would result in a density Both the diaphragm and the case receive air from the static
altitude of 1,232 feet. An aircraft would perform much better line at existing atmospheric pressure. The diaphragm receives
under these conditions. unrestricted air while the case receives the static pressure via
the metered leak. When the aircraft is on the ground or in level
Instrument Check flight, the pressures inside the diaphragm and the instrument
Prior to each flight, a pilot should examine the altimeter for case are equal and the pointer is at the zero indication. When
proper indications in order to verify its validity. To determine the aircraft climbs or descends, the pressure inside the
the condition of an altimeter, set the barometric scale to the diaphragm changes immediately, but due to the metering
current reported altimeter setting transmitted by the local action of the restricted passage, the case pressure remains
automated flight service station (AFSS) or any other reliable higher or lower for a short time, causing the diaphragm to
source, such as ATIS, AWOS, or ASOS. The altimeter contract or expand. This causes a pressure differential that
pointers should indicate the surveyed field elevation of the is indicated on the instrument needle as a climb or descent.
airport. If the indication is off more than 75 feet from the

7-7

a positive rate of climb and then, once a stabilized climb is
Accelerometer established, a rate of climb can be referenced.

Airspeed Indicator (ASI)
The ASI is a sensitive, differential pressure gauge which
measures and promptly indicates the difference between pitot
(impact/dynamic pressure) and static pressure. These two
I 2 pressures are equal when the aircraft is parked on the ground
.5 UP
3
0 in calm air. When the aircraft moves through the air, the
.5 D
OW
I
N
4 pressure on the pitot line becomes greater than the pressure
2 3 in the static lines. This difference in pressure is registered by
Inlet from static port the airspeed pointer on the face of the instrument, which is
calibrated in miles per hour, knots (nautical miles per hour),
Calibrated leak or both. [Figure 7-7]
Airspeed indicator

Diaphragm
Figure 7-6. An IVSI incorporates accelerometers to help the Long lever Sector

instrument immediately indicate changes in vertical speed.
Pitot connection
When the pressure differential stabilizes at a definite ratio, 50
the needle indicates the rate of altitude change. Pitot tube
100

The VSI displays two different types of information:
150
• Trend information shows an immediate indication of
an increase or decrease in the aircraft’s rate of climb 200
or descent.
• Rate information shows a stabilized rate of change in
altitude.
Ram air Static air line Handstaff pinion

The trend information is the direction of movement of the
VSI needle. For example, if an aircraft is maintaining level Figure 7-7. Airspeed indicator (ASI).
flight and the pilot pulls back on the control yoke causing the
nose of the aircraft to pitch up, the VSI needle moves upward The ASI is the one instrument that utilizes both the pitot,
to indicate a climb. If the pitch attitude is held constant, as well as the static system. The ASI introduces the static
the needle stabilizes after a short period (6–9 seconds) and pressure into the airspeed case while the pitot pressure
indicates the rate of climb in hundreds of fpm. The time (dynamic) is introduced into the diaphragm. The dynamic
period from the initial change in the rate of climb, until the pressure expands or contracts one side of the diaphragm,
VSI displays an accurate indication of the new rate, is called which is attached to an indicating system. The system drives
the lag. Rough control technique and turbulence can extend the mechanical linkage and the airspeed needle.
the lag period and cause erratic and unstable rate indications.
Some aircraft are equipped with an instantaneous vertical Just as in altitudes, there are multiple types of airspeeds.
speed indicator (IVSI), which incorporates accelerometers to Pilots need to be very familiar with each type.
compensate for the lag in the typical VSI. [Figure 7-6]
• Indicated airspeed (IAS)—the direct instrument
Instrument Check
reading obtained from the ASI, uncorrected for
As part of a preflight check, proper operation of the VSI must variations in atmospheric density, installation error,
be established. Make sure the VSI indicates near zero prior or instrument error. Manufacturers use this airspeed
to leaving the ramp area and again just before takeoff. If the as the basis for determining aircraft performance.
VSI indicates anything other than zero, that indication can Takeoff, landing, and stall speeds listed in the AFM/
be referenced as the zero mark. Normally, if the needle is POH are IAS and do not normally vary with altitude
not exactly zero, it is only slightly above or below the zero or temperature.
line. After takeoff, the VSI should trend upward to indicate

7-8

• Calibrated airspeed (CAS)—IAS corrected for As shown in Figure 7-8, ASIs on single-engine small aircraft
installation error and instrument error. Although include the following standard color-coded markings:
manufacturers attempt to keep airspeed errors to a • White arc—commonly referred to as the flap operating
minimum, it is not possible to eliminate all errors range since its lower limit represents the full flap stall
throughout the airspeed operating range. At certain speed and its upper limit provides the maximum flap
airspeeds and with certain flap settings, the installation speed. Approaches and landings are usually flown at
and instrument errors may total several knots. This speeds within the white arc.
error is generally greatest at low airspeeds. In the
cruising and higher airspeed ranges, IAS and CAS • Lower limit of white arc (VS0)—the stalling speed
are approximately the same. Refer to the airspeed or the minimum steady flight speed in the landing
calibration chart to correct for possible airspeed configuration. In small aircraft, this is the power-off
errors. stall speed at the maximum landing weight in the
landing configuration (gear and flaps down).
• True airspeed (TAS)—CAS corrected for altitude
and nonstandard temperature. Because air density • Upper limit of the white arc (VFE)—the maximum
decreases with an increase in altitude, an aircraft has speed with the flaps extended.
to be flown faster at higher altitudes to cause the same • Green arc—the normal operating range of the aircraft.
pressure difference between pitot impact pressure Most flying occurs within this range.
and static pressure. Therefore, for a given CAS, TAS
• Lower limit of green arc (VS1)—the stalling speed
increases as altitude increases; or for a given TAS,
or the minimum steady flight speed obtained in a
CAS decreases as altitude increases. A pilot can find
specified configuration. For most aircraft, this is the
TAS by two methods. The most accurate method is
power-off stall speed at the maximum takeoff weight
to use a flight computer. With this method, the CAS
in the clean configuration (gear up, if retractable, and
is corrected for temperature and pressure variation by
flaps up).
using the airspeed correction scale on the computer.
Extremely accurate electronic flight computers are • Upper limit of green arc (V NO )—the maximum
also available. Just enter the CAS, pressure altitude, structural cruising speed. Do not exceed this speed
and temperature, and the computer calculates the TAS. except in smooth air.
A second method, which is a rule of thumb, provides • Yellow arc—caution range. Fly within this range only
the approximate TAS. Simply add 2 percent to the in smooth air, and then, only with caution.
Single-engine airspeed indicator
CAS for each 1,000 feet of altitude. The TAS is the
speed which is used for flight planning and is used
when filing a flight plan. VNE(red line)
VSO
• Groundspeed (GS)—the actual speed of the airplane Yellow arc
over the ground. It is TAS adjusted for wind. GS
decreases with a headwind, and increases with a
F° 120 90 60 30 0 -3
0 VS1
PRESS 0
ALT 5

tailwind. AIRSPEED 10

160 KNOTS 40
15

Airspeed Indicator Markings
140

20

140 60
Aircraft weighing 12,500 pounds or less, manufactured after VNO White arc
H
MP
H

120 100
1945, and certificated by the FAA, are required to have ASIs
MP

120
120

marked in accordance with a standard color-coded marking 80
100 .S.
system. This system of color-coded markings enables a pilot 100 80
T.A TS
K

to determine at a glance certain airspeed limitations that are Green arc
VFE
important to the safe operation of the aircraft. For example,
if during the execution of a maneuver, it is noted that the
airspeed needle is in the yellow arc and rapidly approaching
Figure 7-8. Airspeed indicator (ASI).
the red line, the immediate reaction should be to reduce
airspeed.

7-9

• Red line (VNE)—never exceed speed. Operating above sure the pitot tube cover is removed. Then, check the pitot and
this speed is prohibited since it may result in damage static port openings. A blocked pitot tube affects the accuracy
or structural failure. of the ASI, but, a blockage of the static port not only affects
the ASI, but also causes errors in the altimeter and VSI.
Other Airspeed Limitations
Some important airspeed limitations are not marked on the Blocked Pitot System
face of the ASI, but are found on placards and in the AFM/ The pitot system can become blocked completely or only
POH. These airspeeds include: partially if the pitot tube drain hole remains open. If the pitot
• Design maneuvering speed (V A)—the maximum tube becomes blocked and its associated drain hole remains
speed at which the structural design’s limit load can clear, ram air no longer is able to enter the pitot system. Air
be imposed (either by gusts or full deflection of the already in the system vents through the drain hole, and the
control surfaces) without causing structural damage. remaining pressure drops to ambient (outside) air pressure.
It is important to consider weight when referencing Under these circumstances, the ASI reading decreases to
this speed. For example, VA may be 100 knots when zero, because the ASI senses no difference between ram and
an airplane is heavily loaded, but only 90 knots when static air pressure. The ASI no longer operates since dynamic
the load is light. pressure can not enter the pitot tube opening. Static pressure
is able to equalize on both sides since the pitot drain hole
• Landing gear operating speed (VLO)—the maximum is still open. The apparent loss of airspeed is not usually
speed for extending or retracting the landing gear if instantaneous but happens very quickly. [Figure 7-9]
flying an aircraft with retractable landing gear. Blocked static system

• Landing gear extended speed (VLE)—the maximum
speed at which an aircraft can be safely flown with
the landing gear extended.
• Best angle-of-climb speed (VX)—the airspeed at
which an aircraft gains the greatest amount of altitude
in a given distance. It is used during a short-field Pitot tube Static port
takeoff to clear an obstacle.
• Best rate-of-climb speed (VY)—the airspeed that Blockage
provides the most altitude gain in a given period of
time.
Drain hole
• Single-engine best rate-of-climb (VYSE)—the best
rate-of-climb or minimum rate-of-sink in a light
Figure 7-9. A blocked pitot tube, but clear drain hole.
twin-engine aircraft with one engine inoperative. It is
marked on the ASI with a blue line. VYSE is commonly
referred to as “Blue Line.” If both the pitot tube opening and the drain hole should
become clogged simultaneously, then the pressure in the pitot
• Minimum control speed (VMC)—the minimum flight
tube is trapped. No change is noted on the airspeed indication
speed at which a light, twin-engine aircraft can be
should the airspeed increase or decrease. If the static port
satisfactorily controlled when an engine suddenly
is unblocked and the aircraft should change altitude, then a
becomes inoperative and the remaining engine is at
change is noted on the ASI. The change is not related to a
takeoff power.
change in airspeed but a change in static pressure. The total
pressure in the pitot tube does not change due to the blockage;
Instrument Check however, the static pressure will change.
Prior to takeoff, the ASI should read zero. However, if there
is a strong wind blowing directly into the pitot tube, the ASI Because airspeed indications rely upon both static and
may read higher than zero. When beginning the takeoff, make dynamic pressure together, the blockage of either of these
sure the airspeed is increasing at an appropriate rate. systems affects the ASI reading. Remember that the ASI has
a diaphragm in which dynamic air pressure is entered. Behind
Blockage of the Pitot-Static System this diaphragm is a reference pressure called static pressure
Errors almost always indicate blockage of the pitot tube, the that comes from the static ports. The diaphragm pressurizes
static port(s), or both. Blockage may be caused by moisture against this static pressure and as a result changes the airspeed
(including ice), dirt, or even insects. During preflight, make indication via levers and indicators. [Figure 7-10]

7-10

Blocked pitot system with clear static system

tube always provides static pressure in addition to dynamic
pressure.

Therefore, the airspeed indication is the result of two
pressures: the pitot tube static and dynamic pressure within
the diaphragm as measured against the static pressure in case.
Blockage What does this mean if the pitot tube is obstructed?
Static port

If the aircraft were to descend, the pressure in the pitot
Pitot tube system including the diaphragm would remain constant. It is
clogged and the diaphragm is at a single pressure. But as the
descent is made, the static pressure would increase against
Drain hole the diaphragm causing it to compress thereby resulting in an
indication of decreased airspeed. Conversely, if the aircraft
were to climb, the static pressure would decrease allowing
the diaphragm to expand, thereby showing an indication of
greater airspeed. [Figure 7-10]
Clim
b
The pitot tube may become blocked during flight due to
visible moisture. Some aircraft may be equipped with pitot
heat for flight in visible moisture. Consult the AFM/POH for
specific procedures regarding the use of pitot heat.

cen
t Blocked Static System
Des
If the static system becomes blocked but the pitot tube remains
clear, the ASI continues to operate; however, it is inaccurate.
The airspeed indicates lower than the actual airspeed when
the aircraft is operated above the altitude where the static
ports became blocked, because the trapped static pressure is
Figure 7-10. Blocked pitot system with clear static system. higher than normal for that altitude. When operating at a lower
altitude, a faster than actual airspeed is displayed due to the
For example, take an aircraft and slow it down to zero knots relatively low static pressure trapped in the system.
at given altitude. If the static port (providing static pressure)
and the pitot tube (providing dynamic pressure) are both Revisiting the ratios that were used to explain a blocked pitot
unobstructed, the following claims can be made: tube, the same principle applies for a blocked static port. If
1. The ASI would be zero. the aircraft descends, the static pressure increases on the pitot
side showing an increase on the ASI. This assumes that the
2. There must be a relationship between both dynamic
aircraft does not actually increase its speed. The increase in
and static pressure. At zero speed, dynamic pressure
static pressure on the pitot side is equivalent to an increase
and static pressure are the same: static air pressure.
in dynamic pressure since the pressure can not change on
3. Because both dynamic and static air pressure are equal the static side.
at zero speed with increased speed, dynamic pressure
must include two components: static pressure and If an aircraft begins to climb after a static port becomes
dynamic pressure. blocked, the airspeed begins to show a decrease as the aircraft
continues to climb. This is due to the decrease in static pressure
It can be inferred that airspeed indication must be based upon on the pitot side, while the pressure on the static side is held
a relationship between these two pressures, and indeed it is. constant.
An ASI uses the static pressure as a reference pressure and
as a result, the ASI’s case is kept at this pressure behind the A blockage of the static system also affects the altimeter and
diaphragm. On the other hand, the dynamic pressure through VSI. Trapped static pressure causes the altimeter to freeze
the pitot tube is connected to a highly sensitive diaphragm at the altitude where the blockage occurred. In the case of
within the ASI case. Because an aircraft in zero motion the VSI, a blocked static system produces a continuous zero
(regardless of altitude) results in a zero airspeed, the pitot indication. [Figure 7-11]

7-11

Blocked static system
displayed for pilot reference. An additional pilot-controlled
Inaccurate airspeed indications
airspeed bug is available to set at any desired reference speed.
As on traditional analogue ASIs, the electronic airspeed tape
Constant zero indication on VSI
displays the color-coded ranges for the flap operating range,
normal range, and caution range. [Figure 7-12] The number
Frozen altimeter
value changes color to red when the airspeed exceeds VNE to
warn the pilot of exceeding the maximum speed limitation.

Attitude Indicator
29.8
29.9
30.0

One improvement over analogue instrumentation is the
larger attitude indicator on EFD. The artificial horizon spans
the entire width of the PFD. [Figure 7-12] This expanded
Pitot tube
instrumentation offers better reference through all phases of
flight and all flight maneuvers. The attitude indicator receives
Blockage
its information from the Attitude Heading and Reference
System (AHRS).
Static port

Altimeter
Figure 7-11. Blocked static system.
The altimeter is located on the right side of the PFD.
Some aircraft are equipped with an alternate static source [Figure 7-12] As the altitude increases, the larger numbers
in the flight deck. In the case of a blocked static source, descend from the top of the display tape, with the current altitude
opening the alternate static source introduces static pressure being displayed in the black box in the center of the display tape.
from the flight deck back into the system. Flight deck static The altitude is displayed in increments of 20 feet.
pressure is lower than outside static pressure. Check the
aircraft AOM/POH for airspeed corrections when utilizing Vertical Speed Indicator (VSI)
alternate static pressure. The VSI is displayed to the right of the altimeter tape and can
take the form of an arced indicator or a vertical speed tape.
Electronic Flight Display (EFD) [Figure 7-12] Both are equipped with a vertical speed bug.
Advances in digital displays and solid state electronic Heading Indicator
components have been introduced into the flight decks
The heading indicator is located below the artificial horizon
of general aviation (GA) aircraft. In addition to the
and is normally modeled after a Horizontal Situation
improvement in system reliability, which increases overall
Indicator (HSI). [Figure 7-12] As in the case of the attitude
safety, electronic flight displays (EFD) have decreased
indicator, the heading indicator receives its information from
the overall cost of equipping aircraft with state-of-the-art
the magnetometer which feeds information to the AHRS unit
instrumentation. Primary electronic instrumentation packages
and then out to the PFD.
are less prone to failure than their analogue counterparts. No
longer is it necessary for aircraft designers to create cluttered
Turn Indicator
panel layouts in order to accommodate all necessary flight
The turn indicator takes a slightly different form than the
instruments. Instead, multi-panel digital flight displays
traditional instrumentation. A sliding bar moves left and right
combine all flight instruments onto a single screen which is
below the triangle to indicate deflection from coordinated
called a primary flight display (PFD). The traditional “six
flight. [Figure 7-12] Reference for coordinated flight comes
pack” of instruments is now displayed on one liquid crystal
from accelerometers contained in the AHRS unit.
display (LCD) screen.
Tachometer
Airspeed Tape
The sixth instrument normally associated with the “six pack”
Configured similarly to traditional panel layouts, the ASI
package is the tachometer. This is the only instrument that is
is located on the left side of the screen and is displayed as
not located on the PFD. The tachometer is normally located
a vertical speed tape. As the aircraft increases in speed, the
on the multi-function display (MFD). In the event of a display
larger numbers descend from the top of the tape. The TAS is
screen failure, it is displayed on the remaining screen with
displayed at the bottom of the tape through the input to the air
the PFD flight instrumentation. [Figure 7-13]
data computer (ADC) from the outside air temperature probe.
Airspeed markings for VX, VY, and rotation speed (VR) are

7-12

Slip/Skid Indicator Turn Rate Indicator
The slip/skid indicator [Figure 7-12] is the horizontal The turn rate indicator, illustrated in Figure 7-12, is typically
line below the roll pointer. Like a ball in a turn-and-slip found directly above the rotating compass card. Tick marks to
indicator, a bar width off center is equal to one ball width the left and right of the luber line denote the turn (standard-
displacement. rate versus half standard-rate). Typically denoted by a trend
line, if the trend vector is extended to the second tick mark
the aircraft is in a standard-rate turn.

NAV1 108.00 113.00 WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK _ _ _°T TRK 360° 134.000 118.000 COM1
NAV2 108.00 110.60 Indicator
Attitude Slip Skid Indicator 123.800 Altimeter
118.000 COM2

4000
130 4300
2

120 4200
Vertical Speed Indicator (VSI)
1
Air Speed Indicator 110 4100
1 20
100 4000
3900
9 80
90 3900 1

80 3800
270° 2

70
TAS 106KT 4300

Turn Indicator
VOR 1

Horizontal Situation Indicator Slip/Skid Indicator
270°
270°
OAT 6°C Turn Rate Indicator Tick Marks 5537 IDNT LCL10:12:34
XPDR
INSET PFD OBS CDI DME XPDR IDENT TMR/REF NRST ALERTS

Turn Rate Trend Vector

Figure 7-12. Primary flight display. Note that the actual location of indications vary depending on manufacturers.

Figure 7-13. Multi-function display.

7-13

Individual panel displays are able to be configured for a variety
of aircraft simply by installing different software packages.
[Figure 7-14] Manufacturers are also able to upgrade existing
instrument displays in a similar manner, eliminating the need
to replace individual gauges in order to upgrade.

200 210 220 230 240 250 260 270

58
140

UY
120 70
102
100 10 10 6 710
3 0 .3 0
00:03:29 65
UX 80
VS
60 10 2320B 10 W Figure 7-15. Teledyne’s 90004 TAS/Plus Air Data Computer (ADC)
S
40 70 computes air data information from the pitot-static pneumatic
E
N system, aircraft temperature probe, and barometric correction
MA2 3 9 58 0 0 ’
device to help create a clear picture of flight characteristics.
IFR APPR ANG 239 A 2 3 9 2. 3NM

autopilot control system. In the event of system malfunction,
the ADC can quickly be removed and replaced in order to
decrease down time and maintenance turn-around times.

Altitude information is derived from the static pressure port
just as an analogue system does; however, the static pressure
does not enter a diaphragm. The ADC computes the received
barometric pressure and sends a digital signal to the PFD to
display the proper altitude readout. Electronic flight displays
also show trend vectors which show the pilot how the altitude
and airspeed are progressing.

Trend Vectors
Trend vectors are magenta lines which move up and down
both the ASI and the altimeter. [Figures 7-16 and 7-17] The

Figure 7-14. Chelton’s FlightLogic (top) and Avidyne’s Entegra Airspeed trend vector

1
(bottom) are examples of panel displays that are configurable.
150

0
Air Data Computer (ADC) 140
Electronic flight displays utilize the same type of instrument
inputs as traditional analogue gauges; however, the processing 130
system is different. The pitot static inputs are received by an 1
ADC. The ADC computes the difference between the total 20
120
pressure and the static pressure, and generates the information 9
necessary to display the airspeed on the PFD. Outside air 110
temperatures are also monitored and introduced into various
components within the system, as well as being displayed on
100
the PFD screen. [Figure 7-15]
90
TAS 120KT

The ADC is a separate solid state device which, in addition to
providing data to the PFD, is capable of providing data to the Figure 7-16. Airspeed trend vector.

7-14

There are two fundamental properties of gyroscopic action:
Airspeed trend vector rigidity in space and precession.

1 150
Rigidity in Space

0
140 Rigidity in space refers to the principle that a gyroscope
remains in a fixed position in the plane in which it is spinning.
130 An example of rigidity in space is that of a bicycle wheel.
1 As the bicycle wheels increase speed, they become more and
20
120 more stable in their plane of rotation. This is why a bicycle is
9 very unstable and very maneuverable at low speeds and very
110 stable and less maneuverable at higher speeds.

100 By mounting this wheel, or gyroscope, on a set of gimbal
rings, the gyro is able to rotate freely in any direction. Thus,
90
TAS 120KT if the gimbal rings are tilted, twisted, or otherwise moved,
the gyro remains in the plane in which it was originally
Figure 7-17. Altimeter trend vector. spinning. [Figure 7-18]
ADC computes the rate of change and displays the 6-second
projection of where the aircraft will be. Pilots can utilize
the trend vectors to better control the aircraft’s attitude. By
including the trend vectors in the instrument scan, pilots are
able to precisely control airspeed and altitude. Additional
information can be obtained by referencing the Instrument
Flying Handbook or specific avionics manufacturer’s training
material.

Gyroscopic Flight Instruments
Several flight instruments utilize the properties of a gyroscope
for their operation. The most common instruments containing
gyroscopes are the turn coordinator, heading indicator, and
the attitude indicator. To understand how these instruments
operate requires knowledge of the instrument power systems,
gyroscopic principles, and the operating principles of each
instrument.

Gyroscopic Principles Figure 7-18. Regardless of the position of its base, a gyro tends to
Any spinning object exhibits gyroscopic properties. A wheel remain rigid in space, with its axis of rotation pointed in a constant
or rotor designed and mounted to utilize these properties is direction.
called a gyroscope. Two important design characteristics of an
instrument gyro are great weight for its size, or high density, Precession
and rotation at high speed with low friction bearings. Precession is the tilting or turning of a gyro in response to a
deflective force. The reaction to this force does not occur at
There are two general types of mountings; the type used the point at which it was applied; rather, it occurs at a point
depends upon which property of the gyro is utilized. A freely that is 90° later in the direction of rotation. This principle
or universally mounted gyroscope is free to rotate in any allows the gyro to determine a rate of turn by sensing the
direction about its center of gravity. Such a wheel is said to amount of pressure created by a change in direction. The rate
have three planes of freedom. The wheel or rotor is free to at which the gyro precesses is inversely proportional to the
rotate in any plane in relation to the base and is balanced so speed of the rotor and proportional to the deflective force.
that, with the gyro wheel at rest, it remains in the position
in which it is placed. Restricted or semi-rigidly mounted
gyroscopes are those mounted so that one of the planes of
freedom is held fixed in relation to the base.

7-15

Using the example of the bicycle, precession acts on the or pressure required for instrument operation varies, but is
wheels in order to allow the bicycle to turn. While riding usually between 4.5 "Hg and 5.5 "Hg.
at normal speed, it is not necessary to turn the handle bars
in the direction of the desired turn. A rider simply leans in One source of vacuum for the gyros is a vane-type engine-
the direction that he or she wishes to go. Since the wheels driven pump that is mounted on the accessory case of
are rotating in a clockwise direction when viewed from the the engine. Pump capacity varies in different airplanes,
right side of the bicycle, if a rider leans to the left, a force is depending on the number of gyros.
applied to the top of the wheel to the left. The force actually
acts 90° in the direction of rotation, which has the effect of A typical vacuum system consists of an engine-driven
applying a force to the front of the tire, causing the bicycle vacuum pump, relief valve, air filter, gauge, and tubing
to move to the left. There is a need to turn the handlebars at necessary to complete the connections. The gauge is mounted
low speeds because of the instability of the slowly turning in the aircraft’s instrument panel and indicates the amount
gyros, and also to increase the rate of turn. of pressure in the system (vacuum is measured in inches of
mercury less than ambient pressure).
Precession can also create some minor errors in some
instruments. [Figure 7-19] Precession can cause a freely As shown in Figure 7-20, air is drawn into the vacuum system
spinning gyro to become displaced from its intended plane by the engine-driven vacuum pump. It first goes through
of rotation through bearing friction, etc. Certain instruments a filter, which prevents foreign matter from entering the
may require corrective realignment during flight, such as the vacuum or pressure system. The air then moves through the
heading indicator. attitude and heading indicators, where it causes the gyros
to spin. A relief valve prevents the vacuum pressure, or
Pla suction, from exceeding prescribed limits. After that, the air
n eo is expelled overboard or used in other systems, such as for
fR
ota inflating pneumatic deicing boots.
tio ce
n of For
Plane
FOR
CE It is important to monitor vacuum pressure during flight,
because the attitude and heading indicators may not provide
P

reliable information when suction pressure is low. The
la
n
e
o
f

vacuum, or suction, gauge is generally marked to indicate
P
re
ce
ss

the normal range. Some aircraft are equipped with a warning
io
n

light that illuminates when the vacuum pressure drops below
the acceptable level.

When the vacuum pressure drops below the normal operating
range, the gyroscopic instruments may become unstable and
inaccurate. Cross checking the instruments routinely is a
good habit to develop.

Figure 7-19. Precession of a gyroscope resulting from an applied Turn Indicators
deflective force. Aircraft use two types of turn indicators: turn-and-slip
indicator and turn coordinator. Because of the way the gyro
Sources of Power is mounted, the turn-and-slip indicator shows only the rate of
In some aircraft, all the gyros are vacuum, pressure, or turn in degrees per second. The turn coordinator is mounted
electrically operated. In other aircraft, vacuum or pressure at an angle, or canted, so it can initially show roll rate. When
systems provide the power for the heading and attitude the roll stabilizes, it indicates rate of turn. Both instruments
indicators, while the electrical system provides the power for indicate turn direction and quality (coordination), and also
the turn coordinator. Most aircraft have at least two sources serve as a backup source of bank information in the event an
of power to ensure at least one source of bank information is attitude indicator fails. Coordination is achieved by referring
available if one power source fails. The vacuum or pressure to the inclinometer, which consists of a liquid-filled curved
system spins the gyro by drawing a stream of air against the tube with a ball inside. [Figure 7-21]
rotor vanes to spin the rotor at high speed, much like the
operation of a waterwheel or turbine. The amount of vacuum

7-16

Typical vacuum system

Heading Indicator Vacuum relief valve
33 3

30

6
Overboard vent line
24

I2
2I I5

Vacuum pump

20 20
I0 I0

I0 I
I0
20 20

STBY PWR TEST

4 6
SUCTION
CT
2 8
INCHES MERCURT

0 I0
Suction Attitude Indicator
Gauge

Vacuum air filter

Figure 7-20. Typical vacuum system.

Turn-and-Slip Indicator invalid. Certain instruments have speciﬁc pitch and bank
The gyro in the turn-and-slip indicator rotates in the vertical limits that induce a tumble of the gyro.
plane, corresponding to the aircraft’s longitudinal axis. A
single gimbal limits the planes in which the gyro can tilt, and Turn Coordinator
a spring tries to return it to center. Because of precession, a The gimbal in the turn coordinator is canted; therefore, its
yawing force causes the gyro to tilt left or right, as viewed gyro can sense both rate of roll and rate of turn. Since turn
from the pilot seat. The turn-and-slip indicator uses a pointer, coordinators are more prevalent in training aircraft, this
called the turn needle, to show the direction and rate of turn. discussion concentrates on that instrument. When rolling into
The turn-and-slip indicator is incapable of “tumbling” off or out of a turn, the miniature aircraft banks in the direction
its rotational axis because of the restraining springs. When the aircraft is rolled. A rapid roll rate causes the miniature
extreme forces are applied to a gyro, the gyro is displaced aircraft to bank more steeply than a slow roll rate.
from its normal plane of rotation, rendering its indications

Horizontal gyro

Gimbal Gyro rotation
Gimbal rotation Gimbal rotation
Gyro rotation

Canted gyro
Standard rate turn index

Standard rate turn index
Inclinometer Inclinometer

Turn coordinator Turn-and-slip indicator

Figure 7-21. Turn indicators rely on controlled precession for their operation.

7-17

The turn coordinator can be used to establish and maintain turn is too great for the angle of bank, and the ball moves
a standard-rate turn by aligning the wing of the miniature to the outside of the turn. To correct for these conditions,
aircraft with the turn index. Figure 7-22 shows a picture of a and improve the quality of the turn, remember to “step on
turn coordinator. There are two marks on each side (left and the ball.” Varying the angle of bank can also help restore
right) of the face of the instrument. The first mark is used to coordinated flight from a slip or skid. To correct for a slip,
reference a wings level zero rate of turn. The second mark decrease bank and/or increase the rate of turn. To correct for
on the left and right side of the instrument serve to indicate a skid, increase the bank and/or decrease the rate of turn.
a standard rate of turn. A standard-rate turn is defined as a
turn rate of 3° per second. The turn coordinator indicates only Yaw String
the rate and direction of turn; it does not display a specific One additional tool which can be added to the aircraft is a
angle of bank. yaw string. A yaw string is simply a string or piece of yarn
attached to the center of the wind screen. When in coordinated
D.C. D.C.
flight, the string trails straight back over the top of the wind
screen. When the aircraft is either slipping or skidding,
ELEC. ELEC.

the yaw string moves to the right or left depending on the
direction of slip or skid.
TURN COORDINATOR TURN COORDINATOR

L R L R
2 MIN.
NO PITCH
INFORMATION
2 MIN.
NO PITCH
INFORMATION
Instrument Check
During the preflight, check to see that the inclinometer is
full of fluid and has no air bubbles. The ball should also be
Slipping turn Skidding turn
resting at its lowest point. When taxiing, the turn coordinator
should indicate a turn in the correct direction while the ball
moves opposite the direction of the turn.
D.C.
ELEC.

Attitude Indicator
TURN COORDINATOR
The attitude indicator, with its miniature aircraft and horizon
L R
2 MIN.
NO PITCH
bar, displays a picture of the attitude of the aircraft. The
INFORMATION
relationship of the miniature aircraft to the horizon bar is
the same as the relationship of the real aircraft to the actual
Coordinated turn horizon. The instrument gives an instantaneous indication of
even the smallest changes in attitude.
Figure 7-22. If inadequate right rudder is applied in a right turn, a
slip results. Too much right rudder causes the aircraft to skid through The gyro in the attitude indicator is mounted in a horizontal
the turn. Centering the ball results in a coordinated turn. plane and depends upon rigidity in space for its operation.
The horizon bar represents the true horizon. This bar is
fixed to the gyro and remains in a horizontal plane as the
Inclinometer
aircraft is pitched or banked about its lateral or longitudinal
The inclinometer is used to depict aircraft yaw, which is
axis, indicating the attitude of the aircraft relative to the true
the side-to-side movement of the aircraft’s nose. During
horizon. [Figure 7-23]
coordinated, straight-and-level flight, the force of gravity
causes the ball to rest in the lowest part of the tube, centered
The gyro spins in the horizontal plane and resists deflection of
between the reference lines. Coordinated flight is maintained
the rotational path. Since the gyro relies on rigidity in space,
by keeping the ball centered. If the ball is not centered, it can
the aircraft actually rotates around the spinning gyro.
be centered by using the rudder.
An adjustment knob is provided with which the pilot may
To center the ball, apply rudder pressure on the side to which
move the miniature aircraft up or down to align the miniature
the ball is deflected. Use the simple rule, “step on the ball,” to
aircraft with the horizon bar to suit the pilot’s line of vision.
remember which rudder pedal to press. If aileron and rudder
Normally, the miniature aircraft is adjusted so that the wings
are coordinated during a turn, the ball remains centered in the
overlap the horizon bar when the aircraft is in straight-and-
tube. If aerodynamic forces are unbalanced, the ball moves
level cruising flight.
away from the center of the tube. As shown in Figure 7-22, in
a slip, the rate of turn is too slow for the angle of bank, and
the ball moves to the inside of the turn. In a skid, the rate of

7-18

Attitude indicator
The pitch and bank limits depend upon the make and model
Bank index of the instrument. Limits in the banking plane are usually
Gimbal rotation
from 100° to 110°, and the pitch limits are usually from 60°
to 70°. If either limit is exceeded, the instrument will tumble
or spill and will give incorrect indications until realigned. A
2 20
number of modern attitude indicators do not tumble.
20 I0 I0
I0 0

I I0
Every pilot should be able to interpret the banking scale
I0
20
0 20 illustrated in Figure 7-24. Most banking scale indicators on
the top of the instrument move in the same direction from
TEST
BY PW
R
that in which the aircraft is actually banked. Some other
models move in the opposite direction from that in which the
Roll gimbal aircraft is actually banked. This may confuse the pilot if the
Horizon reference arm
indicator is used to determine the direction of bank. This scale
Gyro should be used only to control the degree of desired bank.
Pitch gimbal
The relationship of the miniature aircraft to the horizon bar
should be used for an indication of the direction of bank.
Figure 7-23. Attitude indicator.

20
20
I0 20 20
20 I0
I0 I0 20
I0
I0
I0
20 I0
I0 I0 I0 20
I0
20 20 20
20

STBY PWR TEST STBY PWR TEST STBY PWR TEST

Climbing left bank Straight climb Climbing right bank

Pointer
10° Ba
20° n
30°
k
sc

20
20
a
le

I0
20
45° I0
I0 20
I0
I0 0
20 I0
I0 20 20 60° I0
20

20
I0 I0 20

STBY PWR TEST
90° STBY PWR TEST

I0 I0
I
20 20

STBY PWR TEST

Level left bank Level right bank
Artificial horizon Adjustment knob

20
20
20 20
I0 I0
20 I0 I0
I0 20
I0
I0
20 I0
I0
I0 I0
0 20
20 20 0
I0
20
20

STBY PWR TEST STBY PWR TEST STBY PWR TEST

Descending left bank Straight descent Descending right bank

Figure 7-24. Attitude representation by the attitude indicator corresponds to the relation of the aircraft to the real horizon.

7-19

The attitude indicator is reliable and the most realistic flight caused by friction, the heading indicator may indicate as
instrument on the instrument panel. Its indications are very much as 15° error per every hour of operation.
close approximations of the actual attitude of the aircraft.
Some heading indicators referred to as horizontal situation
Heading Indicator indicators (HSI) receive a magnetic north reference from
The heading indicator is fundamentally a mechanical a magnetic slaving transmitter, and generally need no
instrument designed to facilitate the use of the magnetic adjustment. The magnetic slaving transmitter is called a
compass. Errors in the magnetic compass are numerous, magnetometer.
making straight flight and precision turns to headings difficult
to accomplish, particularly in turbulent air. A heading Attitude and Heading Reference System (AHRS)
indicator, however, is not affected by the forces that make Electronic flight displays have replaced free-spinning gyros
the magnetic compass difficult to interpret. [Figure 7-25] with solid-state laser systems that are capable of flight at
Heading indicator any attitude without tumbling. This capability is the result
of the development of the Attitude and Heading Reference
Main drive gear Compass card gear System (AHRS).
Gimbal rotation
The AHRS sends attitude information to the PFD in order
to generate the pitch and bank information of the attitude
33 3 indicator. The heading information is derived from a
magnetometer which senses the earth’s lines of magnetic
6

flux. This information is then processed and sent out to the
PFD to generate the heading display. [Figure 7-26]
I2

I5
2I

Gimbal Adjustment gears

Gyro Adjustment knob

Figure 7-25. A heading indicator displays headings based on a 360°
azimuth, with the final zero omitted. For example, “6” represents
060°, while “21” indicates 210°. The adjustment knob is used to
align the heading indicator with the magnetic compass.

The operation of the heading indicator depends upon the
principle of rigidity in space. The rotor turns in a vertical
plane and fixed to the rotor is a compass card. Since the rotor
remains rigid in space, the points on the card hold the same
position in space relative to the vertical plane of the gyro. The Figure 7-26. Attitude and heading reference system (AHRS).
aircraft actually rotates around the rotating gyro, not the other
way around. As the instrument case and the aircraft revolve The Flux Gate Compass System
around the vertical axis of the gyro, the card provides clear As mentioned earlier, the lines of flux in the Earth’s magnetic
and accurate heading information. field have two basic characteristics: a magnet aligns with
them, and an electrical current is induced, or generated, in
Because of precession caused by friction, the heading any wire crossed by them.
indicator creeps or drifts from a heading to which it is set.
Among other factors, the amount of drift depends largely The flux gate compass that drives slaved gyros uses the
upon the condition of the instrument. If the bearings are worn, characteristic of current induction. The flux valve is a small,
dirty, or improperly lubricated, the drift may be excessive. segmented ring, like the one in Figure 7-27, made of soft iron
Another error in the heading indicator is caused by the fact that readily accepts lines of magnetic flux. An electrical coil
that the gyro is oriented in space, and the Earth rotates in is wound around each of the three legs to accept the current
space at a rate of 15° in 1 hour. Thus, discounting precession

7-20

Figure 7-28. The current in each of the three pickup coils changes
with the heading of the aircraft..

The slaving control and compensator unit has a push button
that provides a means of selecting either the “slaved gyro”
or “free gyro” mode. This unit also has a slaving meter
and two manual heading-drive buttons. The slaving meter
indicates the difference between the displayed heading and
the magnetic heading. A right deflection indicates a clockwise
Figure 7-27. The soft iron frame of the flux valve accepts the flux
error of the compass card; a left deflection indicates a
from the Earth’s magnetic field each time the current in the center
counterclockwise error. Whenever the aircraft is in a turn
coil reverses. This flux causes current to flow in the three pickup
and the card rotates, the slaving meter shows a full deflection
coils.
to one side or the other. When the system is in “free gyro”
induced in this ring by the Earth’s magnetic field. A coil mode, the compass card may be adjusted by depressing the
wound around the iron spacer in the center of the frame has appropriate heading-drive button.
400 Hz alternating current (AC) flowing through it. During
the times when this current reaches its peak, twice during A separate unit, the magnetic slaving transmitter is mounted
each cycle, there is so much magnetism produced by this remotely, usually in a wingtip to eliminate the possibility of
coil that the frame cannot accept the lines of flux from the
Earth’s field.

As the current reverses between the peaks, it demagnetizes
the frame so it can accept the flux from the Earth’s field. As
this flux cuts across the windings in the three coils, it causes
current to flow in them. These three coils are connected in
such a way that the current flowing in them changes as the
heading of the aircraft changes. [Figure 7-28]

The three coils are connected to three similar but smaller coils
in a synchro inside the instrument case. The synchro rotates
the dial of a radio magnetic indicator (RMI) or a HSI.

Remote Indicating Compass
Remote indicating compasses were developed to compensate
for the errors and limitations of the older type of heading
indicators. The two panel-mounted components of a typical
system are the pictorial navigation indicator and the slaving
control and compensator unit. [Figure 7-29] The pictorial Figure 7-29. Pictorial navigation indicator (HSI, top), slaving meter
navigation indicator is commonly referred to as an HSI. (lower right), and slaving control compensator unit (lower left).

7-21

magnetic interference. It contains the flux valve, which is The bank and pitch limits of the heading indicator vary
the direction-sensing device of the system. A concentration with the particular design and make of instrument. On some
of lines of magnetic force, after being amplified, becomes heading indicators found in light aircraft, the limits are
a signal relayed to the heading indicator unit, which is also approximately 55° of pitch and 55° of bank. When either of
remotely mounted. This signal operates a torque motor in these attitude limits is exceeded, the instrument “tumbles”
the heading indicator unit that processes the gyro unit until or “spills” and no longer gives the correct indication until
it is aligned with the transmitter signal. The magnetic slaving reset. After spilling, it may be reset with the caging knob.
transmitter is connected electrically to the HSI. Many of the modern instruments used are designed in such
a manner that they do not tumble.
There are a number of designs of the remote indicating
compass; therefore, only the basic features of the system are An additional precession error may occur due to a gyro not
covered here. Instrument pilots must become familiar with spinning fast enough to maintain its alignment. When the
the characteristics of the equipment in their aircraft. vacuum system stops producing adequate suction to maintain
the gyro speed, the heading indicator and the attitude indicator
As instrument panels become more crowded and the pilot’s gyros begin to slow down. As they slow, they become more
available scan time is reduced by a heavier flight deck susceptible to deflection from the plane of rotation. Some
workload, instrument manufacturers have worked toward aircraft have warning lights to indicate that a low vacuum
combining instruments. One good example of this is the situation has occurred. Other aircraft may have only a vacuum
RMI in Figure 7-30. The compass card is driven by signals gauge that indicates the suction.
from the flux valve, and the two pointers are driven by an
automatic direction finder (ADF) and a very high frequency Instrument Check
(VHF) omni-directional radio range (VOR). As the gyro spools up, make sure there are no abnormal
sounds. While taxiing, the instrument should indicate turns in
the correct direction, and precession should not be abnormal.
At idle power settings, the gyroscopic instruments using the
vacuum system might not be up to operating speeds and
precession might occur more rapidly than during flight.

Compass Systems
The Earth is a huge magnet, spinning in space, surrounded
by a magnetic field made up of invisible lines of flux. These
lines leave the surface at the magnetic north pole and reenter
at the magnetic South Pole.

Lines of magnetic flux have two important characteristics:
any magnet that is free to rotate will align with them, and
an electrical current is induced into any conductor that cuts
across them. Most direction indicators installed in aircraft
Figure 7-30. Driven by signals from a flux valve, the compass card make use of one of these two characteristics.
in this RMI indicates the heading of the aircraft opposite the upper
center index mark. The green pointer is driven by the ADF. Magnetic Compass
One of the oldest and simplest instruments for indicating
direction is the magnetic compass. It is also one of the basic
Heading indicators that do not have this automatic
instruments required by Title 14 of the Code of Federal
northseeking capability are called “free” gyros, and require
Regulations (14 CFR) part 91 for both VFR and IFR flight.
periodic adjustment. It is important to check the indications
frequently (approximately every 15 minutes) and reset the
A magnet is a piece of material, usually a metal containing
heading indicator to align it with the magnetic compass
iron, which attracts and holds lines of magnetic flux.
when required. Adjust the heading indicator to the magnetic
Regardless of size, every magnet has two poles: north and
compass heading when the aircraft is straight and level at a
south. When one magnet is placed in the field of another, the
constant speed to avoid compass errors.
unlike poles attract each other, and like poles repel.

7-22

An aircraft magnetic compass, such as the one in Figure 7-30, A compensator assembly mounted on the top or bottom
has two small magnets attached to a metal float sealed inside a of the compass allows an aviation maintenance technician
bowl of clear compass fluid similar to kerosene. A graduated (AMT) to create a magnetic field inside the compass housing
scale, called a card, is wrapped around the float and viewed that cancels the influence of local outside magnetic fields.
through a glass window with a lubber line across it. The card This is done to correct for deviation error. The compensator
is marked with letters representing the cardinal directions, assembly has two shafts whose ends have screwdriver slots
north, east, south, and west, and a number for each 30° accessible from the front of the compass. Each shaft rotates
between these letters. The final “0” is omitted from these one or two small compensating magnets. The end of one shaft
directions. For example, 3 = 30°, 6 = 60°, and 33 = 330°. is marked E-W, and its magnets affect the compass when the
There are long and short graduation marks between the letters aircraft is pointed east or west. The other shaft is marked
and numbers, each long mark representing 10° and each short N-S and its magnets affect the compass when the aircraft is
mark representing 5°. pointed north or south.

Magnetic Compass Induced Errors
The magnetic compass is the simplest instrument in the
panel, but it is subject to a number of errors that must be
considered.

Variation
The Earth rotates about its geographic axis; maps and charts
are drawn using meridians of longitude that pass through the
geographic poles. Directions measured from the geographic
poles are called true directions. The magnetic North Pole to
which the magnetic compass points is not collocated with
the geographic North Pole, but is some 1,300 miles away;
Figure 7-31. A magnetic compass. The vertical line is called the directions measured from the magnetic poles are called
lubber line. magnetic directions. In aerial navigation, the difference
between true and magnetic directions is called variation. This
same angular difference in surveying and land navigation is
The float and card assembly has a hardened steel pivot in its called declination.
center that rides inside a special, spring-loaded, hard glass
jewel cup. The buoyancy of the float takes most of the weight Figure 7-32 shows the isogonic lines that identify the number
off the pivot, and the fluid damps the oscillation of the float of degrees of variation in their area. The line that passes near
and card. This jewel-and-pivot type mounting allows the float Chicago is called the agonic line. Anywhere along this line
freedom to rotate and tilt up to approximately 18° angle of the two poles are aligned, and there is no variation. East
bank. At steeper bank angles, the compass indications are of this line, the magnetic North Pole is to the west of the
erratic and unpredictable. geographic North Pole and a correction must be applied to a
compass indication to get a true direction.
The compass housing is entirely full of compass fluid. To
prevent damage or leakage when the fluid expands and
contracts with temperature changes, the rear of the compass
case is sealed with a flexible diaphragm, or with a metal
bellows in some compasses.

The magnets align with the Earth’s magnetic field and the
pilot reads the direction on the scale opposite the lubber line.
Note that in Figure 7-31, the pilot sees the compass card from
its backside. When the pilot is flying north as the compass
shows, east is to the pilot’s right. On the card, “33”, which
represents 330° (west of north), is to the right of north. The
reason for this apparent backward graduation is that the card
remains stationary, and the compass housing and the pilot turn
around it, always viewing the card from its backside. Figure 7-32. Isogonic lines are lines of equal variation.

7-23

Flying in the Washington, D.C., area, for example, the variation
is 10° west. If a pilot wants to fly a true course of south (180°),
the variation must be added to this, resulting in a magnetic course
of 190° to fly. Flying in the Los Angeles, California, area, the
variation is 14° east. To fly a true course of 180° there, the pilot
would have to subtract the variation and fly a magnetic course
of 166°. The variation error does not change with the heading of
the aircraft; it is the same anywhere along the isogonic line.
Figure 7-34. A compass correction card shows the deviation
Deviation correction for any heading.
The magnets in a compass align with any magnetic field.
complete the compass coreection card. If the pilot wants to
Local magnetic fields in an aircraft caused by electrical
fly a magnetic heading of 120° and the aircraft is operating
current flowing in the structure, in nearby wiring or any
with the radios on, the pilot should fly a compass heading
magnetized part of the structure, conflict with the Earth’s
of 123°.
magnetic field and cause a compass error called deviation.
The corrections for variation and deviation must be applied
Deviation, unlike variation, is different on each heading,
in the correct sequence and is shown below, starting from
but it is not affected by the geographic location. Variation
the true course desired.
error cannot be reduced or changed, but deviation error can
be minimized when an AMT performs the maintenance task
Step 1: Determine the Magnetic Course
known as “swinging the compass.”
True Course (180°) ± Variation (+10°) = Magnetic Course (190°)
Most airports have a compass rose, which is a series of lines
The magnetic course (190°) is steered if there is no deviation
marked out on a ramp or maintenance runup area where there
error to be applied. The compass card must now be considered
is no magnetic interference. Lines, oriented to magnetic north,
for the compass course of 190°.
are painted every 30°, as shown in Figure 7-33.
Step 2: Determine the Compass Course
Magnetic Course (190°, from step 1) ± Deviation (–2°, from
True north correction card) = Compass Course (188°)

330 N NOTE: Intermediate magnetic courses between those listed
300 030 on the compass card need to be interpreted. Therefore, to
steer a true course of 180°, the pilot would follow a compass
W 060 course of 188°.

240 E
To find the true course that is being flown when the compass
course is known:
210 120 Compass Course ± Deviation = Magnetic Course ± Variation=
True Course
S 150

Dip Errors
Figure 7-33. Utilization of a compass rose aids compensation for The lines of magnetic flux are considered to leave the Earth
deviation errors. at the magnetic North Pole and enter at the magnetic South
The AMT aligns the aircraft on each magnetic heading and Pole. At both locations the lines are perpendicular to the
adjusts the compensating magnets to minimize the difference Earth’s surface. At the magnetic equator, which is halfway
between the compass indication and the actual magnetic between the poles, the lines are parallel with the surface. The
heading of the aircraft. Any error that cannot be removed magnets in a compass align with this field, and near the poles
is recorded on a compass correction card, like the one in they dip, or tilt, the float and card. The float is balanced with
Figure 7-34, and placed in a cardholder near the compass. The a small dip-compensating weight, to dampen the effects of
pilot can taxi the aircraft to the compass rose and maneuver dip when operating in the middle latitudes of the northern
the aircraft to the headings prescribed by the AMT, and if hemisphere. This dip (and weight) causes two very noticeable
authorized to do so, the AMT can also taxi and maneuver the errors: northerly turning error and acceleration error.
aircraft; however, only the AMT can adjust the compass or

7-24

The pull of the vertical component of the Earth’s magnetic field When an aircraft is flying on a heading of south and begins
causes northerly turning error, which is apparent on a heading a turn toward east, the Earth’s magnetic field pulls on the
of north or south. When an aircraft flying on a heading of north end of the magnet that rotates the card toward east, the same
makes a turn toward east, the aircraft banks to the right, and direction the turn is being made. If the turn is made from
the compass card tilts to the right. The vertical component of south toward west, the magnetic pull starts the card rotating
the Earth’s magnetic field pulls the northseeking end of the toward west—again, in the same direction the turn is being
magnet to the right, and the float rotates, causing the card to made. The rule for this error is: when starting a turn from a
rotate toward west, the direction opposite the direction the turn southerly heading, the compass indication leads the turn.
is being made. [Figure 7-35]
In acceleration error, the dip-correction weight causes the end
If the turn is made from north to west, the aircraft banks to of the float and card marked N (the south-seeking end) to be
the left and the compass card tilts down on the left side. The heavier than the opposite end. When the aircraft is flying at
magnetic field pulls on the end of the magnet that causes the a constant speed on a heading of east or west, the float and
card to rotate toward east. This indication is again opposite to card is level. The effects of magnetic dip and the weight are
the direction the turn is being made. The rule for this error is: approximately equal. If the aircraft accelerates on a heading
when starting a turn from a northerly heading, the compass of east [Figure 7-36], the inertia of the weight holds its end of
indication lags behind the turn. the float back and the card rotates toward north. As soon as the

Figure 7-35. Northerly turning error.

Figure 7-36. Effects of acceleration error.

7-25

speed of the aircraft stabilizes, the card swings back to its east Lags or Leads
indication. If, while flying on this easterly heading, the aircraft When starting a turn from a northerly heading, the compass
decelerates, the inertia causes the weight to move ahead and the lags behind the turn. When starting a turn from a southerly
card rotates toward south until the speed again stabilizes. heading, the compass leads the turn.

When flying on a heading of west, the same things happen. Eddy Current Damping
Inertia from acceleration causes the weight to lag, and the
The decreased amplitude of oscillations by the interaction
card rotates toward north. When the aircraft decelerates on
of magnetic fields. In the case of a vertical card magnetic
a heading of west, inertia causes the weight to move ahead
compass, flux from the oscillating permanent magnet
and the card rotates toward south.
produces eddy currents in a damping disk or cup. The
magnetic flux produced by the eddy currents opposes the flux
A mnemonic, or memory jogger, for the effect of acceleration
from the permanent magnet and decreases the oscillations.
error is the word “ANDS” (acceleration—north, deceleration—
south). Acceleration causes an indication toward north; Outside Air Temperature (OAT) Gauge
deceleration causes an indication toward south.
The outside air temperature (OAT) gauge is a simple and
Oscillation Error effective device mounted so that the sensing element is
exposed to the outside air. The sensing element consists
Oscillation is a combination of all of the other errors, and it
of a bimetallic-type thermometer in which two dissimilar
results in the compass card swinging back and forth around
materials are welded together in a single strip and twisted
the heading being flown. When setting the gyroscopic
into a helix. One end is anchored into protective tube and the
heading indicator to agree with the magnetic compass, use
other end is affixed to the pointer, which reads against the
the average indication between the swings.
calibration on a circular face. OAT gauges are calibrated in
degrees °C, °F, or both. An accurate air temperature provides
The Vertical Card Magnetic Compass
the pilot with useful information about temperature lapse rate
The floating magnet type of compass not only has all the errors
with altitude change. [Figure 7-38] gauge
Outside air temperature
just described, but also lends itself to confused reading. It is
easy to begin a turn in the wrong direction because its card
appears backward. East is on what the pilot would expect to be 40
20 60
the west side. The vertical card magnetic compass eliminates
0
some of the errors and confusion. The dial of this compass 0 20 80
is graduated with letters representing the cardinal directions, -20

numbers every 30°, and tick marks every 5°. The dial is rotated -20 40 100
by a set of gears from the shaft-mounted magnet, and the nose -40
of the symbolic aircraft on the instrument glass represents the -40 60 120
lubber line for reading the heading of the aircraft from the dial. C
-60 140
Eddy currents induced into an aluminum-damping cup damp, F
Vertical card compass
or decrease, oscillation of the magnet. [Figure 7-37]

Figure 7-38. Outside air temperature (OAT) gauge.

N 3
33 Chapter Summary
W 30

6

Flight instruments enable an aircraft to be operated with
maximum performance and enhanced safety, especially when
E 12

flying long distances. Manufacturers provide the necessary
24

15 flight instruments, but to use them effectively, pilots need
S 21 to understand how they operate. As a pilot, it is important to
become very familiar with the operational aspects of the pitot-
static system and associated instruments, the vacuum system
and associated instruments, the gyroscopic instruments, and
the magnetic compass.

Figure 7-37. Vertical card compass.

7-26

Flight instruments chapter 07

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Flight instruments chapter 07