Aircraft ctrl systems

AIRCRAFT CONTROL SYSTEMS
UNIT - II

What is an Aircraft Control System?
• A control system is a collection of mechanical
and electronic equipment that allows an
aircraft to be flown with exceptional precision
and reliability.
• A control system consists of cockpit controls,
sensors, actuators (hydraulic, mechanical or
electrical) and computers.

Control Column (or) Control Yoke

Aircraft Primary Flight Controls In
Motion

Conventional Flight Control System
Components
Push Pull Rods

Turnbuckles
A turnbuckle, stretching screw or bottlescrew is
a device for adjusting the tension or length
of ropes, cables, tie rods and other tensioning
systems.

Torque Tube
A tube in an aircraft control
system that transmits a
torsional force from the
operating control to the
control surface. Torque
tubes are often used to
actuate ailerons and flaps.

Bell Crank
A double lever in an aircraft
control system used to
change the direction of
motion. Bell cranks are
normally used in aileron
controls and in the steering
system of nosewheels.

Fairleads
A fairlead is a device to guide a line, rope or cable around
an object, out of the way or to stop it from moving laterally.
Typically a fairlead will be a ring or hook. The fairlead may
be a separate piece of hardware, or it could be a hole in the
structure.

Mechanical Flight Control System
• Basic method of controlling an aircraft
• Used in early aircraft and currently in small aircraft
where the aerodynamic forces are not excessive.
• It uses a collection of mechanical parts such as rods,
tension cables, pulleys, counterweights, and
sometimes chains to transmit the forces applied from
the cockpit controls directly to the control surfaces

Push Pull Rod System for Elevator Control

Cables & Pulleys System for Elevator Control

• Gust locks are often used on parked aircraft
with mechanical systems to protect the control
surfaces and linkages from damage from wind

• Increases in the control surface area required
by large aircraft or higher loads caused by high
airspeeds in small aircraft lead to a large
increase in the forces needed to move them,
consequently complicated mechanical gearing
arrangements were developed to extract
maximum mechanical advantage in order to
reduce the forces required from the pilots. This
arrangement can be found on bigger or higher
performance propeller aircraft such as the
Fokker 50.

• Some mechanical flight control systems use Servo tabs
that provide aerodynamic assistance. Servo tabs are small
surfaces hinged to the control surfaces. The flight control
mechanisms move these tabs, aerodynamic forces in turn
move, or assist the movement of the control surfaces
reducing the amount of mechanical forces needed. This
arrangement was used in early piston-engined transport
aircraft and in early jet transports. The Boeing 737
incorporates a system, whereby in the unlikely event of
total hydraulic system failure, it automatically and
seamlessly reverts to being controlled via servo-tab.

Servo Tabs
• In large aircrafts the control surfaces are operated by
power operated hydraulic actuators controlled by
valves moved by control yoke and rudder pedals. An
artificial feel system gives the pilot resistance that is
proportional to the flight loads on the surfaces.
• In the event of hydraulic system failure , the control
surfaces are controlled by servo tabs in a process
known as manual reversion.
• In the manual mode the flight control column moves
the tab on the c/surface and the aerodynamic forces
caused by the deflected tab moves the main control
surface

Flight Control Surfaces On An Modern
Advanced Aircraft

Need for Powered Control System
• The Complexity and Weight of the system
(Mechanical) increased with Size and
Performance of the aircraft.
• When the pilot’s action is not directly
sufficient for the control, the main option is a
powered system that assists the pilot.
• The hydraulic system has demonstrated to be a
more suitable solution for actuation in terms of
reliability, safety, weight per unit power and
flexibility, with respect to the electrical system

Powered Assisted Control System
• The pilot, via the cabin components, sends a
signal, or demand, to a valve that opens ports
through which high pressure hydraulic fluid
flows and operates one or more actuators.
• The valve, that is located near the actuators,
can be signalled in two different ways:
mechanically or electrically
• Mechanical signalling is obtained by push-pull
rods, or more commonly by cables and pulleys
• Electrical signalling is a solution of more
modern and sophisticated vehicles

• The basic principle of the hydraulic control is
simple, but two aspects must be noticed when a
powered control is introduced:
• The system must control the surface in a
proportional way, i.e. the surface response
(deflection) must be function to the pilot’s
demand (stick deflection, for instance)
• The pilot that with little effort acts on a control
valve must have a feedback on the maneuver
intensity

• The first problem is solved by using (hydraulic)
servo-mechanisms, where the components are
linked in such a way to introduce an actuator
stroke proportional to the pilot’s demand

• The pilot, in normal hydraulic operating
conditions, is requested for a very low effort,
necessary to contrast the mechanical frictions of
the linkage and the movement of the control
valve
• The pilot is then no more aware of the load
condition being imposed to the aircraft.
• An artificial feel is introduced in powered
systems, acting directly on the cabin control stick
or pedals.

• The simplest solution is a spring system, then
responding to the pilot’s demand with a force
proportional to the stick deflection; this solution
has of course the limit to be not sensitive to the
actual flight conditions.
• A more sophisticated artificial feel is the so-
called Q feel. This system receives data from the
pitot-static probes, reading the dynamic pressure,
or the difference between total (pt) and static (ps)
pressure, that is proportional to the aircraft speed
v through the air density ρ:

• This signal is used to modulate a hydraulic
cylinder that increases the stiffness in the
artificial feel system, in such a way that the pilot
is given a contrast force in the pedals or stick
that is also proportional to the aircraft speed.

Disadvantages of Mechanical and
Hydro-Mechanical Systems
• Heavy and require careful routing of flight
control cables through the aircraft using
pulleys, cranks, tension cables and hydraulic
pipes.
• They require redundant backup to deal with
failures, which again increases weight.
• Limited ability to compensate for changing
aerodynamic conditions

Disadvantages of Mechanical and
Hydro-Mechanical Systems
• Dangerous characteristics such as stalling, spinning
and pilot-induced oscillation (PIO), which depend
mainly on the stability and structure of the aircraft
concerned rather than the control system itself, can
still occur with these systems
• By using electrical control circuits combined with
computers, designers can save weight, improve
reliability, and use the computers to mitigate the
undesirable characteristics mentioned above. Modern
advanced fly-by-wire systems are also used to control
unstable fighter aircraft

Fly –By –Wire System (FBW)
• The term "fly-by-wire" implies a purely
electrically-signalled control system
• It is a computer-configured controls, where a
computer system is interposed between the
operator and the final control actuators or
surfaces
• It modifies the manual inputs of the pilot in
accordance with control parameters
• These are carefully developed and validated in
order to produce maximum operational effect
without compromising safety

FBW – Introduction
• The FBW architecture was developed in 1970’s
• Initially starting as an analogue technique and
later on transformed into digital.
• It was first developed for military aviation,
where it is now a common solution
• The supersonic Concorde can be considered a
first and isolated civil aircraft equipped with a
(analogue) fly-by-wire system

FBW – Introduction
• In the 80’s the digital technique was imported
from military into civil aviation by Airbus, first
with the A320, then followed by A319, A321,
A330, A340, Boeing 777 and A380 (scheduled
for 2005).
• This architecture is based on computer signal
processing

Operation
• The pilot’s demand is first of all transduced
into electrical signal in the cabin and sent to a
group of independent computers (Airbus
architecture substitute the cabin control
column with a side stick)
• The computers sample also data concerning
the flight conditions and servo-valves and
actuators positions
• The pilot’s demand is then processed and sent
to the actuator, properly tailored to the actual
flight status.

Operation
• The flight data used by the system mainly
depend on the aircraft category; in general the
following data are sampled and processed:
– pitch, roll, yaw rate and linear accelerations
– Angle of attack and sideslip
– Airspeed/Mach number, Pressure, Altitude
and radio altimeter indications
– Stick and pedal demands
– Other cabin commands such as landing gear
condition, thrust lever position, etc.

Operation
• The full system has high redundancy to restore
the level of reliability of a mechanical or
hydraulic system, in the form of multiple
(triplex or quadruplex) parallel and
independent lanes to generate and transmit the
signals, and independent computers that
process them

FBW – Basic Operation
• When a pilot moves the control, a signal is sent
to a computer, this is analogous to moving a
game controller, the signal is sent through
multiple wires (channels) to ensure that the
signal reaches the computer.
• When there are three channels being used this is
known as 'Triplex'.
• The computer receives the signals, performs a
calculation (adds the signal voltages and divides
by the number of signals received to find the
mean average voltage) and adds another
channel.

FBW – Basic Operation
• These four 'Quadruplex' signals are then sent
to the control surface actuator and the surface
begins to move.
• Potentiometers in the actuator send a signal
back to the computer (usually a negative
voltage) reporting the position of the actuator.
• When the actuator reaches the desired position
the two signals (incoming and outgoing)
cancel each other out and the actuator stops
moving (completing a feedback loop).

FBW – Stability
• Three gyroscopes fitted with sensors are fitted
in the aircraft to sense movement changes in
the pitch, roll and yaw axes.
• Any movement (from straight and level flight
for example) results in signals being sent to the
computer which again moves the relevant
control actuators, however, the input is done
without the pilot's knowledge; the cockpit
controls do not move

FBW – Safety and Redundancy
• Aircraft systems may be quadruplexed (four
independent channels) in order to prevent loss
of signals in the case of failure of one or even
two channels.
• High performance aircraft that have FBW
controls (also called CCVs or Control-
Configured Vehicles) may be deliberately
designed to have low or even negative
aerodynamic stability in some flight regimes,
the rapid-reacting CCV controls compensating
for the lack of natural stability

FBW – Safety and Redundancy
• Pre-flight safety checks of a fly-by-wire
system are often performed using Built-In Test
Equipment (BITE).
• On programming the system, either by the
pilot or ground crew, a number of control
movement steps are automatically performed.
• Any failure will be indicated to the crews

FBW – Advantages
• Flight envelope protection (the computers will
reject and tune pilot’s demands that might
exceed the airframe load factors)
• Increase of stability and handling qualities
across the full flight envelope, including the
possibility of flying unstable vehicles
• Turbulence suppression and consequent
decrease of fatigue loads and increase of
passenger comfort

FBW – Advantages
• Use of thrust vectoring to augment or replace
lift aerodynamic control, then extending the
aircraft flight envelope
• Drag reduction by an optimised trim setting
• Higher stability during release of tanks and
weapons

FBW – Advantages
• Easier interfacing to auto-pilot and other
automatic flight control systems
• Weight reduction (mechanical linkages are
substituted by wirings)
• Maintenance reduction
• Reduction of airlines’ pilot training costs
(flight handling becomes very similar in an
whole aircraft family)

F-8C Crusader
Digital fly-by-wire test bed
(1972)
The Airbus A320,
First airliner with Digital fly-by-wire controls
(1984)
A Dassault Falcon 7X,
The first business jet with Digital fly-by-
wire controls
(2005)

Digital Fly-By-Wire (DFBW)
• A digital fly-by-wire flight control system is
similar to analog system. However, the signal
processing is done by digital computers and
the pilot literally can "fly-via-computer".
• Increases in flexibility of the flight control
system, since the digital computers can receive
input from any aircraft sensor (such as the
altimeters and the pitot tubes).
• Increase in electronic stability - system is less
dependent on the values of critical electrical
components in an analog controller

• The computers "read" position and force inputs
from the pilot's controls and aircraft sensors.
• They solve differential equations to determine
the appropriate command signals that move the
flight controls in order to carry out the
intentions of the pilot
• The programming of the digital computers
enable flight envelope protection.

• Aircraft designers precisely tailor an aircraft's
handling characteristics, to stay within the
overall limits of what is possible given the
aerodynamics and structure of the aircraft.
• Flight-control computers continuously "fly"
the aircraft, pilot's workloads can be reduced
• In military and naval applications, it is now
possible to fly military aircraft that have
relaxed stability.

• Better maneuverability during combat and
training flights and " carefree handling"
because stalling, spinning. and other
undesirable performances are prevented
automatically by the computers
• Enable inherently unstable combat aircraft,
such as the F-117 Nighthawk and the B-2
Spirit flying wing to fly in usable and safe
manners

DFBW - Redundancy
• If one of the flight-control computers crashes -
or is damaged in combat; or suffers from
"insanity" caused by electromagnetic pulses -
the others overrule the faulty one (or even two
of them), they continue flying the aircraft
safely, and they can either turn off or re-boot
the faulty computers.
• Any flight-control computer whose results
disagree with the others is ruled to be faulty,
and it is either ignored or re-booted.

DFBW - Redundancy
• Most of the early digital fly-by-wire aircraft
also had an analog electrical, a mechanical, or
a hydraulic back-up flight control system
• The Space Shuttle has, in addition to its
redundant set of four digital computers running
its primary flight-control software, a fifth
back-up computer running a separately
developed, reduced-function, software flight-
control system - one that can be commanded to
take over in the event that a fault ever affects
all of the computers in the other four.

DFBW - Redundancy
• This back-up system serves to reduce the risk
of total flight-control-system failure ever
happening because of a general-purpose flight
software fault has escaped notice in the other
four computers.
• For airliners, flight-control redundancy
improves their safety
• Fly-by-wire control systems also improve
economy in flight because they are lighter, and
they eliminate the need for many mechanical,
and heavy, flight-control mechanisms

DFBW - Redundancy
• Most modern airliners have computerized
systems that control their jet engine throttles,
air inlets, fuel storage and distribution system,
in such a way to minimize their consumption
of jet fuel. Thus, digital control systems do
their best to reduce the cost of flights

Engine Control Systems
• To allow the engine to perform at maximum
efficiency for a given condition
• Aids the pilot to control and monitor the
operation of the aircraft's power plant
• Originally, engine control systems consisted of
simple mechanical linkages controlled by the
pilot then evolved and became the
responsibility of the third pilot-certified crew
member, the flight engineer

• By moving throttle levers directly connected to
the engine, the pilot or the flight engineer
could control fuel flow, power output, and
many other engine parameters.
• Following mechanical means of engine control
came the introduction of analog electronic
engine control.
• Analog electronic control varies an electrical
signal to communicate the desired engine
settings

• It had its drawbacks including common
electronic noise interference and reliability
issues
• Full authority analogue control was used in the
1960s.
• It was introduced as a component of the Rolls
Royce Olympus 593 engine of the supersonic
transport aircraft Concorde. However the more
critical inlet control was digital on the
production aircraft.

• In the 1970s NASA and Pratt and Whitney
experimented with the first experimental
FADEC, first flown on an F-111 fitted with a
highly modified Pratt & Whitney TF30 left
engine
F-111C - Fighter - BomberRolls Royce Olympus 593 engine

• Pratt & Whitney F100 – First
Military Engine
• Pratt & Whitney PW2000 -
First Civil Engine fitted with
FADEC
• Pratt & Whitney PW4000 -
First commercial "dual
FADEC" engine.
• The Harrier II Pegasus
engine by Dowty & Smiths
Industries Controls - The
first FADEC in service
Harrier II
Pegasus Engine

Functions
• FADEC works by receiving multiple input
variables of the current flight condition including
air density, throttle lever position, engine
temperatures, engine pressures, and many other
parameters
• The inputs are received by the EEC and analyzed
up to 70 times per second
• Engine operating parameters such as fuel flow,
stator vane position, bleed valve position, and
others are computed from this data and applied as
appropriate

Functions
• It controls engine starting and restarting.
• Its basic purpose is to provide optimum engine
efficiency for a given flight condition.
• It also allows the manufacturer to program
engine limitations and receive engine health and
maintenance reports. For example, to avoid
exceeding a certain engine temperature, the
FADEC can be programmed to automatically
take the necessary measures without pilot
intervention.

Functions
• The flight crew first enters flight data such as
wind conditions, runway length, or cruise
altitude, into the flight management system
(FMS). The FMS uses this data to calculate
power settings for different phases of the flight.
• At takeoff, the flight crew advances the throttle
to a predetermined setting, or opts for an auto-
throttle takeoff if available.
• The FADECs now apply the calculated takeoff
thrust setting by sending an electronic signal to
the engines

Functions
• There is no direct linkage to open fuel flow. This
procedure can be repeated for any other phase of
flight
• In flight, small changes in operation are
constantly made to maintain efficiency.
• Maximum thrust is available for emergency
situations if the throttle is advanced to full, but
limitations can’t be exceeded
• The flight crew has no means of manually
overriding the FADEC

Functions
• True full authority digital engine controls have no form
of manual override available, placing full authority over
the operating parameters of the engine in the hands of
the computer
• If a total FADEC failure occurs, the engine fails
• If the engine is controlled digitally and electronically but
allows for manual override, it is considered solely an
EEC or ECU.
• An EEC, though a component of a FADEC, is not by
itself FADEC. When standing alone, the EEC makes all
of the decisions until the pilot wishes to intervene.

Safety
• With the operation of the engines so heavily
relying on automation, safety is a great concern.
• Redundancy is provided in the form of two or
more, separate identical digital channels.
• Each channel may provide all engine functions
without restriction.
• FADEC also monitors a variety of analog, digital
and discrete data coming from the engine
subsystems and related aircraft systems,
providing for fault tolerant engine control

Applications
• FADECs are employed by almost all current
generation jet engines, and increasingly in piston
engines for fixed-wing aircraft and helicopters.
• The system replaces both magnetos in piston-
engined aircraft, which makes costly magneto
maintenance obsolete and eliminates carburetor
heat, mixture controls and engine priming.
• Since, it controls each engine cylinder
independently for optimum fuel injection and
spark timing, the pilot no longer needs to monitor
fuel mixture.

Applications
• More precise mixtures create less engine wear,
which reduces operating costs and increases
engine life for the average aircraft.
• Tests have also shown significant fuel savings

Advantages
• Better fuel efficiency
• Automatic engine protection against out-of-
tolerance operations
• Safer as the multiple channel FADEC computer
provides redundancy in case of failure
• Care-free engine handling, with guaranteed thrust
settings
• Ability to use single engine type for wide thrust
requirements by just reprogramming the FADECs

Advantages
• Provides semi-automatic engine starting
• Better systems integration with engine and
aircraft systems
• Can provide engine long-term health
monitoring and diagnostics
• Reduces the number of parameters to be
monitored by flight crews

Advantages
• Due to the high number of parameters
monitored, the FADEC makes possible "Fault
Tolerant Systems" (where a system can
operate within required reliability and safety
limitation with certain fault configurations)
• Can support automatic aircraft and engine
emergency responses (e.g. in case of aircraft
stall, engines increase thrust automatically).

Disadvantages
• No form of manual override available, placing
full authority over the operating parameters of
the engine in the hands of the computer.
• If a total FADEC failure occurs, the engine
fails.
• In the event of a total FADEC failure, pilots
have no way of manually controlling the
engines for a restart, or to otherwise control
the engine.
• With any single point of failure, the risk can be
mitigated with redundant FADECs

Disadvantages
• High system complexity compared to
hydromechanical, analogue or manual control
systems
• High system development and validation effort
due to the complexity

COMMUNICATION AND NAVIGATION SYSTEMS

INSTRUMENT
LANDING
SYSTEM (ILS)

What Is ILS?
 ILS is stand for Instrument Landing
System.
 It has been existence for over 60 years.
 But today, it is still the most accurate
approach and landing aid that is used by
the airliners.
 Why need ILS?

History of ILS
The first scheduled passenger airliner to land
using ILS was in 1938.

The Uses of ILS
 To guide the pilot during the approach and landing.
 It is very helpful when visibility is limited and the
pilot cannot see the airport and runway.
 To provide an aircraft with a precision final
approach.
 To help the aircraft to a runway touchdown
point.
 To provide an aircraft guidance to the runway both
in the horizontal and vertical planes.
 To increase safety and situational awareness.

Poor Visibility Landings
 Scheduled service would be impossible without
a way to land in poor weather.

Runway Approach
97
Non-Instrument Runway (NI)
Non-Precision Runway (NP)
Precision Runway (P)
Threshold
Touchdown
zone
Aiming
point

Types of Runway Approach
1.Non-Instrument Runway (NI)
 A runway intended for the operation of aircraft using visual
approach procedure
2. Instrument Runway
 A runway intended for the operation of aircraft using
instrument approach procedures
a) Non-Precision Runway (NP)
• An instrument runway served by visual aids and a non-
visual aid providing at least lateral guidance adequate for a
straight-in approach
b) Precision Runway (P)
• Allow operations with a decision height and
visibility corresponding to Category 1,
or II, or III

Precision Runway (P) Categories
 Runway Threshold: Beginning of runway for
landing.
 Touchdown zone: The first point for the aircraft
should touch the runway during landing.
 Aiming point: serves as a visual aiming point for a
landing aircraft.

ILS Components
 ILS consists of Ground Installations and Airborne
Equipments
 There are 3 equipments for Ground Installations,
which are:
1. Ground Localizer (LLZ) Antenna – To provide
horizontal navigation
2. Ground Glide path (GP) Antenna – To provide
vertical navigation
3. Marker Beacons – To enable the pilot cross check the
aircraft’s height.
 There are 2 equipments for Airborne Equipments,
which are:
1. LLZ and GP antennas located on the aircraft
nose.
2. ILS indicator inside the cockpit

ILS Components
Ground Localizer Antenna
Ground Glide Path Antenna
ILS Indicator inside the
cockpit

ILS Indicator
Localizer
Deviation from runway
centre line
Glidepath
Deviation from optimal
glide path
Signal Integrity Flag
Indicates if instrument is
unreliable
“Dots”
Each “dot” on the
instrument represents 2° of
deviation

How ILS works?
 Ground localizer antenna transmit VHF signal in
direction opposite of runway to horizontally guide
aircraft to the runway centre line.
 Ground Glide Path antenna transmit UHF signal in
vertical direction to vertically guide aircraft to the
touchdown point.
 Localizer and Glide Path antenna located at aircraft
nose receives both signals and sends it to ILS
indicator in the cockpit.
 These signals activate the vertical and horizontal
needles inside the ILS indicator to tell the pilot
either go left/right or go up/down.
 By keeping both needles centered, the pilot can
guide his aircraft down to end of landing runway
aligned with the runway center line and aiming the
touch down.

ILS Components
104
Localizer:
horizontal guidanceGlide Path:
vertical guidance
Marker Beacons: the
height aircraft

Localizer
 Localizer is the horizontal antenna array located at the
opposite end of the runway.
 Localizer operates in VHF band between 108 to 111.975
MHz

How Localizer Works
 Localizer transmit two signals which overlap at the
centre.
 The left side has a 90 Hz modulation and the right has a
150 Hz modulation.
 The overlap area provides the on-track signal.
 For example, if an aircraft approaching the runway
centre line from the right, it will receive more of
the 150 Hz modulation than 90Hz modulation.
 Difference in Depth of Modulation will energizes
the vertical needle of ILS indicator.
 Thus, aircraft will be given the direction to GO LEFT.

How Localizer Works
Right
Left

Localizer
Needle indicates
direction of runway.
Centered Needle =
Correct Alignment

Glide Path Antenna Array
 Glide Path is the vertical antenna located on one side of
the runway about 300 m to the end of runway.
 Glide Path operates in UHF band between 329.15
and 335 MHz

How Glide Path Works
 Glide path produces two signals in the vertical plane.
 The upper has a 90 Hz modulation and the bottom has a
150 Hz modulation.
 For example, if an aircraft approaching the runway too
high, it will receive more of the 90 Hz modulation
than 150Hz modulation.
 Difference in Depth of Modulation will energizes the
horizontal needle of ILS indicator.
 Thus, aircraft will be given the direction to GO DOWN.

Glide Path
Needle indicates
above/below glide path.
Centered Needle =
Correct Glide path

Marker Beacons
 Marker beacons operating at a carrier frequency of
75 MHz are provided.
 When the transmission from a marker beacon is
received it activates an indicator on the pilot's
instrument panel.
 The correct height the aircraft should be at when the
signal is received in an aircraft.

Marker Beacons
Outer marker
 The outer marker should be located about 7.2 km from
the threshold.
 The modulation is repeated Morse-style dashes of a 400
Hz tone.
 The cockpit indicator is a blue lamp that flashes
accordingly with the received audio code.
 The purpose of this beacon is to provide height, distance
and equipment functioning checks to aircraft on
intermediate and final approach.

Marker Beacons
Middle marker
 The middle marker should be located so as to
indicate, in low visibility conditions.
 Ideally at a distance of 1050m from the threshold.
 The cockpit indicator is an amber lamp that flashes
in accordingly with the received audio code.

Marker Beacons
Inner marker
 The inner marker, shall be located so as to indicate
in low visibility conditions.
 This is typically the position of an aircraft on the ILS
as it reaches Category II minima.
 The cockpit indicator is a white lamp that flashes in
accordingly with the received audio code.

ILS Categories
 There are three categories of ILS the operation.
 Category I - A precision instrument approach and landing
with a decision height not lower than 60 m (200 ft) above
touchdown zone elevation and with either a visibility not less
than 800 m or a runway visual range not less than 550 m.
 An aircraft equipped with an Enhanced Flight Vision System
may, under certain circumstances, continue an approach to
CAT II minimums.
 Category II - Category II operation: A precision instrument
approach and landing with a decision height lower than 60 m
(200 ft) above touchdown zone elevation but not lower than
30 m (100 ft), and a runway visual range not less than 350 m.

ILS Categories
 Category III is further subdivided
 Category III A - A precision instrument approach and landing
with:
 a) a decision height lower than 30 m (100 ft) above touchdown
zone elevation, or no decision height; and
 b) a runway visual range not less than 200 m.
 Category III B - A precision instrument approach and landing
with:
 a) a decision height lower than 15 m (50 ft) above touchdown
zone elevation, or no decision height; and
 b) a runway visual range less than 200 m but not less than 50
m.
 Category III C - A precision instrument approach and landing
with no decision height and no runway visual range limitations. A
Category III C system is capable of using an aircraft's autopilot to
land the aircraft and can also provide guidance along the runway.

Advantages of ILS
 The most accurate approach and landing aid that
is used by the airliners.

Disadvantages of ILS
 Interference due to large reflecting objects, other
vehicles or moving objects.
 This interference can reduce the strength of the
directional signals.

VOR : VHF Omnidirectional Range

Introduction
 VOR, short for VHF Omni-directional
Range, is a type of radio navigation system
for aircraft.
 VOR navigation system is one of the most
significant aviation invention.
 With it, a pilot can simply, accurately, and
without ambiguity navigate from Point A to
Point B.

Introduction
 As opposed to the NDB, which transmits a
non-directional signal, the signal
transmitted by the VOR contains
directional information.
 VOR provide MAGNETIC BEARING
information to and from the station.
 “Omni-” means all and an Omni-directional
range means VOR station transmits signal
in all directions.

Signal Transmission
“Omni-” means
all and an Omni-
directional range
means VOR
station transmits
signal in all
directions.

VOR Equipments
 VOR equipments can be divided into three
equipments:
 Aerial / Antenna
 Receiver
 Indicator
 As for aircraft, VOR consist of VOR antenna, at
vertical tail and VOR receiver and indicator inside
cockpit.
 As for ground station (also known as VOR beacon)
consist of antenna (transmitter and receiver).

VOR Ground Antenna
VOR station for broadcast the signal
Rotating
Antennas
Stationary
Antennas

Rotating
Antennas
Stationary
Antennas

VOR ground antenna
 The VOR ground antenna is oriented to
magnetic north.
 Consists of :
 Single Stationary Antenna at the centre
 Rotating antennas
 It produces 360° radials/tracks at 1°
spacing.
 These 360 bearings are known as RADIALS
 VOR ground installations are
strategically located along air routes
and airport to ensure continuity of
guidance.

090
045
135
180
225
270
315
360
RADIALS
Magnetic North
135º

How VOR works
 VOR receiver in the cockpit is tuning to the specific
frequencies assigned for that VOR ‘s airport.
 It is VHF frequency which is between 108-117.95
MHz.
 After entering the frequency, the volume control
should be turned up in order to confirm that the
three letter identification code (Morse
Code) is correct.
 For example, KLIA airport has a VOR
known as VKL-Victor Kilo Lima

How VOR works
 The VOR station on the ground transmits two
signals at the same time; one signal is constant in all
directions, while the other signal is rotated about a point.
 One from stationary antenna, while the other from
rotating antenna.
 When aircraft receives these two signals, an aircraft VOR
receiver electronically measures the phase angle
different between these two signals.
 This phase angle different is translated as the
MAGNETIC BEARING which tell the pilot the aircraft
angle direction to the VOR station.
 This bearing angle also known as RADIALS.

VOR Indicator Display
A Display
A Rotating Course Card, calibrated from 0 to 360°, which
indicates the VOR bearing chosen as the reference to fly TO
or FROM. Here, the 345° radial has been set into the display.
This VOR gauge also digitally displays the VOR bearing,
which simplifies setting the desired navigation track
B Display
The Omni Bearing Selector, or OBS knob, used to manually
rotate the course card.

C Display
The CDI, or Course Deviation Indicator. This needle
swings left or right indicating the direction to turn to
return to course. When the needle is to the left, turn left
and when the needle is to the right, turn right, When
centered, the aircraft is on course. Each dot in the arc
under the needle represents a 2° deviation from the
desired course.

D Display
The TO-FROM indicator. This arrow will point up, or towards
the nose of the aircraft, when flying TO the VOR station.
The arrow reverses direction, points downward, when
flying away FROM the VOR station.
A red flag replaces these TO-FROM arrows when the VOR is
beyond reception range, has not been properly tuned in,
or the VOR receiver is turned off. Similarly, the flag
appears if the VOR station itself is inoperative, or down for
maintenance.

V H F O M N I D I R E C T I O N A L R A N G E ( V O R )

Advantages of VOR
 More accurate & precise flying:
 The accuracy of course alignment of the VOR is excellent,
being generally plus or minus 1 degree.
 Reliable:
 Can be used day and night.
 Multiple number of route :
 Provide multiple number of route ‘towards’ or away
from each station.
 These routes are like invisible highways , which the pilot
can navigate to @ away from any location.

Disadvantages of VOR
 Signals cannot be received at low altitudes
(below 1000ft)
 VORs are sensitive to the interference of
terrain. The nearest mountains and buildings
cause the VOR bearings to be stopped and
interrupted.
 Other disadvantages is VOR equipments are costly
to maintain.

:
Distance
Measuring
Equipment (DME)

Definition
 DME is stand for Distance Measuring
Equipment.
 DME is a type of en-route navigation system for
aircraft.
 DME often installed near VOR stations so as to
provide combined bearing and distance.
 When DME is installed with the VOR, it is referred to
as a VOR/DME.

The uses of DME
 DME provides the physical distance from the
aircraft to the ground DME transponder
expressed in Nautical Miles (NM).
 DME also calculates ground speed and the time
needed to reach the station if the aircraft is
fitted with appropriate computer.

DME System Components:
The DME system consists of three basic components
which are:
 DME antenna on the aircraft body
 DME navigation display unit in aircraft cockpit
 DME transmitter/receiver in the ground

DME Indicator
 DME enables aircraft to establish its range to the
ground station: Distance in nautical miles, Ground
speed in knots, Flying time to the station in minutes

How DME works?
 DME provides distance (slant range) from the
aircraft to the ground DME.
 DME operates on Ultra High Frequency (UHF) which
is between 962 to 1213 MHz.
 DME works based on pulse techniques, where pulse
means a single vibration of electric current.
 The aircraft’s antenna sends out paired pulses at
specific spacing.
 The ground DME station receives the pulses and then
responds with paired pulses at the same spacing but a
different frequency.

How DME works?
 The aircraft receiver measures the time taken to transmit
and receive the signal which is transmitted into distance.
 Beside that, the distance formula is also used by the DME
receiver to calculate the distance from DME station in
Nautical Miles.

Advantages of DME
 DME is extremely accurate: Provide continuous
and accurate indication of the slant range distance.
 Aircraft Handling Capability: The transponder
equipment should be capable of handling 100 to 200
aircrafts.
 Large coverage: DME facility provides coverage
up to 200 NM.

Disadvantages of DME
 As VOR the DME is also restricted to line-of-
sight transmission. For example, the aircraft at
altitude below 10’000 ft is unable to detect the DME
signal.

Disadvantages of DME
 Errors and abnormal indications:
 Slant range
 Speed and time calculation
 Ground system saturation – 100 aircraft
 System error

Automatic Direction Finder
(ADF)
&
Non Directional Beacon (NDB)

INTRODUCTION TO NDB & ADF
Non Directional Beacon
Automatic Directional Finder

Definition
 ADF is stand for Automatic Direction Finder.
 NDB is stand for Non Directional Beacon.
 ADF & NDB is the one of the older types of
radio navigation system that still in use today.
 They still in use today because of its simplicity.
 As it name, the signal transmitted by NDB does not
included directional information, but ADF
automatically searching for NDB signal.

ADF & NDB Equipments
 Non Directional Beacon (NDB) is used in
conjunction with Automatic Direction Finder
(ADF) in the cockpit.
 ADF equipments consists of 1) ADF antenna
(transmitter & receiver) outside aircraft’s body,
2) an ADF indicator inside the cockpit.
 NDB equipment only consist of ground NDB
antenna located near the airport (airfield area).
 ADF determines the direction to ground NDB
station.

ADF & NDB Equipment
Ground NDB stations is the
Tall antenna located near
the airfield
ADF antenna outside
aircraft ‘s body
ADF indicator inside
the cockpit

Purpose
 The purpose of ADF/NDB is to provide aid for
aircraft navigation by provide bearing
information of aircraft location to the
airport. (aircraft direction or heading to the
airport in degrees(angle))
 **Bearing: the angle which measured in a
clockwise direction.
 NDB bearings provide a consistent method for
defining paths aircraft can fly. NDB can define
"airways" in the sky.

NDB Frequencies
 ICAO has assigned Low Frequency (LF) and
Medium Frequency (MF) band for NDB,
 It is within 200 – 1750 KHz.
 However, most of NDB equipments are found
operating within frequency band of 200-525 KHz.

How ADF & NDB works
NDB station radiates a non-directional signal in all
directions around its antenna (transmitter).
Station identification code(callsign) in the form Morse
code is also transmitted by the NDB.
An ADF selector in aircraft will tune to NDB’s frequency
in order to search its signals.
After NDB call sign is identified, the direction of aircraft
in bearing to the NDB station will be indicated.
ADF indicator in the cockpit will display the bearing to
the NDB station relative to the heading of the aircraft.

The uses of NDB
 Used for FLYING FROM NDB or HOMING TO
NDB when maintaining airway centre-lines.
 Used for en-route navigational bearing
 Used for HOLDING system before landing.
 Used as markers for an Instrument Landing
System (ILS) approach

FLYING FROM or HOMING TO NDB
station
Flying From
Homing To

En-Route Navigation
 Aircraft must maintain their heading using the
Automatic Direction Finding (ADF) in the cockpit.
 Pilot must always watch the relative bearing indicator
to maintain the airway center line.

Markers for an ILS approach
NDB also can used as the markers for Instrument
Landing System (ILS) approach.
This type of NDB is also known as LOCATOR.
 Locator is a low power NDB.
 It has signal range within 10 to 25 Nautical Miles.

Advantages of NDB
 NDB signal can be received at low altitudes.
 This is because NDB signal is based on surface wave
propagation (signal not limited to ‘line of sight’ ).
 NDB also can be used as the Back-Up system. For
example, during no signal given by the VHF Omni-
directional Range (VOR) system.
 NDB system only requires low cost for their
maintenances.
 NDB still important for many small airports.

Disadvantages of NDB
 Limited Signal because of several factors including:
1. Interference Effect
2. Thunderstorm Effect
3. Mountain Effect
4. Night Effect
5. Coastal Refractions

 Interference Effect –interference occurs if an
ADF receives two or more signals radiated by
NDB.
 Thunderstorm Effect – Thunderstorm have
very powerful discharges of static electricity that
can interrupt the NDB signal. Needle of ADF
indicator sometimes points toward the storm.
 Mountain Effect – Mountain areas can cause
reflections and diffractions and lead to the error
direction reading by ADF.

 Night Effect – Low signal or no signal during
night time because contamination of radio wave.
 Coastal Refractions– Also known as
Shoreline Effect . Surface wave travel in one
direction over land, but another direction over
water (refraction). This can cause error reading in
ADF indicator.

Accuracy
 The accuracy of NDB is +/- 5 degree for approach
and +/- 10 for en-route.
 The accuracy of an NDB at any given time is difficult
to determine when considering all the factors
creating error.

EMERGENCY LOCATOR TRANSMITTER
(ELT)

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