Engine stock control article

study

STOCK
CONTROL FOR
ENGINES OF
AIRCRAFT

Eng. Mohammed S. Awad
PhD candidature – Aviation Management

The number of engines in stock has a
significant role in deciding the technical
budget for any airline. This has a direct
effect or impact on the overall budget of the
airline. Due to the expansive values of these
engines, purchasing engines that exceeds
the actual company level needs; it affects
the maintenance and holding costs and
the money seized by these extra engines
can be utilized in other useful project.
Nevertheless a number of engines less than
the required service level exposing the airline
to huge loss or risk of losing revenue. Due
to unavailability of engine, such a term as
Aircraft On Ground Cost (AOG) is used to
reflect a loss of revenue. While optimization
techniques are usually used to solve such
a problem, to define the right number of
engine in stocks, by an analysis of U curve
cost analysis. While previously many airlines
work on 1:1 inventory policy for engines
in stocks and numbers of engine of the
fleet, which is a very expensive policy.8

16

study
Study

Pic. No. 1 Fig. No.1

Introduction B) Cost of losing Opportunity expressed by curve C. That take
The important policy of material This cost is demonstrated by a U curve shape, and the lowest
planning division in airline AOG cost – Aircraft On Ground value in the curve demonstrate
companies is to find the right Cost, which arises due to loss of the best decision to be selected.
number of spare parts in the stock revenue since no supported engine Actually the B curve can
which subsequently assists and / available engine in stock as shown be presented by a well-
supports maintenance program in figure (2). So the decision to known statistical distribution
of the airlines. So spare part allocate the right number of spare that reflects the best defects
availability has an important role in engines depends on the minimum behavior of an Engine.
maintaining the operation activity costs of the pre-mentioned
of the fleet and drive the operating parameters. Thus airlines should Poisson distribution
system at high performance. select the right policy that reflects According to the method of
According to the situation, minimum costs. By referring to analysis and a data sampled, a
position and related costs, the the figure (2), the straight line (A) proper distribution is selected.
right stock policy is selected. reflects the increasing cost due to In a statistics a two terms are
Spare parts should be typically / the increasing number of engine. used widely, i.e. time and event
fit, otherwise this policy will be more While the curve B reflects the that may describe the distribution
expansive to support the fleet. gradual decline of AOG Cost of more clearly, also distribution
And there several questions the aircraft. So by increasing the can be continuous or discrete.
should answer clearly as is number of engines in spare, we In our case, Poisson Distribution
there any indication / guide line are expected to minimize the cost represents the best distribution
to indicate that policy?, is that incurred by Aircraft On Ground that reflect the defect characters
policy economic? And what is Cost. The total resulting cost will in terms of engine demand and
the required service level to be be the super imposed of these it’s a specified period of time.
implemented to that spare parts two curves (Functions), which
policy, and how far the repair Estimating the Parameters
and overhaul cost compared to of Poisson distribution
the breakdown cost, and AOG The main parameter of Poisson
(Aircraft On Ground) Cost. Also distribution is expressed by the
are these long-term decisions for total time of the experiments by the
overhaul and replacement issues?, average time the events (defects)
where these topic are usually to happen and can be expressed in
related to U curve techniques. the following equation

U Curve Technique: ^ Total Opreating Time of the Engines
n =
U curve techniques are Average life time of Engines
illustrated by two main factors
Fleet Size × No. of engines in A/C ×
A) Cost Of Offering Service Daily Ultilization × repair Cycle
B) Cost Of Losing Opportunity ^
n = Main Time To Failure
As shown in figure (1) ............. Equation (1)
A) Cost Of Offering Service: Engine PW 4000 (94 inch)
In this case, the cost The Engine PW 4000 (94 inch) is
demonstrated by the ownership one of most world wide commercial
and holding cost (maintenance), engines, it is supported and installed
mathematically, is a linear function in both fleet of Boeing or Airbus, as
represented by a straight line. B747-400, B767-200/300, MD11,
A300-600, A310-300
Figure below shows engine
PW 4000 in a test. 8

17

study

Case Study Table No.1 Inputs
Inputs air bus 310-300
As shown in table (1) the basic
input data
inputs for the analysis as No of
Aircraft, No of Engine , Daily fleet Size 3
Utilization in hours and Repair Cycle OTY oer aircraft 2
by Days and MTBF by manufacture. daily utiliation (HRS) 9
repair cycle (days) 120
Analysis
By implementing U curve techniques MTBD (hours) 10,000
and developing a spread sheet estimationg (poisson) 0.648
of Excel program and using the AOG coast (USD) 100,000
assumption mentioned in table
engine holding cost 2000
(1) and also assuming the failure
distribution is equivalent to the
demand distribution of engines, the Table No.2 Analysis
process is an engine replacement
while the event of failure will occur in
the mid period, all these assumption
reflect best distribution is Poisson.
The calculation is demonstrated by
equation (1) and a Poisson function
implemented from Excel program. So
we evaluate the probability of Engine
failure, and AOG cost as it is shown in
tables (2). The study shows the best
decision is to keep 2 engines in stock.

Effect of Fleet Size On Spare Engines
Referring to the previous study,
let us keep all the pre-mentioned
parameters while the Fleet Size factor
is random variable, as step function,
so multiple U curve are developed
based on the different values of
fleet size as shown in the following
graph, which can represented by a Fig. No.2 U Curve Approach
smooth line with R2= 0.94, so with
AOG
a fleet size of 12-15 Aircraft the
required number will be 5 Engine HOLDING
in stock as shown in Fig (3).
TOTAL
Cost (USD)

Summary
The setting policy, for decided
number of engines in stock, is a
main economic approach to minimize
the cost in aviation industry, due
to the highest purchasing price of
engines. While many arrangements
and policies are setup to minimize
the cost of inventory and cost of
AOG (Aircraft On Ground Cost) by Numbers of Spares Engine
using Poisson Distribution, which
is a statistical tool used / reflects Fig. No.3 Relations of Fleet Size & No. of Engines for Airlines
the failures patterns / and inventory
environments in airline industry.
So all parameters are considered
No. of Engines in Stock

as number of aircrafts, number
of engines in each aircraft, daily
utilization, repair cycle in days, and
average life of the engine i.e (MTTF)
to develop U curve optimum cost
to select the right quantity of spare
engines in stocks based on two
costs – holding cost and AOG cost.
Further we can study all the
input factors and their effects on
inventory of the company as repair
cycle and daily utilizations. n Fleet Size (No. of Aircraft)

18

Civil Aviation & Meteorology Authority, July - September 2010, issue 8

www.camamagazine.com

Engine stock control article

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