Title: 393SYS Airport Engineering Practice Lecture 2 Development of Maintenance Programs
1393SYS Airport Engineering Practice Lecture
2Development of MaintenancePrograms
21.0 Why We Have to Do Maintenance
- Summary of Last Weeks material
- Why We Have To Do Maintenance
- Entropy and the Laws of Thermodynamics
- The Role of the Engineer
- The Role of the Mechanic
- Perfection and Reliability
- Failure Rate Patterns
- Management Techniques for In-Service
Interruptions - Redundant Systems
- Line Replaceable Units
- Minimum Equipment List
31.0 Why We Have to Do Maintenance
What is a component and what is a system ?
- A collection of components organized to
accomplish a specific function or set of
functions. - An assembly of various components designed to
function as a whole. - A collection of interacting subsystems designed
to satisfy a set of requirements. - So a system is composed of components, or
smaller subsystems, and all of it is designed to
provide one or more functions. - A component is the smallest part you cannot
subdivide a component.
41.0 Why We Have to Do Maintenance
Various Systems
51.0 Why We Have to Do Maintenance
UNDERSTANDING SYSTEMS
A system is a collection
ofparts that interact tofunction
purposefullyas a whole.
A collection ofauto
parts isNOT a system A working carIS a
system
61.0 Why We Have to Do Maintenance
parameters - are quantities that define certain
characteristics of systems
- Reliability Parameters
- In last weeks tutorial, we considered the bath
tub failure rate curve
- The question was asked, How do you think
failure rate data is obtained ? - We now consider some reliability parameters
which are in practical use in managing aircraft
maintenance.
71.0 Why We Have to Do Maintenance
- Mean Time Between Failures (MTBF)
- MTBF is the average time between the failures of
a component or system. - Mean Time to Repair (MTTR)
- MTTR is the time taken to repair or replace a
component or system. - Availability (A)
- The availability of a component or system is the
percentage of time when it is operational
MTBF A ----------------------
x 100 MTBF MTTR
81.0 Why We Have to Do Maintenance
Example During a period of 100,000 flight hours
an avionics component on a commercial aircraft
failed 50 times. In this case, MTBF 200,000 /
50 4,000 hours Assume the component can only be
replaced after an interval of time equal to 1
cycle, and that the average cycle time for this
aircraft is 2 hours. Then, the MTTR 2 hours and
the availability is -
MTBF
4000 hours A ---------------------- x 100
------------------------- x 100 MTBF
MTTR 4000 hours 2
hours 99.95
91.0 Why We Have to Do Maintenance
- Reliability
- Component reliability is the probability of no
failure over a specified period of time. - This reliability (R(t)) is given in terms of the
failure rate (?) which is the number of
component failures per unit time - R(t) exp(-?t)
- In this formula, the failure rate, ?, is assumed
to be constant with the age of the component.
101.0 Why We Have to Do Maintenance
Example Logs of aircraft equipment failures
reveal that a particular aircraft component has
experienced 40 failures in 100,000 hours of use.
The failure rate may then be calculated as
follows ? 40/100,000
0.0004 For a 2 hour flight, t 2 and the
reliability then becomes R(t) exp(-?t) 1
?t when ?t is less than 0.001 So R(t) 1
0.0004 x 2 1 0.0008 0.9992 or, expressed as
a percentage, R(t) 0.9992 x 100 99.92
111.0 Why We Have to Do Maintenance
System Reliability The reliability of a system
composed of two components, X and Y, is given by
- Rsystem RX x RY where it is assumed that
the system fails if either item fails, and that
failures are statistically independent. An
example of the type of system where this would
occur is represented as follows -
This is referred to as a series system because
both components operate in series and if one
fails, the whole system fails.
121.0 Why We Have to Do Maintenance
There is another configuration of components
which occurs in systems with redundancy
In a redundant system, the system will still
operate as long one component is still
operational. This arrangement is also called a
parallel system. In this case, Rsystem 1
(1 RX)2
131.0 Why We Have to Do Maintenance
Lets compare the reliability of a system with a
single component with reliability of a series
system and the reliability of a parallel system
Component X
RX R 0.9992
Rsystem RX x RX 0.9992 x 0.9992 0.9984
Rsystem 1 (1 RX)2 1 (1 0.9992)2
0.99999936
141.0 Why We Have to Do Maintenance
System Availability System Availability is
calculated using the same kind of formula as for
reliability but with the reliability of
individual components replaced with the
availability of individual components
Asystem AX x AY
Asystem 1 (1 AX)2
151.0 Why We Have to Do Maintenance
Other Aircraft Reliability Parameters
1. Aircraft (or Schedule) Reliability
Schedule Interruptions R 1
- ---------------------------- Total
Departures
R is the probability of starting and completing a
scheduled flight without interruption. For
example, if there were 80,000 departures over a
given period of time and 4 schedule interruptions
during the same period, then the reliability
would be R 1 (4 / 80,000) 1 0.00005
0.99995
161.0 Why We Have to Do Maintenance
2. Dispatch Reliability R 1 ((delays
cancellations) / total departures) This is the
probability of departing on time. 3. En Route
Reliability R 1 - ((air turnbacks ground
turnbacks diversions) / total departures) where
an air turnback occurs after take-off, and a
ground turnback occurs before take-off.
172.0 Development of Maintenance Programs
182.0 Development of Maintenance Programs
- Introduction
- We have seen in the first lecture that
components and systems fail in different ways
and at different rates. - This results in a requirement for unscheduled
maintenance that is somewhat erratic and
uncertain. - There are periods high workloads and periods
with no work these have to be managed to
smooth out the workload and stabilize the
manpower requirements. - The maintenance programs currently in use in
commercial aviation were developed by the
aviation industry using two diffeent approaches
- the process-oriented approach, and
- the task-oriented approach
192.0 Development of Maintenance Programs
- The Process-Oriented Approach
- This uses three primary maintenance processes to
accomplish the scheduled maintenance actions - Hard-Time (HT)
- Used for components or systems that have
definite life limits. Item is removed at a
predetermined interval, usually specified in
either flight hours or flight cycles. - On-Condition (OC)
- Used for components or systems that have
detectable wear out periods. Item will be
checked at specific intervals (in hours, cycles,
or calendar time) to determine its remaining
serviceability. - Condition Monitoring (CM)
- Used to monitor systems and components that
cannot utilize either HT or OC processes.
Involves monitoring of failure rates, removal
rates, etc. to facilitate maintenance planning
202.0 Development of Maintenance Programs
- HT and OC processes apply to items in categories
A, B and C
A. Infant mortality constant or slightly rising
failure rate definite wear out period (4 ).
B. No infant mortality slightly rising failure
rate definite wear out period (2 ).
C. No infant mortality slightly rising failure
rate no definite wear out period (5 ).
HT definite life limits OC detectable wear
out periods
212.0 Development of Maintenance Programs
- CM items are operated to failure and failure
rates are tracked to aid in failure prediction
or failure prevention efforts. - These are operate to failure items in
categories D, E and F
D. Increasing failure rate at outset constant or
slightly rising failure rate no definite wear
out period (7 ).
E. No infant mortality constant failure rate
throughout life no definite wear out period (14
).
F. Infant mortality constant failure rate
throughout life no definite wear out period (68
).
CM no definite wear out period.
222.0 Development of Maintenance Programs
- The Task-Oriented Approach
- Uses predetermined maintenance tasks to avoid
in-service failures. - Equipment redundancies are sometimes used to
allow in- service failures to occur without
adversely affecting safety and operation. - More on this approach next week.
232.0 Development of Maintenance Programs
- The Maintenance Steering Group (MSG) Approach
- The modern approach to aircraft maintenance
started with the Boeing Company in 1968. - It coincided with the introduction of the Boeing
747 then the largest commercial airplane. - Six industry working groups analysed aircraft
- structures
- mechanical systems
- engines and auxiliary power plants
- electrical and avionics systems
- flight controls and hydraulics
- zonal configurations.
242.0 Development of Maintenance Programs
- This analysis provided them with information on
failure modes, failure effects and failure
causes. - The approach was called bottom up because it
looked at the components as the most likely
causes of equipment malfunction. - The purpose of the analysis was to determine
which of the three processes (HT, OC or CM)
would be required to repair the item and return
it to service. - This resulted in the a generalized maintenance
process called MSG-2 which could be applied to
any aircraft, not just 747s. - The following table summarizes the steps
involved. - Note that the process is sligtly different for
(a) systems and components, (b) structures and
(c) engines
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262.0 Development of Maintenance Programs
- The steps in this table may be generalized as
follows - Step 1 Identify the maintenance or structure
items requiring analysis. - Step 2 Identify the functions and failure
modes associated with the item and the effect of
the failure. - Step 3 Identify those tasks which may have
potential effectiveness. - Step 4 Assess the applicability of those tasks
and select those deemed necessary. - Step 5 For structures only, evaluate initial
sampling thresholds. - The following is a simplified process flow
diagram for MSG-2
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282.0 Development of Maintenance Programs
E.G. If failure of the unit is safety related
(1), and there is a maintenance check available
to detect a reduction in failure resistance (4),
then the item in question is identified as
on-condition.
292.0 Development of Maintenance Programs
- Once the maintenance action is determined, it is
then necessary to determine how often such
maintenance should be done. - Available data on failure rates, removal rates,
etc., are used to determine how often the
maintenance should be performed.
302.0 Development of Maintenance Programs
- Process-Oriented Maintenance
- The Hard-Time Process
- Hard time is a failure prevention process.
- It is applied to items
- having a direct adverse effect on safety
- subject to reliability degredation but having no
possible maintenance check (e.g. rubber
components) - The item has to be removed from the vehicle and
either - completely overhauled, or
- partial overhauled, or
- discarded
- before exceeding a pre specified life time.
312.0 Development of Maintenance Programs
- This life time or interval between each removal
of the item may be specified in terms of - calendar time
- engine or aircraft check intervals
- operating cycles
- flight hours
- If a particular component fails at X hours of
operation, ideally the component would be
replaced at the last scheduled maintenance
period prior to the accumulation of X hours. - This would give the airline the maximum service
hours from the component and the component would
never fail in service.
322.0 Development of Maintenance Programs
- Examples of components which are regulated by
hard time failure prevention are - structural components
- landing gear
- life-limited engine parts e.g. turbine blades
332.0 Development of Maintenance Programs
- Examples of components which are regulated by
hard time failure prevention are - mecahnical linkages
- actuators
- hydraulic pumps and
- motors
- electric motors
- generators.
All of these things and similar items having a
definite wear out period and being safety
related, will be subject to hard time maintenance
regulations.
342.0 Development of Maintenance Programs
Actuators and Pumps
352.0 Development of Maintenance Programs
- The On-Condition (OC) Process
- With on-condition, the item is not removed
periodically. Instead, it is subject to periodic
inspections or tests. - The objective of the inspections or tests is to
determine whether or not the item can continue
in service. - If an item fails an OC check, only then is it
removed for overhaul, repair, or replacement. - OC items are restricted to component / equipment
/ systems on which checks and tests can be
applied without having to remove the item. - These OC checks must be performed within time
limits (intervals) prescribed for each OC check.
362.0 Development of Maintenance Programs
- Examples of OC checks are as follows
- tire tread and brake linings
- scheduled borescope inspections of engines
- engine oil analysis
- in-flight engine performance analysis (using
engine condition monitoring istruments (ECM)
built into the engine)
In each of these cases, the amount of degredation
can be measured and compared with establish norms
to determine how much life or servicability
remains. Borescope -gt
372.0 Development of Maintenance Programs
- For an OC process to be applicable in a
maintenance situation, one of the following must
be true - the OC check must be able to ensure
serviceability with reasonable probability until
the next OC check, or - a satisfactory measurement can be made of the
failure predicting data - Examples
- Break wear indicator pins the wear in these
pins is compared to some reference standard or to
some limit. - Control cables Measure these for diameter,
tension, and broken strands. - Linkages, control rods, pulleys, roller tracks,
jack screws measure these for wear, and or side
play, or backlash.
382.0 Development of Maintenance Programs
Control Rods
Pulleys
Jack Screws
392.0 Development of Maintenance Programs
- The Condition Monitoring (CM) Process
- Unlike HT and OC processes, CM does not really
monitor the condition of a component. - CM systems consist of data collection and data
analysis procedures. - For example, a CM process may collect data on
- unscheduled removals of equipment from aircraft
(i.e. due to some failure), - maintenance log entries,
- pilot reports,
- workshop findings,
- sampling inspections,
- mechanical reliability reports, and
- other sources of maintenance data
402.0 Development of Maintenance Programs
- The CM process is applied when neither the hard
time nor the on-condition process can be applied. - CM is not a failure prevention process as are HT
and OC. - CM components have to be operated to failure, and
replacement of CM items is an unscheduled
maintenance action. - Since CM items are operated to failure, these
items must comply with the following conditions - A CM item has no direct, adverse effect on safety
when it fails. - A CM item must not have any hidden function
(i.e. that cannot be observed by flight crew). - A CM item must be included in a CM program.
412.0 Development of Maintenance Programs
- Typical CM components include
- navigation equipment
- communications equipment
- lights
- instruments
- other items where test or replacement will not
predict approaching failure, nor result in
improved life expectancy. - CM is frequently applied to
- where redundant systems,
- coffee makers,
- lavatories,
- passenger entertainment systems
failure has no effect on safety or air worthiness
422.0 Development of Maintenance Programs
- Summary
- Systems, Subsystems and Components
- Reliability Parameters
- Development of Maintenance Programmes
- The Process Oriented Approach
- Hard Time
- On Condition
- Condition Monitoring
- The Maintenance Steering Group Approach (MSG-2)
- Process Oriented Maintenance
- The Hard Time Process
- The On Condition Process
- The Condition Monitoring Process
43What you need to know for the exam !
- Can you explain what is meant by a component and
system ? - What are the various reliability pararmeters
MTBF, MTTR, Reliability and Availability ? - Can you calculate the reliability of a component
or system given the failure rate for that
component or system ? - Can you calculate the availability of a
component or system given the MTBF and the MTTR
? - Can you calculate the reliability of a system
given the failure rate one or more subsysyem
components operating in series or parallel ? - What are the two main approaches developed by
the aviation industry for developing maintenance
programmes and what maintenance processes do
they use ? - Can you explain MSG-2 ?
- Can you explain Process-Oriented Maintenance ?