393SYS Airport Engineering Practice Lecture 2 Development of Maintenance Programs

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393SYS Airport Engineering Practice Lecture
2Development of MaintenancePrograms
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1.0 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

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1.0 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.

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1.0 Why We Have to Do Maintenance
Various Systems
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1.0 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                                         
     
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1.0 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.

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1.0 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
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1.0 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
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1.0 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.

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1.0 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
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1.0 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.
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1.0 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
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1.0 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
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1.0 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
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1.0 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
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1.0 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.
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2.0 Development of Maintenance Programs
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2.0 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

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2.0 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

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2.0 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
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2.0 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.
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2.0 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.

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2.0 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.

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2.0 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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2.0 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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2.0 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.
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2.0 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.

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2.0 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.

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2.0 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.

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2.0 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

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2.0 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.
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2.0 Development of Maintenance Programs
Actuators and Pumps
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2.0 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.

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2.0 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
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2.0 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.

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2.0 Development of Maintenance Programs
Control Rods
Pulleys
Jack Screws
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2.0 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

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2.0 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.

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2.0 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
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2.0 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

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What 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 ?