1 / 43

393SYS Airport Engineering Practice Lecture 2 Development of Maintenance Programs

393SYS Airport Engineering Practice Lecture 2 Development of Maintenance Programs. 1.0 Why We Have to Do Maintenance. Summary of Last Week’s material Why We Have To Do Maintenance Entropy and the Laws of Thermodynamics The Role of the Engineer The Role of the Mechanic

milly
Télécharger la présentation

393SYS Airport Engineering Practice Lecture 2 Development of Maintenance Programs

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 393SYS Airport Engineering PracticeLecture 2Development of MaintenancePrograms

  2. 1.0 Why We Have to Do Maintenance • Summary of Last Week’s 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

  3. 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.

  4. 1.0 Why We Have to Do Maintenance Various Systems

  5. 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 systemA working carIS a system

  6. 1.0 Why We Have to Do Maintenance parameters - are quantities that define certain characteristics of systems • Reliability Parameters • In last week’s 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.

  7. 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

  8. 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% =======

  9. 1.0 Why We Have to Do Maintenance Note : In general, failure rate is not constant – • 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 (λ) which is the number of component failures per unit time : • R(t) = exp(-λt) • In this formula, the failure rate, λ, is assumed to be constant with the age of the component.

  10. 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 : λ = 40/100,000 = 0.0004 For a 2 hour flight, t = 2 and the reliability then becomes – R(t) = exp(-λt) ≈ 1 – λt [when λt 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%

  11. 1.0 Why We Have to Do Maintenance Component X Component Y 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.

  12. 1.0 Why We Have to Do Maintenance Component X Component X 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

  13. 1.0 Why We Have to Do Maintenance Component X Component X Component X Component Y 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

  14. 1.0 Why We Have to Do Maintenance Component X Component X Component X Component Y 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

  15. 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

  16. 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.

  17. 2.0 Development of Maintenance Programs

  18. 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

  19. 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

  20. 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

  21. 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.

  22. 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.

  23. 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.

  24. 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 :

  25. 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 :

  26. (1) (2) Does the unit’s failure affect flight safety ? Is the failure evident to the flight crew ? No (3) No Is there an adverse relationship between the unit’s age and reliability ? Yes Yes Yes (5) (4) Is there a mainteance check to assure continued function ? Is reduced resistance to failure detectable by a maintenance check ? No Yes No Yes No On Condition On Condition Hard Time Condition Monitoring Hard Time

  27. 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.

  28. 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.

  29. 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.

  30. 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.

  31. 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

  32. 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.

  33. 2.0 Development of Maintenance Programs Actuators and Pumps

  34. 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.

  35. 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 ->

  36. 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.

  37. 2.0 Development of Maintenance Programs Control Rods Pulleys Jack Screws

  38. 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

  39. 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.

  40. 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

  41. 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

  42. 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 ?

More Related