Airworthiness Manual Advisory (AMA)
Date: April 1, 1986
Subject: Reliability Monitoring Programs
1. PURPOSE. This publication provides information on the use of reliabil-ity control methods to monitor the effectiveness of aircraft maintenance programs. Its objective is to provide guidance for the development of reli-ability programs, outline Department of Transport (DOT) standards for the assessment of such programs and give examples of some, but not all, of the statistical calculations and data displays which these programs may employ.
2. REFERENCE AIRWORTHINESS REQUIREMENTS. Airworthiness Manual, Chapter 571 section 571.101 and Chapter 573.
The methods described in this advisory are of maximum value when ap-plied to modern, multi-engined, transport category aircraft, which incorporate in their design provision for system redundancy as a safe-guard against component failure. The initial inspection programs for such aircraft are normally based on the mainte-nance steering group (MSG) logic described herein, and may pres-cribe "condi-tion monitoring" (C.M.) as the primary maintenance process for certain com-ponents. In such cases, the establishment by the operator of a system to monitor the reliability of the C.M. components is a condition of the inspection program approval.
Reliability methods may also be applied to types of aircraft other than those described in (a), to assess system and component per-formance for development of the maintenance program, although C.M. may not be prescribed as the primary maintenance process in these cases.
- Effective application of statistical reliability methods is usual-ly considered to require a fleet of 5 or more aircraft, although this number may vary according to aircraft type and util-ization. To accom-modate the needs of smaller operators, partici-pation in joint reliabil-ity programs may be approved.
The first generation of formal air carrier maintenance programs was based on the belief that each part of an aircraft required periodic overhaul.
Times between component overhaul were strictly controlled, and the entire aircraft was periodically disassembled, overhauled, and reas-sembled, in an effort to maintain the highest level of safety. This was the origin of the process referred to as "hard-time".
As experience was gained, it became apparent that some components did not require overhaul on a fixed time basis. Consequently, a second process evolved, referred to as "on-condition". This des-ignation was assigned to components, the condition of which could be determined by visual inspection, measurement, testing or other means which did not involve disassembly or overhaul.
New methods of maintenance control were developed, which were oriented towards the assessment of mechani-cal performance rather than the prediction of failure. These methods were collectively known as "reliability control" because their major emphasis was upon maintaining failure rates below a predetermined value; i.e., the achievement of an acceptable level of reliability. The analy-tical nature of reliability control also disclosed the exis-tence of aircraft components and systems that did not fit either the hard-time or on-condition process categories. This led to the recogni-tion of a third process category in which no maintenance tasks need be specified; instead, current performance is monitored and analyzed to indicate the need for maintenance program amend-ment. This process, entitled "condition monitoring", was first recognized in the decision logic of the initial maintenance steer-ing group document MSG-1 and was applied to Boeing 747 aircraft.
The experience gained with MSG-1 was used to update its decision logic and create a more universal document for application to other aircraft or powerplants. This document was designated MSG-2. When applied to a particular aircraft type, the MSG-2 logic results in a list of "mainte-nance significant items" (MSI's), to each of which is assigned one or more of the three process categories described above.
After more than a decade of MSG-2 use, experience indicated that a further update was appropriate. As a result, a new industry task force developed MSG-3, which uses the basic philosophies of MSG-1 and MSG-2, but prescribes a different approach in the assignment of maintenance requirements. In lieu of process categories, MSG-3 identifies mainte-nance tasks. The development of this task orien-ted decision logic came about, partly in response to the misunder-standing which had been expe-r-ienced with the terms on-condition and condition monitoring, and partly due to the realization that the reliability monitoring (on a unit basis) of items having only benign failure modes was an economic, rather than a safety re-quirement. Detailed explanations of the MSG-2 and MSG-3 analysis methods may be found in AMA 571.101/3 (Maintenance review boards).
Although primarily intended for the initial development of inspec-tion programs for new aircraft, these methods may also be used, in conjunc-tion with service experience, to modify the programs of earlier air-craft.
- The processes, tasks and intervals arrived at by the use of MSG-1, -2 or -3, or, in the case of earlier aircraft, by the manufac-turers' sub-jective analyses, are used by the operator as the basis of his initial maintenance program. Subsequent amendments to that program must be consistent with the initial logic used, and will be based upon the operators experience with the aircraft type. The means by which that experience is analyzed, quantified, and used to indicate required changes, are collectively known as the operator's "relia-bility pro-gram". Over a period of time the changes implemented as a result of a reliability program can be significant. An example of how the "B" check intervals of a first generation jet aircraft have grown by the use of reliability moni-toring may be found on page 19.
5. RELIABILITY PROGRAM FORMAT
An air carrier reliability program should be tailored to meet the special requirements of the particular operator, and should take into account his operational and environmental circumstances, organizational structure, record keeping system, etc. The scope of each operator's reliability program will be defined in his mainten-ance control manual. All or part of an operator's mainte-nance program may be controlled by use of reliability methods, and a typical program may include segments devoted to systems, compo-nents, powerplants and structures. All segments of the pro-gram may use identical methods, or each may be handled individu-al-ly. A reli-ability program may encompass a select group of items without affecting other controls for the remain-ing items.
- Statistical type programs may be used wherever the frequency of events being monitored is sufficient. This type of program enables the use of alert rates which may be shown on graphic charts (or equiva-lent displays) to identify areas where corrective action may be needed. Where the frequency of events is too low to provide valid statistical data, sampling inspection and defect analysis may be used to assess the relationship between operating time and the failure resis-tance of components. These types of programs are respectively known as "alert" and "non-alert" pro-grams. In practice most reliability programs include elements of both techniques. The description of a program as an "alert" or "non-alert" type generally indicates the predominant method used.
6. PROCESS CATEGORIES AND TASKS
The basis of each operator's inspection program is a list of items, together with the processes or tasks assigned to those items, and the intervals at which action is required. The primary categories of main-tenance pro-cess for MSG-2 based programs are hard-time, on-condition and condition monitoring. MSG-3 tasks are categorized as inspections, functional checks, operational checks, servic-ing/lubrication, restora-tion, dis-card, operating crew moni-toring and "no scheduled" tasks. Each inspec-tion program should include specific definitions of the process cate-gories and/or tasks it uses, and how they are applied.
- There is no hierarchy of processes or tasks, and complex (multi-cell) units may be subject to control by one or more of them. It should be noted, however, that some tasks may be included to meet a safety re-quirement, while others may have a primarily economic purpose. Before amending the inspection program, it may be neces-sary to refer to the initial analysis to determine which of these purposes applies. If not known to the operator, this information can be obtained from the manu-facturer.
7. RELIABILITY PROGRAM ELEMENTS
Both alert and non-alert type programs will usually include the follow-ing elements: (a) data collec-tion, (b) analysis, (c) display and re-porting, (d) responsive action, and (e) program amendment procedures. The intent of the following paragraphs is not to provide rigid specifi-cations, but rather to explain the purpose of each of these elements which the operator may incorporate in his particular program.
Data collection. The data collection system should provide a specific flow of information from identified sources, and proce-dures for trans-mission of data, including the use of forms, com-puter print-outs, etc. Responsibilities within the operator's organiz-ation must be established for each source of data collec-tion. Typical sources of performance information are described below; however, it is not implied that all of these sources need be included in the program, nor does this listing prohibit the use of others.
Pilot reports, more usually known as "Pireps", are reports of occurrences and malfunctions entered in the aircraft journey log by the flight crew. Pireps are among the most signifi-cant sources of information, since they are a direct indica-tion of aircraft reliability as experienced by the crew. It is usual for the journey log entries to be routed to the reliability section at the end of each day, or at some other agreed interval, whereupon each entry is extracted and recorded as a count against the appropriate system. Engine performance (trend) monitoring can also be covered by the Pirep system, and may be used as a source of data in the same way as reports on system malfunctions, however it should be kept in mind that this form of monitoring is primarily inten-ded as a part of the "on-condition" process.
Aircraft delays and cancellations resulting from mechanical defects are normally reported daily by the operator's line maintenance staff. Each report gives the cause of delay and clearly identifies the system or component in which the defect occurred. The details of any corrective action taken and the severity (period) of the delay are also included. The delays are usually listed in Air Transport Association of America Specification 100 (ATA 100) chapter sequence.
Engine in flight shutdowns
Flight crew reports of engine shutdowns usually include details of the indications and symptoms prior to shutdown. When ana-lyzed, these reports provide an overall measure of propulsion system reliability, particularly when coupled with the results of the subsequent investigations and with the records of unscheduled engine removals.
Component unscheduled removals are reported, together with the following information:
Identification of component;
Precise reason for removal;
Aircraft registration and component location;
Date and airframe hours at removal; and
Component hours since new/repair/overhaul/calibration.
- Identification of component;
With the exception of self-evident cases, each unscheduled removal report is followed up by a workshop report in which the reported defect is confirmed or denied. This report is routed to the reliability section. Workshop reports may be compiled from an operator's own "in-house" findings and/or from details sup-plied by component repair/overhaul contrac-tors.
Where a reported malfunction is confirmed, the workshop report will normally include details of the cause of the defect, the corrective action taken and, where relevant, a list of replace-ment items. Many programs utilize the same type of report to highlight structural and general aircraft defects found during routine maintenance checks.
Dependent upon the formation of individual programs, a va-riety of additional reports may be produced on a routine or ad hoc basis. Such reports could range from formal minutes of reliability meet-ings to reports on the sample stripping of components, and also include special reports which have been requested during the investigation of any item which has been highlighted by the pro-gram, such as service difficulty re-ports.
- Pilot reports
Data analysis is the process of evaluating mechanical performance data to identify characteristics which indicate a need for program adjust-ment, revision of maintenance practices or hardware improve-ment (modi-fication). The initial step in analysis is the compari-son of the data to a predetermined standard of performance. This comparison may involve statistical calculations (alert type pro-grams) or other methods (non-alert type programs).
With both alert and non-alert type programs, the objectives of data analysis are to verify acceptable levels of perfor-mance, to identify trends which may need corrective action, and to indicate those tasks and intervals which may be safely eliminated, modi-fied or extended.
Alert type programs
Programs incorporating statistical performance standards use parameters such as delays, Pireps per 1,000 departures or com-ponent removals/failures per 1,000 hours, for each air-craft sys-tem, or total delays/cancellations per 100 depar-tures for the entire aircraft. The choice of units of mea-surement is not critical provided that they are constant throughout the opera-tion of the program and are appropriate to the type and frequency of the events being recorded.
When prepared as a running graphical or tabular display of cur-rent performance, these data depict trends as well as show out--of-limits conditions. The system performance data is usually reinforced by reports of component removals or con-firmed fail-ures.
The data are then compared with a reliability alert level (or equivalent title, e.g. performance standard, control level, reli-ability index, upper limit, hereinafter referred to as an "alert level") which, when exceeded, indicates that there has been an apparent deterioration in the normal behaviour pattern of the system or component with which it is associa-ted. When an alert level is exceeded, appropriate corrective action must be taken. It should be recognized that alert levels are not minimum accept-able airworthiness levels. Rather, they are a means of ident-ify-ing those increases in failure rate which fall outside the bounds of normal distri-bution and therefore warrant further investiga-tion.
Alert levels can range from zero (for critical components, and for those where failures in service have been extremely rare) to perhaps as many as 100 Pireps per 1,000 hours on a systems basis, for less critical systems, such as ATA 25 (equipment/ furnishings). Wherever possible, they should be based on the number of events which have occurred during a representative period of safe operation of the aircraft fleet. Alert levels should be revised periodically to reflect operating experi-ence.
When establishing alert levels based on operating experience, the normal period of operation taken is between two and three years, dependent upon fleet size and utilization. The levels will usually be so calculated as to be appropriate to the numbers of events recorded in one-month or three-month periods of operation. Large fleets will generate sufficient signifi-cant information much sooner than small fleets. Some examples of alert level calculations may be found in Appendix "A".
Where there is insufficient operating experience, or when a pro-gram for a new aircraft type is being established, the following approaches may be used.
For a new aircraft type during the first two years of opera-tion all malfunctions may be considered signifi-cant (i.e. alert level zero) while data is accumulat-ed for future use.
Alternatively, levels may be established based on the degree of system and component in-service reliability assumed in the design of the aircraft. These estima-t-ed values are normally quoted in terms of mean time between unscheduled removals (MTBUR) or mean time between failures (MTBF) for both individual components and complete systems. These initial predictions should be replaced by actual reliabil-ity figures when sufficient in-service experience has been accumulated.
For an established aircraft type with a new operator, the alert levels of other operators may be utilized until the new operator has accumulated suf-ficient ex-perience. Alternatively, experience gained from oper-ation of a similar aircraft model may be used.
Both the method used for establishing an alert level, and the associated qualifying period, apply also when the level is recalculated to reflect current operating experience. However, if during the period between recalculations of an alert level, a significant change in the reliability of an item is experienced, which can be related to some known action(e.g. modification, change in maintenance or operating procedure) then the alert level applicable to the item should be reassess-ed, based upon the data subsequent to the change. The procedures, periods and conditions for recalculation of alert levels must be defined in the program docu-ment and approved by DOT.
- For a new aircraft type during the first two years of opera-tion all malfunctions may be considered signifi-cant (i.e. alert level zero) while data is accumulat-ed for future use.
Non-alert type programs
Programs that do not depend upon statistics for their opera-tion (non-alert type programs) may be used by any size of organization and applied to any size of fleet. If the pro-grams are to be as effective as the statistically based pro-grams however, the number and range of inputs must be equiva-lent to those of the statisti-cal programs, and the operator's organization must have the ca-pability of analyzing the data to arrive at meaningful conclu-sions. This may involve the establishment of a dedicated section for the purpose. Less comprehensive non-alert type programs, which are applied to a limited number of components at any given time, may be handled by existing organizational elements, or even by a single indivi-dual.
Much of the information that is compiled to assist in the day to day operation of the operator's maintenance program may be effec-tively used as a basis for this type of continuous me-chanical performance analysis. Mechanical interruption sum-maries, flight log reviews, powerplant monitoring reports, incident reports, and powerplant and component analysis re-ports are examples of the types of information suitable for this monitoring method. Non--alert type programs also include many elements of maintenance management which are often not considered under the heading of "reliability". Examples in-clude trial programs for different (though previously ap-proved) lubricants, the documented use of different suppliers or overhaulers, and sampling (time trial) programs, whereby the escalation of times between overhaul of engines or other components is based upon satisfactory strip reports following successive trial extensions.
Changes in inspection program basis
Both types of program must include provision for the analysis of changes in the basis of the inspection program. Such changes may take the form of maintenance review board (MRB) report amend-ments, or changes in the maintenance planning document, or other manufacturers' recommenda-tions, transmit-ted by manual amend-ments, service bulletins or other means. Each change must be evaluated to assess its applicability to the operator's program.
- Alert type programs
Data display and reporting
All programs will require some means of displaying and reporting the collected data, and should include a periodic reporting system with appropriate data displays, summarizing the activity of the previous period. The reports should cover all aircraft systems controlled by the pro-gram, in sufficient depth to enable DOT and other recipients to evalu-ate the effectiveness of the affected segments of the maintenance pro-gram. The reports should highlight systems which have fallen short of the established performance standards and discuss any action which has been taken, or is planned, including changes in maintenance and inspection intervals and changes from one process category and/or task to another. Continuing over-alert conditions carried forward from previous reports should be listed, together with details of the progress of any correc-tive action taken. Some examples of the types of data display used in reliabil-ity programs may be found in appendix "B".
The actions to be taken in response to the data analysis should be positive enough to achieve the desired level of performance within a reasonable time. The system must include procedures to ensure DOT approval for any proposed changes in the inspection program, and for notification of the organizational element(s) responsible for taking the action. The sys-tem should also provide periodic feedback until such time as perfor-mance has reached an acceptable level. The procedures of the respon-sive action system may include work forms, special inspection procedures, engineering orders, etc. Special provision should be made for the control of critical items, the failure of which could impair the airworthiness of the aircraft.
Action taken in response to the findings of the reliability pro-gram can include changes in operational procedures or fault find-ing techniques, changes in fuels or lubricants, variation of stor-age conditions, the use of different sources of supply and the im-provement of training standards, etc. The major advantage of reliability control programs however, is that they afford the operator a formal means of substantiating applications for ap-proval to amend his existing maintenance schedules. The program document should include a description of the process by which such applica-tions will be made. The volume of data required to sub-stantiate the extension of a mainte-n-ance interval, or the change or deletion of a maintenance task, will depend both on the fre-quency of the task, and on the reason for its inclusion in the initial program. The minimum level of experience would normally be approximately one year, or one complete interval between the events in question, which-ever is the greater. Thus, high frequen-cy events, such as "A" check items, will require a relatively high volume of data, in the order of 25-50 events or more, while infrequent events, such as "D" check items, will usually require the operator to demonstrate satisfactory completion of at least one complete interval between the tasks under review. Similarly, changes to tasks introduced for safety reasons (e.g. in response to questions c or d of MSG-2, or questions 5 or 8 of MSG-3) will require significantly more substantiating data than those included primarily for economic or operational efficiency reasons. It will be necessary to refer to the original MRB report to determine the reason for each task.
Changes which involve the deletion of a task, or a change in the primary maintenance process (e.g. from on condition to hard time, or vice versa) must be subjected to the same analysis that was used to establish the initial program basis. This is sometimes referred to as the internal MRB procedure.
Changes to tasks designated as "airworthiness limitations", life limits, etc., may NOT be made on the basis of an operator's reli-ability program.
Reliability program amendment. The program should include a des-cription of the procedures for its own revision. The description should identify the organizational elements involved in the revi-sion process and their authority. DOT approval will be required for any revision affecting:
Data collection systems;
Data analysis methods;
Addition or deletion of aircraft types; and
- Procedural and organizational changes concerning the admin-istra-tion of the program.
- Data collection systems;
8. ADMINISTRATION OF PROGRAMS
Participants in the reliability program should be drawn from ap-propriate elements of the organization and should be authorized to act on behalf of those elements. The principal airworthiness in-spector (PAI) assigned to the carrier, or any other DOT represen-tative, may participate in the administration of the program as an observer, but such participation will have no bearing on the ap-proval or rejection of any changes proposed.
The makeup of the administration group may vary considerably from one operator to another. It may consist of a technical board that analyzes performance trends and shop findings to make recom-menda-tions. This board type of administration should have meet-ings scheduled at some specified interval, and should provide for the ad hoc assembly of the board at any time a decision is needed. An organization and data flow chart for this type of system is shown on page 16.
Alternatively, operators with sufficient organizational capability may administer their program by assigning appropriate respon-sibili-ties to each organizational element. With this type of arrange-ment, responsibility for operation of the program should be as-signed to a specific element of the operator's organization.
- The procedures used for controlling each of the elements of the reliability program should be incorporated in appropriate sections of the operator's maintenance control manual. This will provide each organ-izational element, and individuals therein, with in-structions regarding their part in the program. Pre printed forms should be used to document recurring actions that involve several organiza-tional elements, such as the analysis of substandard sys-tems or components, shop analysis of components for purposes of task interval adjustment, and reports relating to aircraft check con-tent amendment.
9. Program document
Reference is made in this advisory to a reliability program docu-ment. In practice this document will most likely take the form of a section of the operator's maintenance control manual, and should include at least the following:
General description of the program;
Organizational structure, duties and responsibilities;
Description of the individual systems;
Derivation of performance standards;
Method of controlling changes to the program;
Copy and explanation of all forms peculiar to the system; and
Revision control system.
- General description of the program;
The document should describe the workings of all systems in suf-ficient detail to provide for proper operation of the program. It should describe any reports relative to the program, and include samples of any forms used with instructions for their use. The organizational element(s) responsible for publishing reports should be identified and the distribution should be stated.
- The document should also include definitions of significant terms used in the program, with particular emphasis on definition of the process categories and/or tasks.
10. Program approval
Private operators may implement a reliability program without prior DOT approval. The program will, however, be subject to review prior to the approval of any inspection program changes which may be based upon it. Air carriers' reliability programs will be reviewed, and where appro-pri-ate, approved by DOT, as a part of the operators overall maintenance program. Approval of revisions to an air carrier's program will be by approval of the associated maintenance control manual amend-ment.
11. Joint Reliability Programs
A fleet size of 5 or more aircraft is usually considered necessary to generate sufficient data for the operation of an effective statistical-ly based reliability program. To accommodate the needs of smaller operators, participation in joint programs may be approved. Such pro-grams fall into two main categories, those primarily intended for the use of another operator, and those managed by a third party (usually either the aircraft manufacturer, or a supplier of computer services). Each of these types of program differs significantly from the other, as shown below.
Participation in the reliability program of another operator will re-quire the approval of the PAI, or equivalent airworthiness au-thority representative, of each operator. The applicants will be required to demonstrate sufficient commonality of aircraft config-uration, operating environment, utilization and route structure, to maintain the integrity of the program.
Responsibility for program management for each of the participat-ing fleets must be clearly defined, and the integration of data must be arranged so as to enable termination of the joint program without loss of applicable data. Data collection and analysis systems must include provision to detect significant differences in the participating fleets.
Third party programs
This type of reliability program is most applicable to those air-craft types, such as business jets, where there may be no single large fleet operator. They may be regarded as data collection and analysis ser-vices, rather than complete programs. Multi partici-pant programs of this type are of most value in the pro-vision of operating data to the Maintenance Review Board to enable revision of the MRB report.
Nevertheless, such programs may meet the CM require-ments of MSG-2, and may be used by individual operators to demonstrate significant differences between their own and the global fleet, in support of changes to their own inspection pro-grams.
Third party programs are not approved as such. Instead, each indivi-dual operator must apply to DOT for approval for his parti-cipation in the program. This is usually considered part of the approval of the operator's inspection program.
12. Maintenance development programs
Certain large air carriers may be authorized by DOT to make specified types of changes in their maintenance programs without prior approval. This privilege, previously known as a "continu-ous maintenance system" is granted by the issuance of a "mainte-nance development pro-gram" (MDP) approval. To qualify for such an approval, an air carrier must:
Operate a large fleet of complex modern transport category air-craft, at a rate of utilization sufficiently high to permit the accumulation of experience at a rate commen-surate with that of the global fleet leaders;
Have an established engineering department, operating as part of a reliability program which meets the general requirements of this advisory;
Demonstrate, over a number of years, the effective manage-ment of their maintenance program, with particular emphasis on the sound-ness of any inspection program changes pro-posed; and
Demonstrate that their reliability program is sufficiently active, in terms of the number and complexity of changes in-volved, to warrant the delegation of the approval respon-sibility to the carrier.
- Operate a large fleet of complex modern transport category air-craft, at a rate of utilization sufficiently high to permit the accumulation of experience at a rate commen-surate with that of the global fleet leaders;
The nature and scope of the approval privileges will be outlined in the air carrier's maintenance control manual. The approval may cover the entire maintenance program, or be restricted to certain specified areas of scheduled maintenance, dependent on the nature of the reliability monitoring techniques used, the carrier's organiz-ation, and the quali-fications and experience of the person-nel.
Maintenance development approval will NOT include the privilege of changing component life limits, or any requirements specified in air-worthiness directives, airworthiness limitations or MRB sampl-ing programs.
- A chart depicting a typical maintenance development organization and data flow sequence is illustrated on page 17, adjacent to that for a typical reliability program, to facilitate comparison.
12. (a) Reliability program flow chart
12. (b) Maintenance development program flow chart
12. (c) B707 "B" check interval growth
GLOSSARY OF TERMS
Maintenance items, compliance with which is mandatory to maintain conformity with the type approval basis for a given type of aircraft.
CONDITION MONITORING (CM)
A maintenance process under which data on the whole population of specified items in service is analyzed to indicate whether some allocation of tech-nical resources is required. Not a preventive maintenance process, condi-tion monitored maintenance allows failures to occur, and relies upon analy-sis of operating experience information to indicate the need for appropriate action.
When shop findings confirm a defect or failure which substantiates the reason for removal.
HARD TIME (HT)
A maintenance process under which a part is removed from service or over-hauled at fixed intervals defined in operating hours, operating cycles, or calendar time.
A program for the scheduled maintenance of aircraft of a given fleet, devel-oped by the operator, and approved by DOT.
The period (in flying hours, operating cycles or calendar time) which is permitted to elapse before a particular maintenance task is required, or between repeats of task.
LIFE LIMITED PARTS
Those items which, as a condition of the type approval, must be removed and discarded after specified time in service.
MAINTENANCE PLANNING DOCUMENT
A manufacturers recommendation on the maintenance of his product, produced in advance of, or concurrent with, the MRB report, and usually based on the same analyses which are submitted to the program development group, to enable prospective operators to establish maintenance schedules prior to the introduc-tion of a new type into service.
Classification of the manner in which a particular item is maintained.
MAINTENANCE REVIEW BOARD (MRB)
The section of an airworthiness authority responsible for the approval of recommendations made by industry on the maintenance requirements of new aircraft types.
MINIMUM EQUIPMENT LIST (MEL)
An approved list of items which may be inoperative for flight under speci-fied conditions.
The recommendations of a program development group, which are submitted to the airworthiness authority's MRB for approval, and thereafter form an ap-proved basis on which a new operator may establish his maintenance schedule.
Logic systems, developed by the maintenance steering group of the Air Trans-port Association of America, for use by program development groups in the analysis of an aircraft's maintenance requirements.
MTBF -MEAN TIME BETWEEN FAILURES
A performance figure calculated by dividing the total unit flying hours accrued in a period by the number of unit failures in that period.
MTBF = Total unit hours this period
Number of units failed this period
NOTE: MTBF does not account for unit hours flown in other periods, sched-uled unit removals, or units still operating.
MTBR -MEAN TIME BETWEEN REMOVALS
A performance figure calculated by dividing the total unit flying hours accrued in a period by the total unit removals (both scheduled and unsched-uled) in that period.
MTBR = Total unit hours this period
Number of units removed this period
NOTE: MTBR does not account for unit hours flown in other periods nor for units still operating.
MTTF -MEAN TIME TO FAILURE
A performance figure calculated by computing the average of the cumulative operating hours for each unit at failure.
MTTF = Total unit hours accrued by all failed units
Total number of units failed
NOTE: MTTF accounts for the average life of failed units only. Scheduled removals are not included.
MTTR -MEAN TIME TO REMOVAL
A performance figure calculated by computing the average of the total unit operating hours at removal for both scheduled and unscheduled removals.
MTTR = Total unit hours accrued by all removed units
Total number of units removed
NOTE: MTTR is the most accurate performance figure for the average "life" realized for all components.
A maintenance process having repetitive inspections or tests to determine the condition of units, systems or portions of structure.
The ability to perform the required functions within acceptable operational standards for the time period specified.
PRIMARY MAINTENANCE PROCESS
The process primarily depended upon to ensure that inherent design reliabil-ity is maintained.
A program applied to major items of equipment which are expected to be sub-ject to progressive deterioration in service, and for which there is insuf-ficient service experience to determine appropriate tasks and intervals. The purpose of the program is to collect data which will enable the identi-fication of the appropriate tasks and intervals, and indicate the need for product improvements.
An action or set of actions intended to restore an item to, or maintain it in, an airworthy condition, including inspection to determine if the item is airworthy.