Appendix 1 20103 The Objectives of an efficient airline maintenance program are:
(a) To prevent deterioration of the inherent design levels of reliability and operating safety of the aircraft, (Б) To accomplish this protection at the minimum practical costs.
These objectives recognize that maintenance programs, as such, cannot correct deficiencies in the inherent design levels of flight equipment reliability. The maintenance program can only prevent deterioration of such inherent levels. If the inherent levels are found to be unsatisfactory, engineering action is necessary to obtain improvement.
2.1.5 The maintenance program itself consists of two types of tasks:
(a) A group of scheduled tasks to be accomplished at specified intervals. The objective of these tasks is to prevent deterioration of the inherent design levels of aircraft reliability, and
(b) A group of nonscheduled tasks which results from:
(i) The scheduled tasks accomplished at specified intervals,
(ii) Reports of malfunctions (usually originated by the flight crew), or
(iii) Condition Monitoring.
The objective of these nonscheduled tasks is to restore the equipment to its inherent level of reliability.
This document describes procedures for developing the scheduled maintenance program. Nonscheduled maintenance results from scheduled tasks, normal operation or condition monitoring.
2.1.6 Maintenance programs generally include one or more of the following primary maintenance processes:
Hard Time Limit: A maximum interval for performing maintenance tasks. These intervals usually apply to overhaul, but also apply to total life of parts or units.
Appendix 1 On Condition: Repetitive inspections, or tests to determine the condition of units or systems or portions of structure (Ref.: FAA Advisory Circular 121-1).
Conditior Monitoring: For items that have neither hard time limits nor on condition maintenance as their primary maintenance process. Condition monitoring is accomplished by appropriate means available to an operator for finding and resolving problem areas. These means range from notices of unusual problems to special analysis of unit performance.
No specific monitoring system is implied for any given unit (Ref.: FAA Procedures 8310.4, paragraph 3033).
This document results in scheduled tasks that fit the hard time limit or on condition maintenance programs or, where no tasks are specified, the item is included in condition monitoring.
2.2 Scheduled Maintenance Program Content The tasks in a scheduled maintenance program may include:
(a) Servicing
(b) Inspection
(c) Testing
(a) Calibration
(e) Replacement
2.2.1 An efficient program is one which schedules only those tasks necessary to meet the stated objectives. It does not schedule additional tasks which will increase maintenance costs without a corresponding increase in reliability protection.
2.2.2 The development of a scheduled maintenance program requires a very large number of decisions pertaining to:
(a) Which individual tasks are necessary,
(b) How frequently these tasks should be scheduled,
(c) What facilities are required to enable these tasks to be accomplished, Page 4
Appendix 1
(d) Where these facilities should be located, and
(e) Which tasks should be accomplished concurrently in the interests of economy.
2•3 Aircraft System Component Analysis Method. The method for determining the content of the scheduled maintenance program for systems and components (parts a and b of Paragraph 2.2.2) uses decision diagrams. These diagrams are the basis of an evaluatory process applied to each system and its significant items using technical data provided (Ref. 2.7). Principally, the evaluations are based on the systems' and items' functions and failure modes.
The purpose is to:
(a) Identify the systems and their significant items*.
(b) Identify their functions*, failure modes*, and failure reliability*.
(c) Define scheduled maintenance tasks having potential effectiveness* relative to the control of operational reliability*.
(d) Assess the desirability of scheduling those tasks having potential effectiveness.
It should be noted that there is a difference between "Potential" effectiveness of a task versus the "desirability" of including this task in the scheduled maintenance program.
The approach taken in the following procedure is to plot a path whereby a final judgment can be made as to whether those potentially effective tasks are worthy of inclusion in an initial maintenance program for a new airplane.
There are three decision diagrams provided (Addendum I, Figures 1 through 3). Figure 1 is used to determine scheduled maintenance tasks having potential effectiveness relative to the control of operational reliability. This determines tasks which can be done.
Figures 2 and 3 are used to assess the desirability of scheduling those tasks having potential effectiveness.
Figure 2 tasks must be done to prevent direct adverse effects on operating safety and to assure availability of hidden functions.
Figure 3 tasks should be done for economic value.
* See Glossary.
Appendix l
2.3.3 The total analysis process is shown diagramatically below.
See Addendum I for details.
Figure 1 Figure 2 Figure 3 (Questions a,b,c,d, e) (Questions I & II) (Questions A, B & C) Flight Crew Tasks a · Tasks N Tasks T c DA B C Tasks d (Eval) S Tasks e These tasks have These tasks must These tasks should potential effec- be done to prevent be done for economic tiveness & can direct adverse value.
be done. effects on operating safety & assure availability of hidden functions.
20304 The following guidelines encourage consideration of failure consequences and the potential effectiveness of scheduled maintenance tasks. In those cases where failure consequences are purely economic, the guidelines lead to consideration of both the cost of the scheduled maintenance and the value of the benefits which will result from the task.
Appendix 1 20305 A decision tree diagram (Figure 1 of Addendum 1) facilitates the definition of scheduled maintenance tasks having potential effectiveness. There are five key questions.
Note: Questions (a), (b), and (c) must be answered for each failure mode, question (d) for each function, and question (e) for the item as a whole.
(a) Is reduction in failure resistance* detectable by routine flight crew monitoring*?
(b) Is reduction in failure resistance detectable by in situ maintenance or unit test?
(c) Does failure mode have a direct adverse effect upon operating safety? (See Addendum 2.)
(a) Is the function hidden from the viewpoint of the flight crew? (See Addendum 3.)
(e) Is there an adverse relationship between age and reliability?
2.306 Each question should be answered in isolation, e.g., in question (c) all tasks which prevent direct adverse effects on operating safety must be listed. This may result in the same task being listed for more than one question.
If the answer to question (a) is Yes, this means there are methods available through monitoring of the normal in-flight instrumentation to detect incipient conditions before undesirable system effects occur. A Yes answer does not require a maintenance task. If the answer is No, there is no in-flight monitoring which can detect reduction in failure resistance. This question is meant to refer to the flight crews' ability to detect deteriorating calibration or systems operation before a failure occurs. NOTE: Tasks resulting from in-flight monitoring are part of nonscheduled
2.3.8 If the answer to question (b) is Yes, it means there is a maintenance task, not requiring item disassembly, that has potential effectiveness in detecting incipient conditions* before undesirable system effects occur. Tasks may include inspection, servicing, testing, etc. NOIE: Tasks resulting from a les answer to question (b) are part of the On Condition maintenance program.
* See Glossary.
Appendix l If the answer to question (c) is Yes, this failure mode has a direct, adverse effect on operating safety. It is necessary to examine the mechanism of failure and identify the single cells or simple assemblies where the failure initiates. Specific total time, total might cycle, time since overhaul and cycle since overhaul limitations may be assigned these single cells or simple assemblies and the.
probability of operational failures will be minimized.
Examples of these actions are turbine engine disc limits, airplane flap link life limits, etc. In many cases, these Limits must be based upon marufacturer's development testing.
Fortunately, there is only a small number of failure modes which have a direct, adverse effect on operating safety.
This results from the fact that failure mode analyses are conducted throughout the process of flight equipment design.
In most cases, it is possible after identification of such a failure mode to make design changes (redundancy, incorporation of protective devices, etc.) which eliminate its direct adverse effect upon operating safety. If no potentially effective task exists, then the deficiency in design must be referred back to the manufacturer. The term "direct adverse effect upon operating safety" is explained in Addendum 2. NOTE: Tasks resulting from a Yes answer to question (c) are part of either the Hard Time limitation maintenance program or the On Condition maintenance program.
Refer to Addendum 3 for explanation of question (d). If the answer to question (d) is Yes, periodic ground test or shop tests may be required if there is no other way of ensuring that there is a high probability of the hidden function being available when required. The frequencies of these tests are associated with failure consequences and anticipated failure probability. A component cannot be considered to have a hidden function if failure of that function results in a system malfunction which is evident to the flight crew during normal operations. In this case, the answer must be No. NOTE: Tasks resulting from a Yes answer to question (d) may be part of either the Hard Time limitation or the On-Condition maintenance program.
2.3.11 If the answer to question (e) is Yes, periodic overhaul may be an effective way of controlling reliability. Whether or not a fixed overhaul time limit will indeed be effective can be determined only by actuarial analysis of operating experience. NOI: Tasks resulting from a Yes answer to question (e) are part of the Hard Time limitation mainte nance program.
Appendix 1
2.3012 It has been found that overall measures of reliability of complex components, such as the premature removal rate, usually are not functions of the age of these components.
in most cases, thereiore, the answer to question (e) is No. In this event, scheduled overhaul cannot improve operating reliability. Engineering action is the only means of improving reliability. These components should be operated, therefore, without scheduled overhaul. NoIE:
Systems or items which require no scheduled tasks are included in Condition Monitoring.
The preceding paragraph is contrary to the common belief that each component has an unique requirement for scheduled maintenance in order to protect its inherent level of reliability. The validity of this belief was first challenged by actuarial analyses of the life histories of parting pograph has ee enth the ely detonate at the by the massive operational experience of many airlines with many different types of components covered by Reliability Programs complying with FAA Advisory Circular 120-17.
It is possible to change the answers to the five questions in the decision diagram by improved technology. it is hoped that Aircraft Integrated Data Systems (AIDS), for example, will reliably indicate reduced resistance to various modes of failure of many components during normal airline operations. If this is determined to be possible, many "No" answers to questions (a) and (b) will become "Yes" answers. Answers may also be changed by various developments in the field of nondestructive test techniques, built-in test equipment, etc.
The questions in Figure l are intended to determine maintenance tasks having potential effectiveness for possible inclusion in a scheduled maintenance program. However, it is probable that many of these "potentially" beneficial scheduled tasks would not be "desirable" even though such tasks could improve reliability. This might be true when operating safety is not affected by failure or the cost of the scheduled maintenance task is greater than the value of such resulting benefits as reduced incidence of component premature removal, reduced incidence of departure delays, etc. Additional diagrams are used to assess the "desirability" of those scheduled maintenance actions which have potential effectiveness. This is accomplished by Figures 2 and 3 of Addendum 1.
Appendix 1
2.3.16 Figure 2 selects those tasks which must be done because of operating safety or hidden function considerations.
Figure 3 selects those tasks which should be done because of economic considerations.
2.3.17 Figure 2 assesses tasks listed against the Yes answers of questions c and d in Figure 1, and selects those tasks which must be done.
2•3.18 For the operating safety question, at least one task must be listed for each failure mode having a Yes answer to question c of Figure 1. An explanation should be given for any question c tasks not selected.
2.3.19 For the hidden function question, normally at least one task must be listed for each hidden function having a Yes answer to Figure 1, question d. If a task is not selected, as permitted by Addendum 3, an explanation must be provided.
2.3.20 Figure 3 assesses tasks listed against the Yes answer in Figure 1, questions b and e and select those tasks which should be done because of economic considerations.
A key question in Figure 3 is the first, ®Does real and applicable data* show desirability of scheduled task?"
a "Yes" answer is appropriate if there is:
Prior knowledge from other aircraft that the scheduled maintenance tasks had substantial evidence of being truly effective and economically worthwhile,
(2) The system/component configurations of the old and new airplanes are sufficiently similar to conclude that the task will be equally effective for the new airplane.
2•3•22 The question "Does failure prevent dispatch" refers to whether the item will be on the Minimum Equipment List (MEL) • The question "iIs elapsed time for correction of failure >0.5 Hr. refers to whether corrective action can be accomplished without a delay during a normal transit stop.
2.3.24 When a task "requires evaluation" it is important that the frequency of the failure and the cost of carrying out the task are taken into consideration.
* See Glossary.
Appendix 1 204 Aircraft Structure Analysis Method. The method for determining the content of the scheduled maintenance program for structure is:
(a) Identify the significant structural itens.*
(b) Identify their failure modes and failure effects.
(c) Assess the potential effectiveness of scheduled inspections of structure.
(d) Assess the desirability of those inspections of structure which do have potential effectiveness.
2•4.1 The static structure will be treated as hereafter described.
Additionally, the mechanical elements of structural components, such as doors, emergency exits, and flight control surfaces will be treated individually by the processes described in Section 2.3.
The decision tree diagram, Figure 1 of Addendum 1, facilitates the definition of scheduled inspections of structure having potential effectiveness. There are five key questions.
(a) Is reduction in failure resistance detectable by routine flight crew monitoring?
(b) Is reduction in failure resistance detectable by in situ maintenance or unit test?
(c) Does failure mode have a direct adverse effect upon operating safety?
(d) Is the function hidden from the viewpoint of the flight crew?
(e) Is there an adverse relationship between age and reliability?
2.4.3 The answer to question (a) is normally No. However, if in-flight instrumentation is developed which permits detection of incipient structural failures then the answer should be Yes.
If the answer to question (b) is les, there are methods available to detect incipient conditions before undesirable conditions occur. It would be expected that all redundant external and internal structure would be in this category.
VIE: Tasks resulting from a Yes answer to question (b) are part of the Structural Inspection program. This program is an On Condition program.
Appendix 1 20405 If the answer to question (c) is Yes, there is a failure mode which has a direct, adverse effect on operating safety for which there is no effective incipient failure detection method. It would be expected that nonredundant primary structure would be in this category. See Addendum 2 for explanation of "direct adverse effect on operating safety."
NOTE: Tasks resulting from a Yes answer to question (c) are part of the Hard Time limitation (usually total time or total cycle limits) maintenance program.
20406 If the answer to question (d) is Yes, there is a function required of this element of structure that is not regularly used during normal flight operations. Some inspection or test is therefore necessary to ensure that this function has a high probability of being available when required.
Tail bumper structure and structure provided for wheelsup landing are typical structural examples. NOTE: Tasks resulting from a Yes answer to question (d) are part of the Structural Inspection program.
20407 Structures would be expected to have a Yes answer to question (e) but only in a very long total time envelope.
The tasks performed as a result of Yes answers to the other questions are capable of detecting deterioration prior to failure of these items.
20408 It is probable that some of these "potentially" beneficial scheduled inspections would not be desirable, even if such tasks would improve reliability. This might be true when airworthiness is not affected by failure and the cost of the scheduled inspection is greater than the value of the resulting benefits. Therefore, additional diagrams are used to assess the desirability of those scheduled tasks which have potential effectiveness. This is accomplished by Figures 2, 4, and 5 of Addendum 1. A No answer to all questions is unlikely for structure. If it occurs, the item is included in Condition Monitoring.
20409 Figure 2 selects those tasks that must be done because of operating safety or hidden function considerations.
Figures 4 and 5 of Addendum 1 establish internal and external class numbers for structural items. The class numbers take into account vulnerability to failure, consequences of failure. The class numbers are to be used as guides for setting internal and external inspection frequencies.
204.11 The items to be evaluated by Figures 4 and 5 are those termed "structurally significant."
Appendix l 2•4.12 Each item is first rated for each of five characteristics per Figure 4 (fatigue resistance, corrosion resistance, crack propagation resistance, degree of redundancy and fatigue test rating)• Each item is then given an overall rating (R No.) per Figure 4 which considers all of the above ratings and combines them by judgment into a single overall rating (R No.) representing a relative level of structural integrity of the item. In general, the overall R No. for an item is equal to or less than the fatigue resistance or corrosion resistance rating for the item, whichever is lesser.
204014 The internal and external class numbers for each item are then determined by reference to Figure 5. Note that some items have both internal and external class numbers. This occurs for those internal items which have some probability of the internal item's condition being evident by some external condition. In these cases the item as described is visible internally and the "internal" inspection specified refers to the item as described. The "external" inspection of this item refers to that portion of the external structure which is adjacent to the internal item and which may yield some indication of the internal item's condition. Therefore, when an external inspection is specified for an internal item, it refers to the adjacent external structure and not the internal item itself.
205 Aircraft Engine Analysis Method. The method for determining the content of the scheduled engine maintenance program is:
(a) Identify the systems and their significant items.
(b) Identify their functions, failure modes, and failure effects.
(c) Define scheduled maintenance tasks having potential effectiveness relative to the control of operational reliability.
(d) Assess the desirability of scheduling those tasks having potential effectiveness.
(e) Determine initial sampling thresholds where appropriate.
20501 The engine as a whole and each significant engine item will be treated as described below.
Appendix 1 20502 The decision tree diagram, Figure 1 of Addendum 1, facilitates the definition of scheduled inspections having potential effectiveness. There are five key questions.
NOTE: Questions (a), (b), and (c) must be answered for each failure mode, question (d) for each function, and question (e) for the item as a whole.
(a) Is reduction in failure resistance detectable by routine flight crew monitoring?
(b) Is reduction in failure resistance detectable by in situ maintenance or unit test?
(c) Does failure mode have a direct adverse effect upon operating safety?
Is the function hidden from the viewpoint of the flight crew?
(e) Is there an adverse relationship between age and reliability?
20503 If the answer to question (a) is Yes, there are methods available through monitoring the normal in-flight instrumentation (including computerized Flight Log Monitoring) to detect incipient conditions before undesirable system effects occur. A Yes answer does not require a maintenance task. If the answer is No, there is no in-flight monitoring which can detect reduction in failure resistance. NOTE:
Tasks resulting from in-flight monitoring are part of If the answer to question (b) is Yes, there is a maintenance task, not requiring engine disassembly, that has potential effectiveness in detecting incipient conditions before undesirable system effects occur. Tasks may include inspection, servicing, testing, etc. NOIE: Tasks resulting from Yes answers to question (b) are part of the On Conditior maintenance program.
If the answer to question (c) is Yes, this engine component has a failure mode with direct, adverse effect on operating safety. It is necessary to examine the mechanism of failure and identify the single cells or simple assemblies where the failure initiated. Specific total time, or total flight cycle, limitations may be assigned these components to minimize the probability of operational failures. NOIL:
lasks resulting from a les answer to question (c) are part of either the Hard Time limitation maintenance program or the On Condition maintenance program.