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.
Appendix 1 2•506 If the answer to question (d) is Yes, there is a function required of this engine component that is not evident to the flight crew when the component fails. Some scheduled task may be necessary to assure a reasonably high probability that this function is available when required.
NOID: Tasks resulting from a Yes answer to question (d) may be part of either the Hard Time limitation or the On Condition maintenance program.
20507 It is expected that the answer to question (e) is always Yes for structural engine components, but that their expected lile is very long relative to the usual engine inspection periods. If tasks defined by questions (a through (d) are inadequate to control wear or deterioration of engine components, additional tasks should be listed here. NOTE: Tasks resulting from a Yes answer to question
(e) are part of either the Hard Time limitation or the On Condition maintenance program.
2.5.8 Engine components for which no scheduled tasks are selected are included in Condition Monitoring.
20509 The questions in Figure 1 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, 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.
2.5.10 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.
2.5.11 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.
2.5.12 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.
Appendix 1 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.
205014 Figure 3 assesses tasks listed against the Yes answer in Figure 1, questions (b) and (e) and selects those tasks which should be done because of economic considerations.
2.5.15 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:
(1) Prior knowledge from other aircraft that the scheduled maintenance tasks had substantial evidence of being truly effective and economically worthwhile, and 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.
The question "Does failure prevent dispatch" refers to whether the item will be on the Minimum Equipment List (MEL).
The answer to question (b) is expected to always be Yes for engine components that cause engine failure.
The question Is 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.
2.5.18 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.
205019 Engine tasks are included in the Threshold Sampling maintenance program. This program is described below.
The Threshold Sampling maintenance program is intended to recognize the On Condition design characteristics of modern Turbo-Jet engines, while sampling to control reliability.
This program uses repetitive sampling to determine:
(1) The condition of engine components.
(2) The advisability for continued operation to the next sampling limit, and
(3) The next sampling limit, threshold, or sampling band.
Appendix 1
2.5.21 Initial sampling thresholds are based on:
(1) The design of the engine under study, the results of developnental testing, and prior service experience.
(2) The results of previous engine programs.
(3) The fact that samples are available from engines removed for all causes at virtually all ages. This means that knowledge of the conditions of engines is available over the complete continuum of time from start of operation to the highest time experienced, and
(4) The fact that most engine design problems become apparent and can be controlled well within any established limits or thresholds.
205022 The Threshold Sampling program establishes the initial sampling threshold. Operators are subsequently responsible for:
(1) Evaluating the samples obtained from the initial threshold
(2) Determining the next sampling threshold, and
(3) Determining the number to be sampled at the next threshold.
205023 Threshold Sampling is normally accomplished by inspecting the parts or systems of engines that are removed and accessible in the shop. These engines provide samples over a full range of ages without waiting for the threshold to be reached. The results of inspecting these samples are used to determine the future program. When samples are not available from engines that are in the shop, scheduled samples or in situ inspections may be required.
206 Program Development Administration. Regulatory Authority participation is encouraged as early and as thoroughly as possible in all phases of working group activity. It is recognized that the Regulatory Authority will later be asked to approve the proposed program resulting from these efforts. Therefore, the Regulatory Authority participation must necessarily be restricted to technical participation, contributing their own knowledge, and observing the activities of the working group. Regulatory Authority approva ol working group recommendations is not implied by the participation of Regulatory Authority members in working group sessions. The following activity phases will apply.
Appendix 1 Phase I. Steering Group general familiarization training.
Phase II. (a) Working Group or Working Activity Training.
*(b) Preparation of first draft Significant Items List. (Ref. 2.7.1) *(c) Establish functions and failure modes applicable to the Significant Items.
(a) Preparation of Figures 1 thru 5 decision diagram replies and supporting data for each system and significant item.
Phase III. (a) Evaluation of manufacturer's technical data and recommended tasks by the Working Groups' airline personnel and meeting with manufacturer to make necessary revisions and prepare task recommendations.
(b) Development of task frequency recommendations.
(This phase is meant to follow Phase III. a).
NOTE: A Steering Group member should participate in all Phase III activity.
Phase IV. Presentation to Steering Group (meeting with each Working Group or Activity Chairman).
Phase V. Preparation and presentation of the Steering Group's proposal to the Regulatory Authority.
2•7 Supporting Technical Data. The following supporting technical data will be provided in printed form, together with adequate crossreferences on the records of replies to the decision diagrams.
Maintenance Significant Items List. This list will include by ATA System, the name, quantity per airplane, prime manuracturer part number, vendor name and part number for each item considered by the Working Group/Activity to require individual analysis.
20702 Significant Items Data.
(a) Description of each significant item and its function(s).
(b) Listing of its failure mode(s) and effects.
* Steering Committee audits are required for these steps before proceeding.
Appendix 1
(c) Expected failure rate.
(a) Hidden functions.
(e) Need to be on M.E.I.
(I) Redundancy (may be unit, system or system management).
(g) Potential indications of reduced failure resistance.
20703 System Data.
(a) Description of each system and its function(s).
(0) Listing of any failure modes and effects not considered in item data.
(c) Hidden functions not considered in item data.
Appendix 1
GLOSSARY Inherent Level of Reliability and Safety - That level which is built into the unit and therefore inherent in its design. This is the highest level of reliability and safety that can be expected from a unit, system, or aircraft. To achieve higher levels of reliability generally requires modification or redesign.
Maintenance Significant Items - Those maintenance items that are judged by the manufacturer to be relatively the most important from a safety or reliability standpoint, or from an economic standpoint.
Structural Significant Items - Those local areas of primary structure which are judged by the manufacturer to be relatively the most important from a fatigue or corrosion vulnerability standpoint or from a failure effects standpoint.
Operational Reliability - The ability to perform the required functions within acceptable operational standards for the time period specified.
Effective Incipient Failure Detection - That maintenance action which will reliably detect incipient failures if they exist. That is, detect the pending failure of a unit or system before that system fails. For example, detection of turbine blade cracks prior to blade failure.
Real and Applicable Data - Those data about real, operating hardware that is similar enough to the hardware under discussion to be applicable to the design of maintenance programs for the current hardware.
Reduction in Failure Resistance - The deterioration of inherent (design) levels of reliability. As failure resistance reduces, failures increase;
resulting in lower reliability. If reduction in failure can be detected, maintenance can be performed prior to the point where reliability is adversely affected.
Function - The characteristic actions of units, systems and aircraft.
Failure Modes - The ways in which units, systems and aircraft deteriorate can be considered to have failed.
Potential Effectiveness - Capable of being effective (maintenance action) to some degree.
Routine Flight Crew Monitoring - That monitoring that is inherent in normally operating the aircraft. For example, the pre-flight check list, or the normal operation of the aircraft and its components. Does not include monitoring of "back-up" equipment that is normally not tested as a part oi a normal flight.
Failure Effects - The consequence of failure.
AIRLINE MAINTENANCE PROGRAM DEVELOPMENT MSG 2 DECISION DIAGRAM 3/27/78 FIGURE I FIGURE 2 FIGURE 3
WHICH TASKS CAN BE DONE WHICH TASKS MUST BE DONE WHICH TASKS SHOULD BE DONE QUESTIONS a, b, & c APPLY TO EACH FAILURE MODE QUESTION I APPLIES TO c TASKS QUESTION A, B, & C'APPLY TO b AND e TASKS
OF ITEM QUESTION d APPLIES TO EACH FUNCTION OF ITEM QUESTION II APPLIES TO d TASKS QUESTION • APPLIES TO ITEM AS A WHOLE IS REDUCTION IN FAILURE RESISTANCE DETECTABLE BY ROUTINE FLIGHT OF IN-FLIGHT MONITORING LIST OF MEANS Figures 1. 2. AND 3.
CREW MONITORING ?
YES NO - IN- SITU MICE IS REDUCTION IN FAILURE RESISTANCE DETECTABLE BY TASKS REF. TO RESISTANCE (CHECK, LIST ALL POTENTIAL REDUCT IN FAILURE MSG 2 DECISION DIAGRAM ADDENDUM 1 OR UNIT TEST ? INSPECT, SERVICE, ES. ETC.J.
NO
WHICH OF THE TASKS RE ADVERSE EFFECT ON DOES FAILURE MODE HAVE A DIRECT LIST ALL POTENTIAL TASKS REF. TO OPERATING SAFETY FERRING TO OPERATING SAFETY MUST BE DONE ? AT LEAST ONE TASK MUST BE DONE, EXPLAIN ANY c OPERATING SAFETY ? IS THE FUNCTION HIDDEN FROM THE VIEWPOINT OF THE FLIGHT CREW ? YES YESİ NO - (TOTAL TIME LIMITS ETC. I CHECK, INSPECT. LIST ALL POTENTIAL TASKS REF. TO HIDDEN FUNCTION (CHECK, INSPECT, FERRING TO HIDDEN FUNCTION MUST BE DONE ? AT LEAST ONE TASK MUST BE DONE EXCEPT AS PERL WHICH OF THE TASKS RE- TASKS NOT SELECTED. DATA SHOW DOES REAL AND APPLICABLE SCHEDULED TASK - DESIRABILITY OF YES-- PREVENT DISPATCH ? DOES FAILURE IS ELAPSED TIME FOR CORRECTION OF FAILURE > 0.5 HOUR ? DISPOSITION OF TASK PROGRAM RELATIVE TO • INCLUDE NOE MITTED BY APP. 3 -EVALUATION REQUIRES Appendix 1 AC 120-17A YES!
IS THERE AN AD - VERSE RELATIONSHIP PERIODIC NO -NO- •I OMIT