Chapter 5.3
Preventive Maintenance
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Table of contents Paragraph |
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2 Policy for the preventive maintenance of aircraft
2.1 Content of preventive maintenance policies
4.1 Scheduled maintenance systems
5 Scheduled maintenance latitudes
6 Anticipation of scheduled maintenance
7 Deferment of scheduled maintenance
7.1 Deferment of scheduled maintenance at Stn/Ship/Units
7.2 Deferment of scheduled maintenance by 4PTs3
8 Maintenance in periods of abnormal usage
8.1 Aircraft not flown for extended periods
9 Contingency maintenance (CTM)
10.1 Training and authority levels
12 Condition-based maintenance
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List of tables
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Table 1. Equalized Maintenance – Example.
Preventive maintenance is one element of the through-life maintenance of materiel within the Military Air Environment (MAE) and is an essential element of the safe operation of aircraft and equipment. Preventive maintenance is systematic, with prescribed work undertaken at pre-determined intervals to reduce the probability of failure and to ensure that performance is not degraded by time or usage. This chapter complements Chapter 5.1.
This chapter is associated with the following publications:
1 Acquisition Operating Framework (AOF).
2 Def Stan 00-40 – Reliability and Maintainability.
4 Def Stan 00-45 - Using reliability centred maintenance to manage engineering failures. Part 2 - Developing an RCM project plan.
5 Def Stan 00-60 – Integrated Logistic Support.
6 JSP 553: Military Airworthiness Regulations.
7 Mil-Std-1629A – Procedures for Performing Failure Modes, Effects and Criticality Analysis.
8 Mil-Std-1843 & 2173(AS) – Reliability Centred Maintenance for Aircraft, Engines, Weapons and Equipment.
2 Policy for the preventive maintenance of aircraft
For aircraft and equipment in the MAE, including support equipment, preventive maintenance comprises 3 distinct types:
1 Flight servicing – contained in Chapter 2.8.
3 Condition-based maintenance.
Preventive maintenance activities incur additional costs; therefore, items should generally be allowed to run to fault, if safe and cost-effective to do so.
2.1 Content of preventive maintenance policies
Aircraft and equipment are to have preventive maintenance policies that:
1 Identify the scheduled and condition-based maintenance requirements.
2 Specify the latitudes to be applied to the various scheduled maintenance/inspection periodicities and component lives.
3 Identify those items that are safety-critical, as declared in the Topic 5 (Component Replacement List (CRL)) or equivalent.
The requirements at paragraph 2.1 are to be implemented for each type and mark as applicable, by the preparation, issue and use of:
1 Maintenance schedules in accordance with Chapter 8.1.1.
2 Maintenance procedures in the maintenance manuals.
The 4Project Team (PT)3 is to determine the preventive maintenance policy in conjunction with the Front Line Command (FLC) during the development phase of the project, using Integrated Logistic Support (ILS), Maintenance Steering Group (MSG) and Reliability Centred Maintenance (RCM) techniques and is to publish and maintain a schedule throughout the life of the equipment.
The overall preventive Maintenance Policy for an aircraft or equipment is to be published in a Support Policy Statement (SPS) in the Topic 2(N/A/R)1 or Topic 5W or equivalent , produced in accordance with Chapter 5.1. Schedules are to be reviewed by the 4PT3 no later than 5 yrs after introduction of the aircraft or equipment into service. The need for subsequent reviews is to be assessed as part of the SPS or Maintenance Policy review process. When reviewing maintenance schedules, an audit trail is to be maintained of the decision-making processes. Further guidance on the need for schedule reviews is given in Chapter 8.1.1. Ordinarily, Maintenance Procedures are validated by use; however, 4PTs3 should consider the implications that changes to equipment might have on the maintenance procedure.
Faults discovered during preventive maintenance are to become corrective maintenance tasks and are to be actioned appropriately.
Scheduled maintenance activities, their forecasting, tracking and recording may be controlled by the use of Information Systems (IS).
Scheduled maintenance is the minimum preventive maintenance that needs to be carried out at predetermined regular intervals to achieve maximum availability of the aircraft or equipment and to minimize deterioration. This is achieved by a schedule of tasks and work, based on calendar time, hours or other usage counts published by the 4PT3 in the Topic 5A1 or equivalent. The 4PT3is also required to consider the need for alternative calendar and flying hour or other usage backstops. Where a calendar time base is employed, the 4PT3 is to stipulate in the SPS and/or Topic 5A1 whether the interval between scheduled maintenance includes or excludes the time spent carrying out the maintenance. Normally, the forecast next due date is calculated from the date when the scheduled maintenance is complete. However, an 4PT 3may decide that due to environmental or other considerations the time spent carrying out the maintenance is to be included. In this circumstance, the forecast next due date is to be calculated from the beginning of the scheduled maintenance.
4.1 Scheduled maintenance systems
There are a number of tried and proven maintenance systems that have been developed over the years in order to meet differing operating requirements. The principles behind these systems are summarized in the following sub-paragraphs, which should guide the 4PT3 and FLC in determining the maintenance system to be used.
4.1.1 Flexible maintenance system
The individual tasks identified by the RCM analysis may be carried out individually at the identified periodicities. These tasks may also be combined into small work packages, carried out within a specified period to coincide with corrective maintenance or other downtime. Each task or work package should have sufficient latitude in its application requirements to permit flexibility in its satisfaction. Flexible maintenance activities may be satisfied and re-forecast individually when carried out on an opportunity basis, or re-calculated upon component replacement when conducting corrective maintenance. This system will tend to generate aircraft quickly, but will involve more frequent requirements for scheduled maintenance.
4.1.2 Grouped maintenance system
The individual tasks identified by the RCM analysis are formally grouped into packages to be carried out at set intervals in a maintenance cycle; generically termed Low, Medium and High frequency maintenance tasks. Each group of scheduled maintenance forms packages of work content that should broadly equate to the capabilities of the organization (Forward/Depth) responsible for carrying it out.
Low frequency task groups provide, through extended downtime, an opportunity to embody time-consuming modifications or upgrades.
Additional groups of scheduled maintenance may be introduced, to provide intermediate frequency groupings (legacy: RAF Primary) to meet maintenance requirements.
4.1.3 Equalized maintenance system
The equalized system employs the high/medium/low frequency groupings identified at paragraph 4.1.2, but the maintenance is carried out progressively throughout the maintenance cycle by completing, for example, ¼ of the low frequency work group in conjunction with each of 4 medium frequency work groups. This system exchanges the protracted down-time normally associated with low frequency scheduled maintenance for slightly extended down-times during the more frequent groupings, allowing better use of resources, although reducing the opportunity to embody time-consuming modifications.
In the example at Table 1 below, the flying hour-based low frequency (legacy: RAF Major) schedule content has been divided into 4 equal work elements which, when combined with the medium frequency content (legacy: RAF Minor), are called E1 to E4; they are carried out progressively through the maintenance cycle. Some also have calendar upper limits, although these are optional.
Table 1. Equalized Maintenance – Example.
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Type of Scheduled Maintenance |
Intermediate (I) |
Medium Frequency plus Equalized Low Frequency Work Elements |
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E1 |
E2 |
E3 |
E4 |
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Normal Flying Hrs Periodicity |
150 |
300 |
600 |
900 |
1200 |
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(Optional) Calendar Upper Limit (see note) |
Every 6 months |
N/A |
3 years |
N/A |
7 years |
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Cycle: |
I |
E1 |
I |
E2 |
I |
E3 |
I |
E4 |
I |
E1 etc |
Note:
For guidance on the use of calendar upper limits also referred to as backstops, see paragraph 4.1.4 below.
4.1.4 Use of maintenance backstops
For aircraft and airborne equipment, the periodicities of the grouped maintenance tasks described at paragraph 4.1.2 are normally expressed in flying hours. However, within a particular maintenance schedule, there may be activities that, should the aircraft or equipment have low utilization, warrant inspection based on calendar time. For example, this may be an inspection or series of inspections for corrosion that are embedded in a flying hour-based group of activities. In such cases, a calendar upper limit may be advisable.
4.1.5 Out-of-phase maintenance
If, when using a grouped or equalized maintenance system, maintenance activities still do not align to groupings or work blocks, they may be forecast and carried out as individual activities. They are known as out-of-phase operations.
5 Scheduled maintenance latitudes
The 4PT3 is responsible for the overall management of scheduled maintenance and is to detail arrangements for the management of scheduled maintenance in the Topic 2(N/A/R)1 or the Topic 5 series. In order to meet engineering or operational requirements, scheduled maintenance may be anticipated or deferred within any upper calendar or usage hour limit for that maintenance. In addition, the upper limit may be extended and the scheduled maintenance carried out or deferred within the revised upper limit. When scheduled maintenance is anticipated, due consideration should be given to the increased use of resources, both in terms of manpower and spares. Deferment of scheduled maintenance is not to jeopardise the airworthiness of the aircraft, or contravene other component lifing policies.
The anticipation and deferment latitudes are set out in paragraph 6 and paragraph 7, together with the procedure for anticipating or deferring scheduled maintenance.
The 4PT3 and, in some instances, the FLC may amend the scheduled maintenance latitudes given in the following paragraphs. All agreed latitudes are to be promulgated in the SPS. In addition, individual operations may be annotated with the agreed latitudes in the Topic 5A1.
6 Anticipation of scheduled maintenance
A4 PT3 may authorize anticipation of scheduled maintenance by any period. A person holding authorization JAP-F351 may anticipate scheduled maintenance by any amount of the relevant periodicity unless otherwise constrained by a4PT3, FLC or Stn/Ship/Unit.
7 Deferment of scheduled maintenance
To meet engineering requirements or flying commitments, scheduled maintenance may be deferred in accordance with paragraph 7.1 and paragraph 7.2. 4PTs3 may limit or extend the maximum deferral of individual, groups of, or all scheduled maintenance activities. FLCs may further constrain the use of deferments by their units or individuals, but only within any limits set by the 4PT3. Special Instructions (Technical) (SI(T)) are not to be considered as scheduled maintenance activities when applying latitudes. Any permitted latitudes for SI(T) will be promulgated as part of the instruction in accordance with Chapter 10.5.4 and Chapter 10.5.5.
7.1 Deferment of scheduled maintenance at Stn/Ship/Units
Unless otherwise limited by an 4PT3 or FLC in accordance with paragraph 7 above, or extended by the 4PT3, an authority level H may defer any scheduled maintenance activity by up to 25% of its periodicity. However, the following constraints apply:
1 The scheduled maintenance activity is to be one for which he is ultimately responsible. If the maintenance activity is one for which he is not ultimately responsible, the agreement of the responsible authority level K, FLC and/or 4PT3 is to be obtained.
2 Any upper usage hour or calendar limit for the scheduled maintenance being deferred is not to be exceeded.
3 Any more frequent scheduled maintenance is either deferred in its own right or carried out as a penalty scheduled maintenance.
4 Due to the complex lifing of some survival equipment and aircrew equipment assemblies, extensions to scheduled maintenance may only be authorized by an authority level K to a maximum of 28 days, with due regard to the details in DAP 108A-0006-2(N/A/R)1.
7.2 Deferment of scheduled maintenance by 4PTs3
The EA in 4a PT 3may grant extensions to any scheduled maintenance or calendar or usage upper limit. However, 4PTs3 are to record in the relevant Airworthiness Management Group (AMG) report:
1 All extensions exceeding 35% of the scheduled maintenance frequency.
2 The percentage of the fleet with the lowest frequency maintenance activity extended by more than 25%.
4PTs3 are to maintain an auditable record of the decision process for all extensions exceeding 25%.
Requests for extension should normally be made by signal. If alternative methods are used, they are to have an auditable trace. Requests for extensions to components are to be submitted to the relevant platform 4PT3 who is to liaise with the appropriate component authority in assessing the Safety Case.
7.3.1 Content of request/approval
A request for an extension is to include the following:
1 Aircraft/component type and Serial Number (CLR reference if applicable).
2 Current airframe hours or component life consumed.
3 Type of maintenance or life due, any extensions already granted and the amount of extension required.
4 Reason for request.
5 Airframe hours or date when maintenance is due or component life expires and, for explosive stores, the storage life and installed life expiry dates.
6 Airframe hours or date when maintenance will be due or component life will expire, if extension granted.
7 Planned date of maintenance or component change.
8 For explosive stores only: Date of Manufacture, Lot Number.
The reply to a request for an extension is to be titled EXTENSION REQUEST APPROVAL/REFUSAL, as appropriate, and is to be tabulated as follows:
9 Aircraft/component type and Serial Number.
10 Originator’s Signal Reference and DTG.
11 Type of maintenance or life.
12 Percentage/amount of extension granted.
13 When event is due with extension.
14 Additional information.
Note:
Additional information may include supplementary maintenance requirements, MOD Form 760 requirements, disposal instructions, etc.
A decision to anticipate or defer scheduled maintenance is to be recorded on the appropriate MOD Form 700 or equivalent IS documentation raised for the scheduled maintenance. The entry is to state the following:
1 The revised timing for the scheduled maintenance.
2 That the condition of the aircraft or equipment justifies the deferment.
3 That where applicable, the authority level K, FLC and/or 4PT3 responsible for carrying out the maintenance have agreed the deferment.
The revised timing for the aircraft scheduled maintenance is also to be recorded in the MOD Form 700, in the appropriate forecast log and maintenance record. In the case of deferments, the entry in the forecast log is to be made using red ink. If applicable, IS-based recording is to be amended in accordance with the appropriate procedures.
Under certain conditions it may be justified to temporarily suspend maintenance whilst not formally placing the aircraft in storage. The procedures for preparing an aircraft for a period of non-flying are to be agreed with the FLC and promulgated by the 4PT3 in the Topic 2(N/A/R)1. Such procedures are to include items that must be carried out at the allotted time and those calendar activities that are to be carried out prior to the aircraft or equipment re-entering service.
8 Maintenance in periods of abnormal usage
The 4PT3 is to specify in the appropriate technical information any additional preventive maintenance required to maintain the condition of aircraft or equipment being operated under adverse conditions, at below-average utilization rates, or which are not flown for extended periods. Examples might include:
1 Cold and hot weather operations. Further details are at Chapter 5.5.
2 Operations in saline environments.
3 Operations in high humidity climates.
4 Operations in continuous high ultra violet conditions.
5 Operations in desert or dusty environments.
6 Embarked operations.
7 Aircraft storage in accordance with JAP 100V-21.
8.1 Aircraft not flown for extended periods
Unless the procedures detailed in JAP 100V-21 have been applied, the condition of aircraft not flown for 28 days is to be assessed to determine the anti-deterioration measures to be taken. A re-assessment is to be carried out each subsequent 28 days.
In addition to the anti-deterioration measures published in the platform technical information, the following non-exhaustive list of general principles should also be considered when carrying out the assessment:
1 Fill all systems to the levels detailed in the aircraft technical information. Note that for vented systems, sufficient air space must be left to allow for any expansion of the fluid caused by variations in ambient temperature.
2 Drain and remove drop tanks and ferry tanks from the aircraft.
3 Depressurize those systems pressurized by pumps, accumulators, etc.
4 Raise aircraft on jacks or trestles to reduce the stress on undercarriage components and fluids.
5 Test fuels and lubricants as detailed in JSP 317.
6 Remove those items that require calendar-based maintenance (eg batteries, first aid kits and explosive stores).
7 Apply short-term reserve standards as detailed in JAP 100V-21.
9 Contingency maintenance (CTM)
In operational or combat conditions it may be appropriate to tolerate additional risk and improve aircraft availability by a combination of:
1 Stopping certain maintenance activities.
2 Decreasing the frequency of scheduled maintenance activities.
3 Allowing greater latitude in the deferment of activities.
The 4PT3 is to annotate those items of preventive maintenance in the Master Maintenance List (MML) that require to be included in CTM and is to task its Schedule Management Agency (SMA) with producing the necessary work procedures.
FLCs and 4PTs3 are to determine the procedure by which CTM may be implemented and terminated, either by the 4PT3 or an FLC, or by an authority level J holder with reference to the FLC and 4PT3. The FLC and 4PT3 are to publish their arrangements for CTM in the Topic 2(NAR)1, with a summary in the SPS. When formulating the policy for a platform, the following points should be considered by the 4PT3:
1 The procedure for initiating and terminating CTM, in association with the FLC.
2 The requirements to carry out or suspend SI(T).
3 The actions to be taken on modifications, including Service Modifications.
4 The procedure for reverting from CTM back to the normal scheduled maintenance cycle.
5 Any changes required in the documentation procedures during CTM.
6 The policy for component maintenance during CTM.
When considering the Implementation of CTM, FLCs should consider the following:
7 During periods of tension or build-up, every effort should be made to maximise the opportunities for scheduled maintenance and component changes, if necessary by anticipating scheduled maintenance activities.
8 The imposition of CTM will place additional emphasis on the activities carried out during flight servicing; therefore, personnel are to apply extra vigilance during these tasks.
Zonal surveys are a product of RCM analysis and are a general visual examination of a specified zone, carried out before and after cleaning, to detect damage, deterioration and discrepancies and assess the general condition of the zone. They are an integral and important element of scheduled maintenance that must be carried out in order to preserve aircraft by identifying and recording all visible faults and detecting any dormant faults that may pose a threat through environmental and accidental damage to aircraft structural and systems integrity. Zones may contain structure, systems, equipment and components that are already subject to individual directed maintenance tasks identified elsewhere in the maintenance schedule.
Zonal surveys look for faults, multiple faults and the interaction of faults. A single fault may be constant seepage: a small fluid leak that may go unnoticed between replenishments, although the cumulative contamination damage induced in adjacent equipment, wiring or structure may be substantial. Similarly, the failure of one cable loom support clip may seem inconsequential; but a multiple failure would occur if several clips fail over a period of time and the loom detached and obstructed a control mechanism, with potentially serious consequences.
Certain failures can be attributed to the combination and interaction of separate system faults and failure modes, such as chafed cable looms, hot gas leaks or damaged/poorly maintained ducts/pipes coming into contact with flammable vapours from leaking pressurized systems. 4JAP(D) 100C-22 3describes the principles for developing zonal surveys.
10.1 Training and authority levels
FLCs/4PTs3 responsible for on-aircraft maintenance are to ensure that their personnel are trained and authorized as necessary to act as zonal survey instructors and zonal surveyors on Stns/Ships/Units.
A current study is reviewing the principles of zonal maintenance training, including zonal survey instructor training, across the MAE. Until the study concludes, the authority levels and training requirements for zonal surveys are as follows:
1 Personnel required to act as Unit Zonal Survey Training Instructors within Air Cmd and JHC are to have completed the revised Zonal Survey Instructor training course and hold authorization JAP-C75.
2 Personnel required to carry out self-supervised zonal surveys are to have completed zonal survey training at their Stn/Ship/Unit and hold authorization JAP-D73.
3 Personnel required to carry out supervised zonal surveys are to have completed zonal survey training at their Stn/Ship/Unit and hold authorization JAP-B74.
Notes:
1 The revised Zonal Survey Instructors training course is sponsored by 4Military Aviation Authority Technical Certification Aircraft Structural Integrity (MAA Tech Cert ASI)3 through MAEI-1 RCM 1 at RAF Wyton. Pending the outcome of the current study, courses are being delivered by DCAE Cosford instructors at selected Air Cmd and JHC units. RAF attendees will be awarded the TQA Q Zonal Fam.
2 For RN personnel, the zonal survey training course (TEM 979) is delivered by specialist instructors at 4RNAS Culdrose and3RNAS Yeovilton followed by aircraft-type specific ‘hot spot’ briefs at the TAS.
The procedure for aircraft condition surveys is to be determined and promulgated by the 4PT3.
12 Condition-based maintenance
Where adequate and realistic condition-monitoring techniques, eg HUMS, are available for a particular item, the 4PT 3is to apply condition-based maintenance to the item in preference to scheduled visual examinations, manual testing and routine component replacement.
This chapter refers to the following publications:
1 JAP(D) 100C-22 – Guide to Developing and Sustaining Preventive Maintenance Programmes.
2 JAP 100V-21 – Aircraft Storage.
3 AP 108 Series 0001-5F(N)s – Survival Equipment Bay Maintenance Schedules.
4 DAP 108A-0006-2(N/A/R)1 – Survival Equipment Support Authority General Orders and Special Instructions.
5 Topic 2(N/A/R)1 – General Orders and Special Instructions.
6 Topic 5A1 – Master Maintenance Schedule.
7 Topic 5W – Summary of Component Repair Plans.