Chapter 5.1.1
Maintenance Policy - Composite Materials
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Table of contents Paragraph |
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1.2 Associated instructions/publications
2 Policy on maintenance of composite materials (CM)
3.2 Factors affecting CM durability
3.4 Inspection and damage assessment
3.5 Incident/accident management
3.8 Health, safety and environmental factors
5.1 Authority to maintain FRP structure and components
5.2 Authorizations to repair and supervise FRP repair
6.1 Structural occurrence recording
6.2 Damage and repair recording
7.2 Equipment sponsor responsibilities
7.3 Assessment, categorization and repair responsibilities
7.4 Front Line Command (FLC) responsibilities
7.5 4Project Team (PT)3 responsibilities
7.6 Stn/Ship/Unit responsibilities
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List of tables
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Table 1. Authorizations to Repair and Supervise/Stage-check FRP Repair.
The use of Composite Materials (CM) within the Military Air Environment (MAE) is continuing to grow because they offer high specific strength; that is, a high strength to weight ratio, high specific stiffness and excellent fatigue resistance, combined with increased design flexibility when compared with traditional aerospace alloys. This chapter details the policy for maintenance and repair of aircraft structure and components utilizing CM employed within the MAE.
General maintenance policy is defined in Chapter 5.1. However, for CM there are aspects that may require more care, additional husbandry or different processes as compared to those employed in the maintenance of aircraft structure and components constructed from more traditional materials. This chapter sets out the policy required to ensure that aircraft utilizing CM in their construction are retained in a serviceable condition or restored to serviceability in the most cost-effective manner. Further detailed descriptions of the processes and infrastructure requirements can be found within AP 101A-0601-1 – Employment and Repair of Aircraft Composite Materials.
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1.2 Associated instructions/publications
This chapter is associated with the following instructions/publications:
1 Def Stan 00-970 – Part 1 - Section 4.
2 AP 100C-22 – Procedures for Developing Preventative Maintenance.
3 AP 100N-0150 – Aircraft and Equipment Support Procedures for the 4Fleet Air Arm (FAA).3
4 AP 100T-0100 – Qualifications for FAA Personnel.
5 AP 3376 – Trade Structure of the Royal Air Force 4(RAF)3.
6 AP 3379 – Manual of Royal Air Force Training.
7 Compendium of Army Aviation Training Policies.
8 4Acquisition Operating Framework.3
This chapter is applicable to the Military Air Environment (MAE).
2 Policy on maintenance of composite materials (CM)
In order that the design properties of CM are retained or recovered in a cost-effective and efficient manner throughout the service life of the aircraft, aircraft CM structures and components are to be maintained and repaired in accordance with the policy and guidance set out in this chapter, with reference to the relevant Aircraft Document Set (ADS).
When used in aerospace applications, the term ‘composite structure’ generally refers to structure manufactured from a very broad band of material types, eg metal alloys, metallic honeycomb sandwich structure and fibre reinforced non-metallic matrix systems. When using the term CM, this chapter is concerned with both monolithic and sandwich structures manufactured from fibre reinforced non-metallic matrix systems, eg Glass Reinforced Plastic (GRP), Carbon (graphite) Fibre Reinforced Plastic (CFRP), Boron fibre reinforced plastic, Aramid (Kevlar ®) reinforced plastic or a hybrid. These materials are generically termed Fibre Reinforced Plastics (FRP). Chapter 5.1 covers the maintenance of metallic-based composite structures.
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3.2 Factors affecting CM durability
When manufactured without flaws and maintained in a pristine condition, components and structure manufactured from FRP have a far superior resistance to fatigue than comparative metal components and structure. However, even low levels of impact energy (eg dropped tools) can lead to fibre damage and delamination that in turn reduces compressive strength and can lead to further cyclic damage growth if left unrepaired.
This phenomenon is due in part to the laminated construction providing little in the way of through-thickness strength; FRP panel edges and fastener holes are extremely vulnerable to impact and crush damage. Following a high-energy impact, the surface is likely to exhibit surface evidence of damage; however, lower energy impacts could result in sub-surface damage that would impair the structural integrity of the item without leaving any external sign of the impact having occurred.
Through either extreme environments or adjacent systems, the strength of FRP can be adversely affected by excess heat and moisture ingress. FRP can also suffer degradation by fuels, oils, lubricants and UV light if left unprotected. If the superior fatigue performance of FRPs is to be maintained throughout the service life of the aircraft structure and components, it is essential to ensure protection from accidental and environmental damage as far as possible.
To ensure the continued structural integrity of aircraft structure and components constructed using FRP, and to reduce maintenance costs, 4Project Teams (PT)3s, FLCs and user units are to put in place procedures to establish and maintain appropriate levels of awareness and husbandry based on the following:
1 When carrying out maintenance on or beside components manufactured from FRP, ensure appropriate precautions are taken to prevent damage to the surface coating and underlying FRP.
2 Panel edges are extremely susceptible to damage. Store FRP components in special-to-type containers (STCs), protective packaging or racking when not fitted to the aircraft in such a manner that prevents damage through contact with other components or equipment.
3 FRP is extremely susceptible to crush damage; therefore rigidly adhere to correct torque settings for fasteners.
4 Due to UV ageing, if damage to the surface coating of FRP structure occurs between the inspection periodicities mandated in the ADS, it should be re-protected as soon as reasonably practicable.
5 Clean chemical/fuel/oil contamination off FRP with ADS-prescribed cleaning agents as soon as practicable and inspect the area for signs of leeching through the surface coating.
6 When chemically stripping the surface finish from metallic structure adjacent to FRP structure, protect the FRP to ensure that it is not contaminated.
7 Keep stored FRP components, whether serviceable or awaiting repair/refurbishment, covered and in a dry and non-humid atmosphere as far as reasonably practicable.
8 Take special care when handling damaged FRP components so as not to cause injury to personnel (eg fibre splinters).
9 Encourage all engineering and non-engineering personnel to report any occurrences or signs of damage to FRP, no matter how insignificant the damage may appear.
3.4 Inspection and damage assessment
The inspection routines for FRP and metallic structure and components may be similar; however, the damage that occurs is significantly different. The type and thickness of the structure will have a bearing on how it reacts to differing impact energies and, as relatively low levels of energy could result in considerable damage if left unrepaired, particular care should be paid when visually inspecting components. For example, a light source used at oblique angles can reveal slight depressions, known as Barely Visible Impact Damage (BVID), which would otherwise go unnoticed.
When investigating an accident or incident involving aircraft with FRP structure, the 4PT3, 4Design Organization (DO)3 and MOD SMEs may need to be consulted. Consideration must be given to the possible extent of unseen damage to FRP following an incident. For example, on CFRP, lightning strikes will result in localized burning of the matrix at both the entry and exit points, and possibly other points in between, with a subsequent reduction in residual strength. Heat transference through metallic structure to FRP may also have a detrimental effect on the adhesive bonds.
The ADS will provide guidance on the manner and methods of inspection within Forward and Depth maintenance organizations, including typical examples of the various types of damage. However, if there is any suspicion that damage is present, specialist NDT advice should be requested in accordance with Chapter 11.5.
3.5 Incident/accident management
Local orders are to include procedures for health, safety and environmental protection from toxic fumes or airborne particles that may be released during an FRP incident/accident involving fire.
Repairs involving FRPs are to be carried out within the following framework:
1 Repairs are to be carried out using 4DO3-approved materials and procedures as detailed in the ADS. Permanent and temporary repairs may be carried out at both Forward and Depth as detailed in the associated aircraft or equipment Support Policy Statement (SPS), provided that the repair process controls can be implemented.
2 Suitable facilities and equipment are to be provided that ensure the appropriate environmental conditions for repair.
3 Where structural damage is beyond the unit capability to categorize or repair, the appropriate 4Repair Organization3 is to be contacted for advice in accordance with Chapter 9.13.1.
4 Stage checks are to be carried out at relevant points to ensure adherence to the process. All work is to be recorded on maintenance documentation in accordance with Chapter 7.2 by suitably qualified and authorized personnel.
5 If time, resources and the overriding need to meet operational requirements dictate, it may be necessary to undertake FRP repairs that are outside the limits detailed in the ADS. These repairs are termed Expedient Repairs (ER) and policy relating to the application of ER is contained in Chapter 9.12.
6 Technological advances with respect to FRP materials, equipment and testing techniques may have an impact on an aircraft’s maintenance policy. 4PTs3 are to review the techniques and facilities applicable to their respective platform where advised by the 4DO3, FLCs or MOD SMEs.
Unlike metallics, once a repair involving FRP has been completed, there is no NDT method currently available to check the durability of the bond between the repair and the substrate. Therefore, durability and bond integrity is to be maintained by strict adherence to the repair process laid down in the relevant ADS and by taking account of the following factors:
1 Repair materials management. All materials have management requirements and these are strictest for pre-mixed resins, adhesive films, pre-impregnated fibres and associated compounds, which often require ‘frost free’ storage at below -18ºC to maintain a usable shelf life. Moreover, prior to use, frozen repair materials must remain sealed for up to 24hrs on removal from chilled storage to reduce moisture absorption during thaw. Adherence to specified storage instructions is to be ensured throughout the supply chain.
2 Environment management. Excessive temperatures and high relative humidity levels will adversely affect cure quality. Dust and contamination will also reduce adhesive performance. Specialist, dedicated, repair facilities, tooling and equipment are required. AP 101A-0601-1 contains a guide to the technical specification for an FRP repair facility.
3 Repair technician skill. The major cause of bond failure is inadequate pre-bonding surface preparation. Specialist training, regular practice and periodic continuation training are required to maintain the technician’s knowledge, skill of hand, attention to detail and competence in the repair process.
4 Material Selection. The repair scheme detailed in the ADS will specify the materials, and any alternatives, to be used. Similar but unqualified adhesives are not to be used, as they may not provide the bond performance required.
If any part of a bonded repair is carried out outside of the detailed process parameters, the strength and durability of that repair cannot be guaranteed. All deviations shall be recorded.
Note:
In order to provide an additional level of confidence in bond integrity, the approved repair scheme may specify that a test piece be manufactured and destructively tested. The test piece is to be manufactured using the same materials and processes at the same time as the repair and cured in tandem to provide representative strength data. A test piece may also be used to provide limited design data that can be used to support a non-standard repair scheme.
3.8 Health, safety and environmental factors
FRP material dust is a health hazard, overheating repair materials could give rise to toxic fumes and/or substances and some associated chemicals and compounds are toxic. However, the risk can be managed by providing appropriate personal protective equipment (PPE), process control and suitable ventilation and dust extraction systems in accordance with the material data sheets. Full risk and COSHH assessments are to be carried out in the work area and full attention paid to the associated regulations and guidance, which include:
1 Health and Safety at Work Act 1974.
2 Classification, Packaging and Labelling of Dangerous Substances Regulations 1984.
3 Control of Substances Hazardous to Health (COSHH) 1994.
4 AP 101A-0601-1, Part 2 Section 1 Chapter 423, Safety Precautions.
Carbon and boron fibrous dust particles are electrically conductive and can cause short-circuiting in electrical, avionic and computer circuitry. Consequently, all electrical equipment, including sockets, panels and fuses, is to be protected from composite dust and debris.
The repair of FRP is a special process within the overall quality assurance regime, requiring the creation of extra controls to provide a quality product. All Stns/Ships/Units maintaining and repairing FRP structure and components are to ensure that their QA regime takes into account Paragraphs 3.6 and 3.7 above.
Disposal advice is to be sought through the appropriate authority and publications. Discarded composite material and uncured resins or adhesives are to be disposed of as Dangerous Engineering Substances. FRP material is NOT to be disposed of by incineration.
All FRP training development and provision is to comply with Chapter 4.1 and encompass:
1 Introduction to FRP. For all engineering officers and technicians, a basic FRP theory module is to be provided at Phase 2 Training, with the addition of a practical module for mechanical tradesmen. This level of training defines the differences between metal and FRP structure with respect to the associated health and safety issues, component handling, damage mechanisms and damage assessment.
2 FRP awareness training. AnFRP awareness brief is to be provided at Unit level to all engineering and support personnel who regularly come into contact with FRP structure or components. Authority for personnel to work on or in support of aircraft containing structurally significant FRP structure is to be managed locally. This training will be presented as a generic package with additions of particular relevance to the aircraft operated and should be sufficient to maintain the health and safety of personnel, in addition to minimizing the occurrence of accidental damage to FRP structure and components.
3 Specific-to type maintenance training. Aircraft 4PTs3 with provisioned specific-to-type maintenance training are to ensure that it encompasses the FRP awareness module at item 2 above.
4 Generic FRP repair training. Tri-Service generic aircraft FRP repair training is provided on course STC2043 at the Harrier Maintenance School, RAF Wittering. Royal Navy-specific aircraft 4Glass Reinforced Plastic (GRP)3 repair training is provided 43at HMS Sultan.
5 Specific-to-type FRP repair training. For platforms with a specific or unique damage assessment, structural monitoring or FRP repair requirement, the aircraft 4PT3 is to develop further specific-to-type training, including continuation training if required, in consultation with the aircraft 4DO3 and MOD SMEs.
All engineering tradesmen required to undertake maintenance tasks on aircraft FRP structure and components are to have completed mechanical trade Phase 2 training, be a minimum of authority level B and have completed FRP awareness training or, if provisioned, specific-to-type maintenance training.
The award of Q-A-FRP (RAF only) post-course number STC2043,43 is to be recorded on the repair tradesman’s personal management documentation in accordance with Chapter 4.3.1 and lifed at 3 years. On expiry, an authority level J is to conduct a review of the tradesman’s level of direct contact with FRP repair processes, taking note of the factors detailed in paragraph 3.7, and re-authorize or withdraw the qualification as necessary.
The completion of additional specific-to-type FRP repair training attracts the same recording and management requirement as generic FRP repair training.
To attain and maintain the capability to repair FRP structure and components, all engineering tradesmen required to undertake FRP repair tasks are to be qualified and authorized as 4shown in table 1. The level of repair for which the authorization is applicable is detailed below:3
1 Type 1. Type 1 repairs include manual resin application to dry fibres with an ambient temperature cure on tertiary components in accordance with the ADS.43
2 Type 2. Type 2 repairs include manual resin application, or use of pre-impregnated fibres, which may require a controlled elevated temperature cure, in accordance with the ADS on secondary structure. 43
3 Type 3. Type 3 repairs are as type 2, but involve novel repairs to primary structure or more complicated processes. 43
4 FRP repair supervisor/stage checker. Personnel who are required to either supervise FRP repairs or carry out FRP repair process stage checks are to 4hold authorization level JAP-C4623 and have completed specific-to-type maintenance training and generic FRP repair training.
Note:
For FW and RW 4ROSUs3, the specialist repair teams (having completed generic FRP repair training) may be locally authorized to complete all types of repair. The authorizing level J is to be satisfied that specific-to-type training is carried out where necessary.
All other engineering and support personnel who regularly come into contact with FRP structure or components during the course of operations are to complete FRP awareness training. Local management is to define which personnel require this awareness training, taking account of the operational and logistic support requirements for that aircraft.
5.1 Authority to maintain FRP structure and components
Authority level B tradesmen that have successfully completed mechanical trade Phase 2 training and FRP awareness training, or specific-to-type maintenance training, may carry out maintenance activities on aircraft comprising significant FRP structure and/or components without requiring additional authorization.
5.2 Authorizations to repair and supervise FRP repair
An FRP repair requires additional quality control procedures to ensure a durable repair. However, the act of repair can be a self-supervised activity. Tradesmen required to carry out, supervise, stage check or authorize repairs to FRP structure are to hold the minimum authority levels detailed in Table 1:
Table 1. Authorizations to Repair and Supervise/Stage-check FRP Repair.
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Task |
Authority Level/Training Requirements |
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Type 1 FRP Repair |
Mechanical tradesman holding 4authorization JAP-B1293 and who has completed mechanical trade Phase 2 training, FRP awareness training or specific-to-type repair training (if provisioned). |
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Type 2 FRP Repair |
Mechanical tradesman to be as 4Type 1, holding authorization JAP-B4603 and who has completed generic FRP repair training and specific-to-type repair training (if provisioned). |
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Type 3 FRP Repair |
Mechanical tradesman to be 4as Type 2, holding authorization JAP-B4603 and who has completed generic FRP repair training, specific-to-type repair training (if provisioned, 4have3 gained relevant experience 4and for repairs that may affect airworthiness, be authorized by an authority level Jfollowing consideration of the factors detailed within this Chapter.3 |
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Supervise/Stage-check FRP Repair |
Mechanical tradesman holding 4authorization JAP-C4623 and who has completed generic FRP repair training and specific-to-type maintenance training. |
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4RO3 mechanical tradesman holding 4authorization JAP-B4603 and who has completed generic FRP repair training and gained relevant experience. 4On completion of a suitable syllabus of training Tradesmen will be granted JAP Auth B463 (Act as RO FRP Repair specialist)
RO Personnel who are required to either supervise FRP repairs or carry out FRP repair process stage checks are to have gained both B460 and B463, they will be awarded JAP Auth C462 (FRP Repair Supervisor/Stage Checker).3 |
Certain operations may require non-mechanical tradesmen to carry out maintenance activities on aircraft with significant FRP structure and/or components. The control of this requirement is to be managed locally, but, at a minimum, is to include the provision of FRP awareness training or specific-to-type maintenance training.
6.1 Structural occurrence recording
The operational life of FRP structure is directly related to the amount of accidental damage and environmental damage (AD/ED) sustained in service. If damage and the subsequent actions are not recorded, future residual strength calculations may be inaccurate or impossible without destructive structural sampling. Furthermore, once an FRP repair is completed, it may be extremely difficult to ascertain its exact location and subsequent repairs may 4conflict with3 previous repairs. This situation could cause additional stress within the structure and decrease the structure’s fatigue and static strength performance. In accordance with JSP 553 Annex K and 4Chapter 16.13, a database is to be in place to record all structural concessions, repairs, modifications and AD/ED. Changes to the configuration of FRP structure and components are also to be included in this record.
6.2 Damage and repair recording
In order to provide the ability to analyse maintenance trends for FRP structure and components, the location, type of damage and repair are to be recorded, either in accordance with AP 100C-02 Annex J on MOD Form 707B - Area 2 – Additional Information Block (AIB), or an equivalent electronic maintenance/repair documentation system. The details entered are to be a true reflection of the damage incurred and the restorative actions; therefore, only a Type 2 FRP repair-authorized tradesman experienced on the platform concerned is to complete the record.
In order to maintain the airworthiness audit trail, the repair process control actions are also to be documented on the maintenance/repair recording system, including:
1 Details of the repair scheme being utilized.
2 Repair material details, batch numbers and expiry dates.
3 Stage checks carried out post damage removal, repair patch lay-up, pre-bonding surface preparation and elevated temperature cure profile (if applicable).
4 Details of any NDT of the repair and test results, including for test pieces (if specified).
5 Confirmation that the repair details have been added to the Structural Occurrence Database.
6 Any deviations from the documented repair process.
7.1 4Military Aviation Authority Technical Certification Aircraft Structural Integrity (MAA Tech Cert ASI)3
4MAA Tech Cert ASI3 is responsible for:
1 Generic FRP maintenance policy for the MAE in consultation with other MOD SMEs and appropriate Industry SMEs.
2 Technical sponsorship of the generic FRP technical publication AP 101A-0601-1.
3 Technical sponsorship of the generic FRP repair course STC2043.
7.2 Equipment sponsor responsibilities
The Equipment Capability Customers (ECCs), or 4Integrated Logistic Support3 (ILS) Managers where they exist, are responsible for producing the initial SPS for their aircraft or equipment, including appropriate policy for maintaining FRP structure and components.
7.3 Assessment, categorization and repair responsibilities
The policy for repairs to FRP structure and components outside the capabilities or resources of the Ship/Stn/Unit is detailed in Chapter 9.13.1.
7.4 Front Line Command (FLC) responsibilities
FLCs, including trade sponsors where appropriate, are responsible for ensuring that:
1 Units with an FRP maintenance and repair requirement are provided with specialist facilities and equipment.
2 Specialist equipment and materiel stocks are adequately maintained to ensure continued serviceability. The 4Aircraft Commodities PT (AC PT)3 and 4GS PT ?3 provide and support generic FRP repair equipment.
3 Units with a requirement are capable of maintaining and repairing FRP structure and components and that the capability is monitored by a QA organization.
4 Generic aircraft FRP awareness and repair training courses are developed and maintained for relevant engineering and support personnel.
7.5 4Project Team (PT)3 responsibilities
4PTs3 are responsible for ensuring that:
1 Maintenance and repair of FRP structure and components is detailed in the SPS and this policy is reviewed and updated as required.
2 Specific-to-type new equipments that do not require a Generic Aircraft Release Process (GARP) or Military Aircraft Release are sponsored in consultation with the ECC.
3 FRP damage assessment and repair techniques are developed for inclusion in the ADS as required.
4 The content of FRP repair equipment and tool kits is developed for their aircraft type, in conjunction with MOD SMEs, the 4AC PT, the GS PT ?3 and relevant 4RO.3
5 Specific-to-type FRP training, including continuation training programme if appropriate, is developed for maintenance and repair technicians employed at aircraft main support units and in deployed teams.
6 Arrangements are in place for 4PT-controlled3 Depth organizations to meet the FRP maintenance and repair requirements of this Chapter.
7 Other 4PTs3 are advised of any repair techniques that may have wider applicability.
7.6 Stn/Ship/Unit responsibilities
Stns/Ships/Units are responsible for:
1 Where required to provide aircraft FRP repair support, establishing an FRP repair continuation training management system for tradesmen.
2 Providing generic aircraft FRP awareness training to engineering and support personnel.
3 Defining which support personnel are to receive FRP awareness training, in accordance with operational procedures.
This chapter refers to the following instructions/publications:
1 JSP 553 – Military Airworthiness Requirements (via mards homepage).
2 AP 100C-02/03 – Maintenance Documentation.
3 AP 101A-0601-1 – Employment and Repair of Aircraft Composite Materials.
4 Health and Safety at Work Act 1974.
5 Classification, Packaging and Labelling of Dangerous Substances Regulations 1984.
6 Control of Substances Hazardous to Health (COSHH) 1994.