Module 7A: Maintenance Practices

1. Module Overview

Module 7A is a foundational module within the EASA Part-66 B1.2 licence syllabus, focusing on the practical application of maintenance knowledge. It bridges theoretical understanding with the hands-on skills and procedures required for the safe, efficient, and compliant maintenance of aircraft. This module is not about learning to fly or design aircraft; it is about learning how to keep them in an airworthy condition.

The module covers a broad spectrum of topics, from fundamental safety precautions and workshop practices to detailed procedures for inspecting, repairing, and testing aircraft systems and structures. A recurring theme is the absolute necessity of following approved data, primarily the Aircraft Maintenance Manual (AMM) and other manufacturer-approved documentation. The module is designed to instil a disciplined, methodical, and safety-first approach to all maintenance tasks.

Key knowledge areas within Module 7A include:

Safety Precautions: Safe working practices, fire safety, handling of hazardous materials, and the use of personal protective equipment (PPE).
Workshop Practices: Use of precision measuring tools, torque procedures, and the correct handling of tools and components.
Standard Maintenance Procedures: Inspection techniques, defect reporting, and the use of approved documentation.
Component Maintenance: Handling, inspection, and repair of specific components like fasteners, fluid lines, hoses, bearings, and control cables.
System-Specific Practices: Procedures for ground handling, jacking, engine handling, and functional testing of various aircraft systems.

2. Key Concepts and Detailed Explanations

This section synthesises the core knowledge from the source questions into a structured, textbook-style format.

2.1 The Primacy of Approved Data

The single most important concept in aircraft maintenance is the use of approved data. This is the foundation upon which all safe and legal maintenance is performed.

Definition: Approved data is any instruction or information that has been accepted by the regulatory authority (e.g., EASA) as the basis for performing maintenance. This includes:
Aircraft Maintenance Manual (AMM): The primary source for all scheduled and unscheduled maintenance procedures, including inspection limits, repair schemes, rigging data, and lubrication schedules.
Component Maintenance Manual (CMM): Provides detailed overhaul, repair, and testing procedures for specific components (e.g., a hydraulic pump, a generator).
Service Bulletins (SBs) and Service Instructions (SIs): Issued by the manufacturer to introduce improvements, modifications, or new inspection requirements.
Airworthiness Directives (ADs): Legally enforceable rules issued by the regulatory authority to correct an unsafe condition.
Airworthiness Limitations Section (ALS): A section of the maintenance manual or Type Certificate Data Sheet (TCDS) that contains mandatory life limits and inspection requirements. Airworthiness Limitation Items (ALIs) are mandatory and must be complied with for continued airworthiness.
Application: When a defect is found, the maintenance engineer must consult the AMM to determine:

Is the damage within allowable limits? (e.g., a dent, a crack, a chafed hose).
If so, is a repair required? (e.g., stop-drilling a crack within limits).
If not, what is the approved repair or replacement procedure?

Example: A crack in an engine cowling is found. The AMM provides a repair scheme for cracks up to 25 mm. A 15 mm crack is within this limit and can be repaired per the AMM. A 150 mm crack exceeds the limit and requires a different, more complex approved repair or replacement of the cowling. A 2 mm dent in a wing leading edge is within the AMM's 3 mm damage limit, so it is acceptable and requires no repair, only documentation.

2.2 Precision Measurement and Inspection

Accurate measurement is critical for determining the serviceability of components.

Precision Measuring Tools:
Micrometer: Used for high-accuracy measurements of external dimensions (e.g., piston pin diameter). The correct procedure involves using the ratchet to apply consistent measuring force and taking multiple measurements at different points to check for out-of-roundness and taper.
Dial Test Indicator (DTI): Used to measure runout, deflection, and alignment. Total Indicated Runout (TIR) is the total movement of the needle during one full revolution of the component.
Vernier Caliper: Used for internal, external, and depth measurements with moderate accuracy.
Interpreting Tolerances: AMM specifications often include a tolerance range. For example, a piston pin diameter may be specified as 25.40 mm +0.03/-0.01 mm. This means the acceptable range is 25.39 mm to 25.43 mm. A measurement of 25.42 mm is within tolerance. Similarly, a torque specification of 25 Nm ±2 Nm means any value between 23 Nm and 27 Nm is acceptable.
Inspection Techniques:
Visual Inspection: The most common method, but limited to surface defects. For control cables, this includes wiping the cable to detect broken wires, discolouration, or pitting that may indicate internal corrosion.
Dye Penetrant Inspection (DPI): A non-destructive testing (NDT) method used to detect surface-breaking cracks in non-porous materials, including non-ferrous metals like aluminium and magnesium alloys. It is the standard method for inspecting aluminium cylinder heads and propeller blades for fine cracks.
Magnetic Particle Inspection (MPI): An NDT method used to detect surface and near-surface cracks in ferromagnetic materials (e.g., steel).
Fluorescent Penetrant Inspection (FPI): A more sensitive version of DPI that uses a fluorescent dye, visible under ultraviolet (UV) light. It is used to detect very fine surface-breaking cracks.

2.3 Fasteners and Locking Devices

The integrity of mechanical joints relies on the correct selection, installation, and locking of fasteners.

Bolts and Nuts:
Material and Finish: Fasteners must be replaced with identical parts as specified in the maintenance data. Substituting material (e.g., using a steel bolt where a cadmium-plated bolt is specified) or finish can affect strength and corrosion protection.
Thread Damage: Cross-threading compromises the integrity of both the bolt and nut. Re-tapping a damaged thread can remove too much material and reduce its strength. The correct action is to replace both the bolt and nut as a set, and to inspect the surrounding structure for hidden damage.
Self-Locking Nuts: These nuts (e.g., with a nylon insert or all-metal design) are generally single-use items. Their locking effectiveness is compromised after removal. Standard practice is to replace them with new ones whenever they are removed.
Torque Procedures:
Standard Torque: Torque must be applied smoothly and accurately using a calibrated torque wrench. The specified value and tolerance in the AMM must be followed.
Torque-to-Yield: This method involves tightening a fastener to a specified initial torque, followed by a specified angle of rotation (e.g., 90 degrees). This achieves plastic deformation of the bolt, creating a consistent and high clamping force. Accurate measurement of the angle is essential, using a torque-angle gauge or a protractor/marked socket if permitted by the AMM.
Torque in Soft Materials: When torquing bolts into soft materials like magnesium alloy castings, the low end of the torque range should be used to prevent thread stripping.
Locking Devices:
Split Pins: Used with castellated nuts. The legs must be bent over the flats of the nut, not into the threads of the bolt, to prevent thread damage and ensure security.
Safety Wire: Must be installed so that it pulls the fastener in the tightening direction. If a safety wire is found missing, a new one must be installed. If the hole in the bolt is damaged, the bolt must be replaced.
Locking Devices are Single-Use: All locking devices (split pins, safety wire, lock washers) are single-use and must be replaced with identical parts. Reusing or altering them is not permitted.

2.4 Fluid Lines, Hoses, and Fittings

Proper installation and maintenance of fluid systems are critical for safety and reliability.

Hose Inspection:
Surface Crazing: A common condition on rubber hoses. Acceptability is determined by the AMM/CMM, which provides specific limits for crazing depth and extent.
Chafing: Chafing of the outer cover that does not penetrate the reinforcement braid must be addressed to prevent future failure. The correct action is to relieve the interference and possibly install a protective sleeve. Taping over is not an approved repair.
Exposed Braid: Chafing that exposes or breaks the wire reinforcement braid indicates structural compromise and requires hose replacement.
Hose Installation:
Preventing Twisting: Hoses must be installed without twist. A straight line painted on the hose can help detect twisting during installation. Fittings must be torqued while the hose is held straight to prevent twisting.
Fire Sleeves: Fire sleeves are required components for fire protection in engine compartments. A hose must not be installed without its required fire sleeve.
Fittings and Leaks:
Leaks: Leaks at fittings are commonly due to worn seals (O-rings or gaskets). The standard repair is to replace the seal. Over-tightening can damage the fitting and is not a correct repair method.
Fluid Compatibility: It is critical to use the correct hydraulic fluid specified in the AMM. Mixing incompatible fluids (e.g., mineral-based MIL-PRF-5606 with phosphate ester-based Skydrol) can cause seal degradation and system failure.
System Testing: After hose or component replacement, a proof pressure test at 1.5 times the normal operating pressure is common to verify system integrity. A static test pressurises the system without moving components to detect external leaks.

2.5 Ground Handling, Storage, and Safety

Safe ground operations are essential to protect personnel and the aircraft.

Lifting and Jacking: Aircraft must be jacked using all specified jacking points per the AMM to avoid structural damage. When removing wheels, the axle must be supported by a safety stand to prevent the aircraft from settling.
Engine Handling and Storage:
Pre-Oiling: If an engine has been inactive for an extended period (e.g., more than 30 days), it must be pre-oiled before starting to prevent dry-start damage. This is done by cranking the engine with the starter, with the ignition off and fuel shut off, until oil pressure is indicated.
Engine Oil Level: Operating with an overfull oil level can cause foaming, increased oil consumption, and possible engine damage. The excess oil must be drained to the correct level.
Hazardous Materials:
Storage: Flammable materials must be stored in approved containers and cabinets per fire safety regulations.
Disposal: Hydraulic filters contain residual fluid and are hazardous waste. They must be drained and disposed of in accordance with environmental regulations.

2.6 Structural and Component Repairs

Sheet Metal Repairs: Cracks in sheet metal can sometimes be repaired per an approved AMM scheme, which may involve stop-drilling and patching. Stop-drilling alone is a temporary measure, not a permanent repair. Welding aluminium cowlings is generally not recommended due to loss of strength.
Engine Mounts and Shock Mounts: Deteriorated shock mounts are often replaced as a set to maintain even support and alignment. A cracked engine mount is a serious defect that requires an approved repair design. The aircraft must be grounded until an approved repair is accomplished.
Exhaust Manifolds: Cracks in exhaust manifolds are often not repairable due to high thermal stresses and the risk of carbon monoxide leakage. If the AMM states they are not repairable, replacement is the only correct action.
Propeller Maintenance:
Nicks and Dents: Minor nicks on metal propeller blades can be dressed out with a file, provided they are within the limits specified in the propeller maintenance manual and the blade thickness remains above the minimum.
Track Check: If a blade is out of track beyond the AMM limit, it can often be corrected by adjusting blade pitch for adjustable-pitch propellers.
Crack Detection: Dye penetrant is the standard method for detecting surface cracks in metal propeller blades.
Bearing Removal and Installation: When removing a bearing from an aluminium housing, heating the housing expands it, reducing the interference fit and allowing removal without damage. When installing a bearing on a shaft, force must be applied to the inner race to avoid damaging the rolling elements.

2.7 System Functional Tests and Troubleshooting

Functional tests verify that a system operates correctly and within specified parameters.

Landing Gear Retraction Test:
Normal System Function: During a retraction test with the aeroplane on jacks, the gear-up warning horn will sound because the throttle is retarded and the gear is not down and locked. This is a normal function, and the test should continue as per the AMM.
Failure to Lock: If the gear retracts but does not lock up, it usually indicates rigging issues such as improper adjustment of the drag links or actuator travel, preventing the over-centre lock from engaging.
Indicator Issues: If the gear is mechanically down and locked but the indicator does not show it, the issue is likely in the electrical indication system (e.g., a faulty microswitch or wiring).
Flight Control Systems:
Rigging: Control surface deflection limits are found in the AMM. If deflection is out of tolerance, rigging adjustment is required, typically using turnbuckles.
Functional Tests: If a system operates unevenly (e.g., flaps), the test must be halted and the system checked per the AMM to prevent structural damage.
Engine Testing:
Compression Test: The throttle must be open to allow maximum air intake for a true reading. The highest reading after several revolutions indicates the cylinder's peak compression. A low reading in one cylinder typically indicates a valve issue (burnt or leaking), while worn rings would affect multiple cylinders.
Oil Filter Inspection: Fine metallic particles in the oil filter can indicate abnormal wear. The source must be identified before return to service. Oil analysis and borescope are NDT methods used to evaluate internal engine condition.

2.8 Weight and Balance

Centre of Gravity (CG) Calculation: The CG is calculated by dividing the total moment (weight × arm) by the total weight.
Formula: CG = Total Moment / Total Weight
Example: An aircraft has an empty weight of 1,500 lbs and a CG at 40.0 inches aft of datum. A 5 lb radio is installed at a station of 100 inches.
Original Moment = 1500 lbs × 40.0 in = 60,000 lb-in.
Added Moment = 5 lbs × 100 in = 500 lb-in.
Total Moment = 60,500 lb-in.
Total Weight = 1,505 lbs.
New CG = 60,500 lb-in / 1,505 lbs = 40.2 inches aft of datum.
Weighing Procedure: The AMM specifies the exact configuration for weighing, typically with all fluids and standard equipment, but with empty fuel tanks or as specified.

2.9 Defect Reporting and Documentation

Reporting: Any defect found during maintenance must be reported to certifying staff. The aircraft must not be released to service until the defect is properly rectified using approved data.
Documentation: All maintenance actions, including inspections, repairs, and component replacements, must be recorded for traceability.
Release to Service: A B1.2 certifying mechanic can only release an aircraft to service after maintenance that is performed in accordance with approved data. Unapproved repairs or deferrals of structural damage are not permitted.

3. Important Regulations and References

Regulation (EU) No 1321/2014, Annex III (Part-66): This regulation establishes the requirements for the certification of maintenance staff. It defines the privileges of a B1.2 certifying mechanic and the knowledge requirements (Appendix I) for the B1.2 licence.
Part-66.25: This section requires certifying staff to have sufficient language proficiency to read and understand maintenance documentation and to communicate effectively.
Regulation (EU) No 1321/2014, Annex II (Part-145): This regulation covers the requirements for maintenance organisations. Part-145.A.50 requires that a certificate of release to service is issued after maintenance is performed, and that any defect found is reported and rectified.
Part-21.A.16: This regulation requires that the Airworthiness Limitations Section (ALS) be included in the maintenance manual or TCDS.
AC 43.13-1B (Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair): While a US FAA document, it is widely recognised as an acceptable source of standard maintenance practices for general aviation aircraft.

4. Common Relationships Between Concepts

Approved Data is the Central Hub: All maintenance actions, from finding a defect to performing a repair, are governed by the AMM. The AMM provides the limits for damage, the procedures for repair, and the specifications for parts and materials.
Inspection Leads to Action: A thorough inspection (visual, dimensional, NDT) is the first step in identifying a defect. The findings of the inspection are then compared against the limits in the AMM to determine the required action (accept, repair, or replace).
Fastener Integrity is Paramount: The correct selection, installation, and locking of fasteners are critical for structural integrity. Torque, safetying, and material compatibility are all interconnected to ensure the fastener performs its function.
System Function is the Ultimate Test: Functional tests are the final verification that a maintenance task has been completed correctly. They confirm that the system operates within its specified parameters and that all components work together as intended.
Safety is a Continuous Process: Safety precautions are not a separate task but are integrated into every step of a maintenance procedure, from the initial risk assessment to the final functional test.

5. Typical Exam Focus Points

When preparing for the Module 7A exam, focus on the following areas:

The "What would you do?" Scenario: Many questions present a specific defect (e.g., a chafed hose, a cracked cowling, a damaged thread) and ask for the correct action. The answer is almost always found by applying the principle of "consult the AMM and follow its instructions."
Acceptable vs. Unacceptable Damage: Be able to distinguish between damage that is within AMM limits (acceptable, document) and damage that is beyond limits (requires repair or replacement).
Single-Use Items: Understand which items are single-use (self-locking nuts, split pins, safety wire, O-rings, gaskets) and must be replaced, not reused.
Correct Use of Tools: Know the correct procedures for using precision measuring tools (micrometer, DTI) and torque wrenches, including torque-to-yield procedures.
Safety Procedures: Be familiar with safety precautions for jacking, engine handling, fuel leaks, and hazardous materials.
System-Specific Procedures: Understand the normal and abnormal indications during functional tests (e.g., landing gear warning horn during retraction test) and the likely causes of failures (e.g., gear not locking up).
Weight and Balance Calculations: Be able to perform basic CG calculations using the formula CG = Total Moment / Total Weight.
Regulatory Knowledge: Understand the roles of Part-66, Part-145, and the importance of approved data (AMM, CMM, SB, AD, ALS).