Module 7A: Maintenance Practices — Overview

Module 7A covers the fundamental practical knowledge required for certifying staff working on turbine-engine aeroplanes. It bridges the gap between theoretical engineering principles and the hands-on procedures, safety protocols, and regulatory requirements that govern daily maintenance activities. The module emphasises the correct use of approved data, adherence to safety precautions, proper tool usage, and the disciplined documentation of all maintenance actions. It is not merely about knowing how to perform a task, but understanding why it must be done in a specific, approved manner to ensure continued airworthiness and safety.

1. Safety Precautions and Health & Safety

1.1 General Principles

Safety is the paramount consideration in all maintenance activities. The maintenance environment presents numerous hazards, including high-pressure systems, flammable materials, heavy components, and electrical energy. A systematic approach to safety is required, starting with a thorough understanding of the task, the associated risks, and the mandatory precautions.

1.2 Hazardous Materials and Solvents

Many materials used in aircraft maintenance are hazardous. The primary source of information is the Safety Data Sheet (SDS), which is a mandatory document for all hazardous substances.

SDS Information: The SDS provides critical data, including:
Physical properties (e.g., flash point, boiling point).
Health hazards (e.g., skin irritant, toxic, carcinogenic).
Fire and explosion risks.
Handling and storage requirements.
First-aid measures.
Personal Protective Equipment (PPE) requirements.
Flammable Solvents: Solvents with a flash point below 38°C are considered highly flammable. When using them, the following are mandatory:
Adequate ventilation to prevent vapour accumulation.
Elimination of all ignition sources (e.g., open flames, sparks, non-explosion-proof electrical equipment).
Use of approved, explosion-proof lighting in areas where vapours may be present.
Proper disposal of rags and waste in approved, fire-resistant containers.
Personal Protective Equipment (PPE): The SDS dictates the required PPE. This may include:
Chemical-resistant gloves: Essential when handling solvents and skin irritants. Cotton or leather gloves may absorb chemicals or offer no protection.
Flame-resistant clothing: Required when handling flammable materials.
Eye protection: Safety glasses or goggles.
Respiratory protection: A dust mask is not appropriate for solvent vapours; a suitable vapour respirator may be required.
Spill Management: Spills must be contained immediately using appropriate absorbent materials. The contaminated absorbent material must be disposed of as hazardous waste, not in regular waste bins, to prevent environmental contamination and fire risk.

1.3 Tools and Equipment Safety

Hydraulic Presses: These tools exert extreme forces. A safety guard is essential to protect the operator from flying debris in case of component failure. A pressure gauge is necessary to control the applied force. Gloves can be a hazard if caught in the mechanism.
Work Lighting: In areas where flammable vapours may be present (e.g., fuel tanks, battery compartments), only explosion-proof lighting is permitted. Using standard work lights or damaged cords is a significant fire hazard.
Tool Control: All tools must be in a serviceable condition. If a tool is dropped onto a critical component (e.g., a turbine blade), the component must be inspected for damage (nicks, cracks) and the finding reported and evaluated per the AMM before the aircraft is released to service.

1.4 Environmental and Hangar Safety

Ventilation: If a task requires ventilation (as per the SDS) and the hangar ventilation system is malfunctioning, the task must not proceed. Opening doors may not achieve the required ventilation rate. PPE does not replace engineering controls.
Housekeeping: A clean and tidy work area is fundamental to safety and FOD prevention.

2. Approved Data, Documentation, and Regulatory Compliance

2.1 The Hierarchy of Maintenance Data

The performance of any maintenance task must be in accordance with approved data. The primary source of this data is the Aircraft Maintenance Manual (AMM).

Aircraft Maintenance Manual (AMM): This is the manufacturer's approved document containing all procedures for scheduled and unscheduled maintenance, including:
Detailed task instructions (e.g., removal/installation, inspection, testing).
Specific torque values, tolerances, and clearances.
Approved materials, parts, and consumables (e.g., greases, sealants, thread-locking compounds).
Lubrication charts and intervals.
Allowable damage limits (often cross-referenced with the SRM).
Safety precautions specific to each task.
Structural Repair Manual (SRM): Contains detailed data for the evaluation and repair of structural damage, including allowable damage limits for dents, cracks, and corrosion.
Component Maintenance Manual (CMM): Provides overhaul and repair data for specific components (e.g., wheels, brakes, actuators).
Illustrated Parts Catalogue (IPC): Used to identify part numbers and ensure correct parts are installed.
Minimum Equipment List (MEL): An approved document that lists equipment that may be inoperative for dispatch, subject to specific conditions and limitations. It is the authority for dispatch with a defect, not for performing the repair.
Airworthiness Limitations Section (ALS): Contains mandatory life limits and inspection intervals for components.

2.2 Regulatory Framework (Part-145 and Part-M)

Part-145.A.40 (Tools and Equipment): All tools used for maintenance must be controlled and calibrated at specified intervals. Using an out-of-calibration tool (e.g., a torque wrench) is a violation and the tool must be quarantined.
Part-145.A.45 (Maintenance Data): The organisation must ensure that all maintenance is performed using the latest applicable approved data.
Part-145.A.50 (Certification of Maintenance): A Certificate of Release to Service (CRS) can only be issued by appropriately authorised certifying staff when all maintenance has been performed in accordance with approved data and the aircraft is airworthy. Certifying staff must only certify work they have personally performed, supervised, or verified.
Part-M.A.801: Requires that all maintenance be performed in accordance with approved data and that records are kept.

2.3 Defect Recording and Reporting

Any defect found during maintenance, even if within allowable limits, must be documented for traceability. This is a fundamental principle.

Damage Within Limits: If damage (e.g., a dent) is found to be within the allowable limits defined in the AMM or SRM, the aircraft can be released to service, but the finding must be recorded in the technical log or maintenance record.
Damage Beyond Limits: If damage exceeds allowable limits, the aircraft is unserviceable until a repair is performed using approved data.
MEL Deferrals: When a defect is deferred using the MEL, the certifying staff must ensure that the deferral is properly documented, and any associated 'O' (operational) or 'M' (maintenance) procedures are completed or communicated to the flight crew as required.

2.4 Maintenance Records

Maintenance records must be retained until the information is superseded by new data or for the life of the component/aircraft. This ensures full traceability of all maintenance actions.

3. Precision Measuring Tools and Unit Conversion

3.1 Calibration

All precision measuring tools (e.g., torque wrenches, multimeters, micrometers) must be within their calibration interval. A tool with an expired calibration sticker must be removed from service and quarantined.

3.2 Unit Conversions

The AMM may specify values in either SI or Imperial units. The technician must be able to convert between them accurately. The AMM is the controlling document; if it specifies a value in a particular unit, the tool used must be calibrated in that unit to avoid conversion errors.

Common Conversions:

Torque:
1 lbf.in = 0.113 N.m
1 lbf.ft = 1.356 N.m
Pressure:
1 bar = 14.5 psi
1 psi = 6.895 kPa

Example: An AMM specifies a torque of 50–60 in-lb.

50 in-lb × 0.113 = 5.65 N.m
60 in-lb × 0.113 = 6.78 N.m
The correct range is 5.65–6.78 N.m.

Example: An AMM specifies a tyre pressure of 100 psi.

100 psi ÷ 14.5 psi/bar = 6.9 bar.

Example: An AMM specifies a torque of 200 lbf.in.

200 lbf.in × 0.1129848 N.m/lbf.in = 22.6 N.m.

3.3 Torque Application

Torque values are mandatory and must be taken from the AMM. A 'typical' value from memory is not acceptable.
The final torque value must be observed during the final tightening motion. If the value is not observed, the fastener must be loosened and re-torqued while observing the reading.
Torque wrenches must be used correctly, applying a smooth, steady pull.

4. Aircraft Handling, Storage, and Ground Operations

4.1 Jacking

The first step in any jacking procedure is to consult the approved maintenance data (AMM). The AMM provides the specific jacking points, required equipment, safety precautions, and step-by-step procedures.

4.2 Fluid Leaks

Fluid leaks (hydraulic, oil, fuel) are a common finding during inspections.

Initial Action: Any leak must be reported to certifying staff. The area should be cleaned to allow for accurate monitoring.
Evaluation: The AMM or CMM provides allowable leak rates for specific components. A light film of oil may be considered normal seepage if within limits. An actively dripping leak may exceed limits and render the aircraft unserviceable.
Hydraulic Leaks: Before condemning a component for a leak, the actual leak rate must be verified per AMM procedures, often by cleaning and pressurising the system.
Oil Leaks: Oil leaks can be a fire hazard and indicate a potential system failure. The AMM provides leak limits; if exceeded, the aircraft is unserviceable.

4.3 Tyres and Wheels

Tread Depth: Minimum tread depth is specified in the AMM or CMM. If the measured depth is below the minimum, the tyre must be replaced.
Damage: A cut in the tread that exposes the cord indicates structural damage. The tyre is not airworthy and must be replaced.
Wheel Nuts: Corroded or damaged wheel nuts are not airworthy and must be replaced. Reuse could lead to failure under load.
Axle Nut Torque: The torque for the axle nut is critical and must be taken from the AMM. A calibrated torque wrench is mandatory.

4.4 Control Cables

Inspection: Control cables must be inspected for broken wires. The AMM specifies the maximum allowable broken wire limits, usually given per cable length or per lay.
Action: If a broken wire is found, it must be measured and counted, then compared with the AMM limit. A single broken wire may be within limits. Taping is not an approved repair.

5. Materials, Fasteners, and Components

5.1 Fastener Identification

Colour Coding: Fasteners are often colour-coded to indicate material type. For example, a blue-coloured anodised fastener typically indicates corrosion-resistant steel (stainless steel). Always refer to the IPC/AMM for exact identification.
High-Torque Fasteners: These are designed for critical applications and require precise torque values as per the AMM.

5.2 Thread-Locking Compounds

Application: Thread-locking compounds require clean, dry, oil-free surfaces to achieve proper adhesion and curing. A small amount is applied to the threads before tightening to the specified torque.
Approved Materials: The AMM specifies the exact compound to be used. If the colour of the available compound does not match the AMM specification, it is not to be used. The AMM or CMM provides approved cross-reference charts.

5.3 Safety Wiring

Safety wiring is a locking method used to prevent fasteners from turning due to vibration. It does not increase clamping force, provide bonding, or seal threads. Correct installation is critical and follows AMM/standard practices.

5.4 Hoses and Lines

Chafing: Chafing of a hydraulic hose against a structure can lead to hose failure and fluid loss. The AMM provides specific instructions for installing chafe guards or repositioning hoses. Temporary fixes like tape are not acceptable.
Service Life: Hoses have a shelf life and a service life limit. Installing a hose with an expired date of manufacture is non-compliant and can lead to failure.

5.5 Seals (O-Rings)

When installing a new O-ring, it should be lubricated with the correct lubricant (specified in the AMM) to reduce friction and prevent it from being cut or pinched during installation. Dry installation can cause damage.

5.6 Grease Fittings

A damaged grease fitting must be replaced to ensure proper lubrication and prevent contamination. Forcing grease or skipping lubrication compromises the maintenance task.

5.7 Hydraulic Systems

Depressurisation: Before disconnecting any hydraulic lines or removing components, the system must be depressurised to prevent injury and environmental contamination.
Capping: Lines must be capped immediately after disconnection to prevent contamination and fluid loss.
Lockout/Tagout: Engine isolation (lockout/tagout) prevents accidental rotation during maintenance.

6. Inspection and Troubleshooting

6.1 Types of Visual Inspection

General Visual Inspection (GVI): A visual examination to detect obvious damage, leaks, or insecure attachments. It may require opening panels, using ladders, or access platforms, but does not involve detailed or non-destructive inspection techniques.

6.2 Damage Evaluation

Dents: Any damage, including dents, must be recorded and reported. The certifying staff will determine if the dent is within allowable limits using the SRM or AMM. Damage within limits is airworthy but must be documented for traceability.
Cracks: Cracks in structural skin panels must be evaluated against AMM/SRM limits. If a repair is required and the AMM states it must be accomplished by an approved repair station, a line mechanic cannot perform it.

6.3 Troubleshooting

Systematic troubleshooting is essential to avoid unnecessary part replacement.

First Steps: Start with simple checks (power, wiring, connectors) before replacing components.
Use of Manuals: Follow the logic of the AMM/TSM (Troubleshooting Manual).
Example: If a landing gear retracts normally but the 'gear up' indication does not illuminate, the first step is to check the indicator bulb, wiring, and connectors, not to replace the gear actuator.

6.4 Electrical Measurements

Continuity: Continuity tests must be performed on de-energised circuits to prevent meter damage and incorrect readings.
Resistance: Resistance measurements must be compared to the AMM specified tolerance. A reading outside the specified tolerance requires further investigation per the manual.

7. Lubrication

7.1 Approved Data

The correct type of grease, quantity, and lubrication interval are found in the AMM and its lubrication charts.

7.2 Lubrication Techniques

Grease Gun: The AMM may specify 'lubricate until new grease appears at the bearing'. This indicates that old grease has been displaced and the bearing is freshly lubricated. Over-lubricating can damage seals; under-lubricating leads to wear.
Approved Substitutes: If the specified grease is unavailable, a substitute is only permitted if it is listed in the AMM as an approved alternative. Using a grease with a different base oil or NLGI grade without approval is not acceptable.

8. Typical Exam Focus Points

Approved Data: The AMM is the primary source for all maintenance tasks. Knowing what document to consult for what purpose (AMM vs. SRM vs. MEL vs. CMM) is a frequent exam topic.
Safety Procedures: Depressurisation of hydraulic systems, use of SDS, PPE requirements, and handling of flammable materials.
Defect Recording: Any defect, even if within limits, must be documented.
Torque: Correct application, unit conversion, and the requirement for calibrated tools.
MEL Philosophy: Understanding that the MEL is for dispatch decisions, not for repair procedures.
Certification: The CRS can only be signed by authorised staff for work performed or verified in accordance with approved data.
Tool Control: All tools must be within calibration and serviceable.
Unit Conversions: Be prepared to convert between imperial and SI units for torque, pressure, and other measurements.
Damage Limits: The difference between damage within limits (record and release) and damage beyond limits (ground and repair).
Material Substitution: Substitutes are only allowed if approved in the AMM or via an approved engineering document.