Module 7A: Maintenance Practices

1. Module Overview

Module 7A is a cornerstone of the EASA Part-66 B1.1 licence, covering the fundamental practical skills and theoretical knowledge required for safe and effective aircraft maintenance. This module is not about a single system, but rather the universal practices, techniques, and safety protocols that underpin all maintenance tasks. It bridges the gap between theoretical engineering principles and the hands-on application required to keep aeroplanes airworthy. The syllabus is designed to instil a deep understanding of how and why maintenance is performed, emphasising the use of approved data, correct tooling, and meticulous attention to detail. This module covers everything from workshop safety and precision measurement to specific procedures for fasteners, bearings, and structural repairs.

2. Key Concepts Explained in Detail

2.1 Safety Precautions and Workshop Practices (7.1, 7.2)

Safety is the paramount concern in all maintenance activities. This section establishes the non-negotiable rules for protecting personnel, equipment, and the aircraft.

Personal Safety: This includes the correct use of Personal Protective Equipment (PPE) such as safety glasses, hearing protection, gloves, and steel-toe-capped footwear. Technicians must be aware of hazards like rotating machinery (e.g., tie back long hair, no loose clothing), compressed air (which can cause serious injury if directed at the body), and high-pressure fluids (which can penetrate the skin).
Workshop Safety:
Fire Safety: Flammable materials like solvents and fuels must be stored in approved, flame-proof cabinets. Waste materials (oily rags) must be disposed of in sealed, fire-resistant containers. Fire extinguishers appropriate for the potential fire class (e.g., dry powder, CO2 for electrical/flammable liquid fires) must be readily accessible and serviced.
Housekeeping: A clean and organised workspace is essential for Foreign Object Damage (FOD) prevention. A rigorous tool control program is mandatory in a Part-145 environment. This involves shadow boards, tool inventories, and accounting for all tools before and after a task to ensure nothing is left inside the aircraft.
FOD Prevention: Any tool, fastener, or debris left in an aircraft can cause catastrophic damage. This is a primary reason for strict tool control and meticulous work practices.
Aircraft Safety:
Grounding and Bonding: When refuelling or working with fuel systems, the aircraft, fuel bowser, and dispensing equipment must be electrically bonded and grounded to equalise potential and prevent static sparks that could ignite fuel vapours.
System Safety: Before working on any system, it must be made safe. This includes de-energising electrical circuits, isolating hydraulic or pneumatic systems (and relieving pressure), and installing safety devices like locks and tags (e.g., "DO NOT OPERATE" tags).
Careful Removal of Components: Before applying force, heat, or using extractors on seized components, the approved maintenance data (AMM/SRM) must be consulted. This ensures the method is acceptable and identifies any restrictions, such as material compatibility (e.g., magnesium alloys are flammable and sensitive to heat) or structural integrity.

2.2 Precision Measuring Instruments (7.4)

Accurate measurement is fundamental to determining serviceability and performing correct installations.

Micrometer: Used for high-precision external measurements (e.g., piston pin diameter, brake disc thickness). The correct technique involves:

Ensuring the workpiece and anvil/spindle faces are clean.
Using the ratchet or friction thimble to apply a consistent, standardised measuring pressure, preventing operator-induced variations.
Taking multiple readings at different points to assess wear or out-of-roundness.

Dial Test Indicator (DTI): Used to measure small variations in surface position, such as shaft runout. The DTI stylus must be set perpendicular to the surface being measured. The shaft is rotated, and the total indicated runout (TIR) is the difference between the maximum and minimum readings.
Go/No-Go Gauge: A pass/fail gauge used to verify that a dimension (e.g., hole diameter, thread form) is within a specified tolerance. The "Go" side must fit, and the "No-Go" side must not. It provides a binary result, not a numerical value.
Multimeter: Used for electrical measurements.
Resistance: Must be measured on a de-energised circuit. The component should be isolated to prevent parallel paths from affecting the reading. A low resistance (near 0 Ω) indicates continuity; a high or infinite reading indicates an open circuit.
Voltage and Current: Measured on live circuits with the meter set to the correct function and range.
Torque Wrench: A precision tool that must be within its calibration validity period (often 12 months). It should be used with a smooth, steady pull perpendicular to the handle. When using adapters:
A crowfoot extension at 90° does not change the effective lever arm length, so the torque reading is accurate.
An extension not at 90° increases the effective length, requiring a correction factor to avoid over-torquing.

2.3 Fasteners and Hardware (7.2, 7.3, 7.6)

Correct fastener selection, installation, and locking are critical for structural integrity and system safety.

Torque Application:
Dry vs. Wet Torque: Lubrication reduces friction. A "wet" torque value is lower than a "dry" torque value to achieve the same clamping force. The AMM specifies which method is required. Using the wrong method leads to incorrect preload.
Torque + Angle Method: Used for critical fasteners (e.g., engine cylinder head bolts) to achieve a more accurate preload. The fastener is torqued to a low "snug" value, then rotated by a specified number of degrees. This method controls bolt stretch more accurately than torque alone, which is affected by friction variations.
Torque Check: If a fastener moves before the specified torque is reached, it indicates a loss of preload, thread damage, or debris in the hole. The fastener must be removed, inspected, and replaced. Simply re-torquing can mask damage.
Torque Seal: A paint stripe applied across the fastener head and surrounding structure. It is not a locking device but a visual indicator that shows at a glance if the fastener has rotated, indicating possible loosening.
Self-Locking Nuts: These nuts use a deformed collar or a nylon insert to provide prevailing torque, resisting loosening.
Serviceability: The primary criterion is the prevailing torque. If the nut moves easily or its locking torque is below the manufacturer's minimum, it is unserviceable and must be replaced. A cracked or damaged nylon insert also renders the nut unserviceable.
Reuse: They can be reused only if they still meet the specified locking torque or friction requirements as defined in the AMM.
Castellated Nuts and Cotter Pins: Used on critical applications like wheel bearings and control linkages.
Installation: Torque the nut to the specified value. If the cotter pin hole does not align with a slot, the nut should be tightened (never loosened) to the next alignment. Loosening reduces the clamping force. At least one full thread should be visible above the nut to ensure proper engagement.
Safety Wire: Used to lock fasteners, typically in groups. The wire must be installed so that it exerts a tension that tightens the fastener. The wire must be pulled tight and twisted, and the direction of pull must be such that it tends to rotate the bolt in the tightening direction.
Rivets:
Solid Rivets: Used for high-strength structural joints where both sides are accessible.
Blind Rivets: Designed for installation from one side only, making them ideal when access to the rear of the joint is limited.
Rivet Hole Repair: If a hole is oversized, it must be reamed to the next standard size and filled with an oversized rivet as per the SRM/AMM. Installing a standard rivet in an oversized hole is not acceptable.
Hardware Standards: MS (Military Standard) and AN (Army-Navy) hardware are not always interchangeable, even if they appear similar. The AMM or IPC specifies the exact part number to use.

2.4 Fluid Lines, Hoses, and Fittings (7.7)

Proper installation and inspection of fluid systems prevent leaks and catastrophic failures.

Hose Installation: Flexible hoses should be installed without twisting. A slight twist may be acceptable if the AMM allows it, but the primary requirement is to avoid contact with hot surfaces, sharp edges, and to prevent kinks. Chafing against structure must be prevented with protective chafe guards.
Hose Inspection: Chafing on the outer cover may be acceptable if the reinforcement braid is not exposed or damaged. The AMM provides specific limits. If the reinforcement is exposed, the hose must be replaced.
O-Ring Installation: O-rings should be lubricated with the recommended fluid or grease to prevent damage during installation and to ensure proper sealing.

2.5 Bearings and Lubrication (7.3)

Correct handling and lubrication of bearings are essential for their longevity and function.

Cleaning: Bearings should be cleaned in a suitable solvent to remove all old grease and contaminants. Rotating the bearing while submerged helps to flush out particles. Never use compressed air to spin a bearing dry, as this can cause damage due to high-speed rotation without lubrication.
Inspection: After cleaning, bearings must be inspected for fretting corrosion, pitting, brinelling, and wear. Fretting corrosion on the inner race bore or axle contact surface indicates bearing rotation on the axle (creep), which is an unacceptable condition requiring bearing replacement and axle inspection.
Removal: A bearing puller is the correct tool to remove a press-fit bearing, as it applies even, controlled force, minimising the risk of damage. Hammering is not acceptable.
Lubrication: Only the grease specified in the AMM should be used. Substituting greases, even with the same NLGI grade, can lead to incompatibility and component failure. The correct amount of grease is specified. Over-packing can cause overheating and premature failure.
Wrong Grease Application: If the wrong grease is accidentally used, the bearing must be removed, completely cleaned to remove all traces of the wrong grease, and then re-lubricated with the correct grease.

2.6 Inspection Techniques and Structural Damage (7.5, 7.9)

Inspection is the primary means of detecting defects and determining airworthiness.

Visual Inspection: The most common NDT method. It is used to detect leaks, cracks, corrosion, chafing, and general condition. Any defect found must be evaluated against the limits in the AMM or SRM.
Dye Penetrant Inspection: A non-destructive test for detecting surface-breaking cracks. The developer acts like a blotter, drawing the penetrant out of the defect to create a visible indication.
Damage Evaluation:
Cracks: Any crack is a defect that must be reported and evaluated per the SRM or AMM. Drilling stop holes or temporary repairs are not authorised without proper engineering approval.
Dents: The SRM provides specific allowable damage limits for dents on aerofoils and other structures. If the dent is within limits, it may be retained. If it exceeds limits, a repair or replacement is required.
Tires: A cut that exposes the cord indicates structural damage, making the tire unserviceable and requiring replacement.
Control Cables: Broken wires are a serious defect. The AMM specifies the maximum allowable number of broken wires over a given length. If the limit is exceeded, the cable must be replaced. Soldering or taping is not an approved repair.
Composite Panels: Delamination must be evaluated against SRM limits. The action is to report and consult the SRM for disposition.
Corrosion: Light surface corrosion that can be removed by light sanding is typically treated by removal, chemical treatment, and re-priming. Fretting corrosion on bearing surfaces requires component replacement.
Leak Checks: Leak rates (e.g., oil drips per minute) are often specified in the AMM. A small leak may be within limits, but it must be checked against the manual. The first step before any repair is to identify the source and rate of the leak, often by cleaning the area and performing a controlled run.

2.7 Weight and Balance (7.19)

Accurate weight and balance data is critical for the safe operation of the aeroplane.

Weighing Procedure: The aeroplane must be weighed in a level attitude with all required equipment and fluids as per the AMM to obtain accurate weight and CG.
Reference Document: The Weight and Balance Report contains the official empty weight and CG location of the aircraft and is used as the baseline for all calculations.

2.8 Maintenance Documentation and Procedures (7.16, 7.14)

All maintenance actions must be performed using approved data and properly documented.

Approved Data: The AMM, SRM, CMM, and service bulletins are examples of approved data. All repairs and modifications must be performed in accordance with this data.
Documentation: All defects and repairs must be documented in the aircraft logbook or maintenance record. This is a legal requirement for maintaining airworthiness.
Functional Tests: After maintenance, a functional test is performed to verify that the system operates correctly within specified limits. This includes checking for smooth operation, correct sequencing, and proper indications. A reading outside the specified tolerance (e.g., flap transit time) requires troubleshooting before return to service.

3. Important Formulas, Regulations, and Procedures

Key Formulas and Conversions

Torque: Torque (τ) = Force (F) × Distance (d) from the pivot point. Units: N·m, lbf·ft, lbf·in.
Unit Conversions:
1 ft-lb = 12 in-lb
1 psi ≈ 6.895 kPa
1 psi ≈ 0.06895 bar
3000 psi ≈ 206.85 bar
150 psi ≈ 1034.25 kPa

Regulatory References

Regulation (EU) No 1321/2014, Annex III (Part-66): Defines the licensing requirements for maintenance certifying staff.
Part-145: Defines the requirements for maintenance organisations, including tool control, approved data, and documentation procedures.
AMC (Acceptable Means of Compliance) and GM (Guidance Material): Provide acceptable methods and guidance for complying with the regulations.

Standard Procedures

Torque Check Procedure: If a fastener moves before the specified torque is reached, it must be removed, inspected, and replaced.
Castellated Nut Installation: Torque to the specified value. If the slot does not align, tighten (never loosen) to the next alignment.
Bearing Cleaning: Clean in solvent, rotating to flush out particles. Never spin dry with compressed air.
Hose Installation: Route to avoid heat, chafing, and kinks. Install without twisting unless the AMM allows it.

4. Common Relationships Between Concepts

Torque and Preload: The purpose of torque is to create a specific clamping force (preload) on the fastener. The relationship is affected by friction, which is why dry vs. wet torque values differ and why the torque-angle method is used for critical fasteners.
Fastener Locking and Vibration: Fasteners are locked (safety wire, cotter pins, self-locking nuts) to prevent them from loosening due to vibration. The choice of locking method depends on the application and accessibility.
Inspection and Airworthiness: The purpose of inspection is to find defects. The disposition of a defect (e.g., "within limits", "repair", "replace") is always determined by comparing the defect to the limits in the approved data (AMM/SRM).
Safety and Maintenance: All maintenance practices are designed to ensure the safety of personnel and the aircraft. This includes FOD prevention (tool control), fire safety (solvent storage), and system safety (grounding, de-energising).
Lubrication and Component Life: Correct lubrication reduces friction and wear, extending the life of components like bearings. Using the wrong grease or over-packing can cause premature failure.

5. Typical Exam Focus Points

Correct Actions for Defects: The exam heavily focuses on the correct action to take when a defect is found. The answer is almost always to consult the AMM/SRM and, if the damage exceeds limits, replace the component. Temporary repairs are rarely acceptable.
Torque Wrench Usage: Understanding the effect of extensions (90° vs. non-90°), the difference between dry and wet torque, and the correct action if a fastener moves before reaching the specified torque.
Fastener Locking: The serviceability criteria for self-locking nuts (prevailing torque) and the correct installation of cotter pins (tighten, never loosen, to align).
Safety and FOD Prevention: The importance of tool control, proper storage of flammable materials, and grounding/bonding procedures.
Precision Measurement: The correct techniques for using micrometers (ratchet pressure) and DTIs (perpendicular stylus).
Unit Conversions: Be prepared to convert between psi, bar, and kPa, and between ft-lb and in-lb.
Bearing Handling: The correct methods for cleaning, inspection (fretting corrosion), removal (puller), and lubrication (correct grease and amount).
Hose and Line Inspection: The criteria for serviceability (chafing on outer cover vs. exposed reinforcement).
Documentation: The requirement to document all defects and repairs in the aircraft logbook.
The "First Action" Principle: For many troubleshooting scenarios, the first action is to consult the approved data (AMM/SRM) or to identify the source of the problem (e.g., leak) before proceeding with a repair.