Module 7A: Maintenance Practices — B2 Avionics

1. Module Overview

Module 7A, "Maintenance Practices," is a foundational module within the EASA Part-66 B2 avionics syllabus. It establishes the essential practical knowledge and safety principles required for all aircraft maintenance activities, with a specific focus on the unique demands of avionics systems. This module is not about any single aircraft system but rather the universal practices, procedures, and precautions that govern how maintenance is performed safely and correctly. It bridges the gap between theoretical knowledge of avionics and the hands-on application required in a maintenance environment.

The module covers a wide range of topics, from general safety precautions and workshop practices to specific techniques for handling electrical wiring, connectors, and sensitive electronic components. For a B2 certifying staff member, this module is critical as it provides the rules and standards that ensure the continued airworthiness of electrical and electronic systems. The content is heavily aligned with the requirements of Part-145 and the procedures detailed in an Aircraft Maintenance Manual (AMM) and Standard Wiring Practices Manual (SWPM).

2. Key Concepts Explained in Detail

2.1 General Maintenance Principles and Safety (Module 7.1, 7.2)

This section forms the bedrock of all maintenance activities. It is not just about knowing how to do a task, but understanding the correct and safe way to do it.

Defect Recording and Reporting: The cornerstone of maintenance is the proper handling of defects. When a certifying staff member or technician discovers any defect, damage, or abnormal condition during an inspection or task, the first action is always to record and report it in accordance with the organisation's procedures (Part-145.A.50). This includes documenting the finding in the aircraft logbook or maintenance record. The decision to repair, replace, or defer a defect is a separate step that follows an assessment against approved data (AMM, CMM, MEL).
Use of Approved Data and Tools: All maintenance must be performed using the data, tools, and parts specified in the approved maintenance documentation (e.g., AMM, CMM, Service Bulletins). This is a fundamental principle of Part-145.
Tools: Tools must be calibrated and have an accuracy that meets or exceeds the requirements specified in the maintenance data. For example, if an AMM specifies a torque wrench with an accuracy of ±3%, using a wrench with ±5% accuracy is not permitted. The tool must be within the specified tolerance. The general standard for torque wrench accuracy is ±10%, but specific tasks may require higher precision (e.g., ±5% or ±3%).
Parts and Materials: Only the exact parts and materials specified in the maintenance data may be used. Substituting a star washer for a specified split lock washer, or using a different type of connector, is not permitted without engineering approval.
Safety Precautions: Safety is paramount. This includes:
De-energising and Isolating: Before any work on electrical or electronic systems, the circuit must be de-energised and isolated. This is the mandatory first step before any repair to prevent electric shock or fire. This involves opening circuit breakers, switching off power, and tagging the system to prevent accidental re-energisation.
Electrostatic Discharge (ESD): Many avionics components are sensitive to static electricity. Correct ESD practices include using grounded workstations, wrist straps, and anti-static bags when handling these components.
Hazardous Materials: Special precautions are required for components containing hazardous materials. For example, an LRU containing a lithium battery must be de-energised and the battery disconnected before removal to prevent short circuits and fires. Damaged lithium batteries must be stored in a fireproof metal container.
Radiation Hazards: Testing systems like weather radar or transponders can emit harmful radiation. The correct procedure is to use a dummy load or radiate into an RF absorber. Never radiate into occupied areas. Radiation badges are for monitoring, not a substitute for safe procedures.
Hearing Protection: Functional tests, such as the public address system, must not exceed specified volume limits to protect hearing and prevent acoustic feedback.
Fasteners and Torque: Fasteners are used to secure components and must be tightened to the correct torque value. The purpose of using a torque wrench is to ensure the correct clamping force without over-stressing the fastener or the component. Over-torquing can strip threads or crack components; under-torquing can lead to loosening due to vibration or, in electrical connections, high resistance and overheating. Torque values are specified in the AMM.
Safety Wiring and Locking: Safety wire (lockwire) is a positive locking method used to prevent fasteners from backing out due to vibration. It is a critical practice for components where loosening could have serious consequences. Similarly, lock washers are used to secure fasteners. The correct type of locking device, as specified in the AMM, must always be used.

2.2 Bonding and Earthing (Module 7.2, 7.4)

Electrical bonding is the intentional electrical connection of all metallic, non-current-carrying parts of an aircraft to the primary structure. Its purpose is to provide a low-resistance return path for electrical current, prevent static charge build-up, and provide a path for lightning strikes.

Purpose: Good bonding is critical for:
Avionics Performance: It provides a stable reference potential (ground) for sensitive electronic equipment, reducing noise and interference.
Lightning Protection: It provides a low-impedance path for lightning currents to travel safely to the aircraft's discharge points, preventing arcing and structural damage.
Static Discharge: It prevents the build-up of static electricity, which can interfere with radio communications and create a fire hazard.
Bonding Resistance Limits: The AMM specifies maximum acceptable resistance values for bonding connections. These are often very low, in the milliohm range.
A common general limit for bonding between an airframe and an avionics component is 0.1 ohm, though many installations require less than 1 ohm.
For critical bonds, such as those for equipment racks, the limit is often much lower, e.g., 2.5 milliohms (0.0025 ohms).
Bonding Jumper Inspection: Bonding jumpers (braids or straps) must be inspected for corrosion, broken strands, and secure attachment.
Corrosion: Corrosion increases resistance and can compromise the bond. The initial action upon finding corrosion is to measure the bond resistance against the AMM limit. If within limits, the bond is acceptable; if not, corrective action (cleaning or replacement) is required.
Broken Strands: The AMM often specifies a maximum percentage of broken strands (e.g., 10%). If the damage is within the limit, the jumper may be serviceable. If it exceeds the limit, it must be replaced.
Bonding Test Procedure: To accurately measure very low resistances (milliohms), a 4-wire Kelvin method must be used. This eliminates the resistance of the test leads from the measurement. A standard multimeter is not accurate enough for these measurements.
Static Wicks: These are part of the static discharge system. A missing static wick is a defect that must be rectified before dispatch, as it is required for safe operation, especially in instrument meteorological conditions (IMC).

2.3 Electrical Cables and Connectors (Module 7.4, 7.5, 7.8)

This is a core area for B2 certifying staff, covering the handling, repair, and installation of wiring and connectors.

Wire Inspection and Damage:
Insulation Damage: Brittle, cracked, or chafed insulation is a serious defect. It can lead to short circuits, arcing, and fires. The correct action is to evaluate the damage against the limits in the AMM or SWPM. If the damage exceeds the limits, the wire must be repaired or replaced. Temporary fixes like tape are not acceptable.
Chafing: A wire bundle chafing against a bracket is a hazard. The correct action is to eliminate the chafing by repositioning the bundle and/or adding protective sleeving or clamps.
Wire Repair and Splicing:
Damaged Insulation: If a wire has damaged insulation near the end, the correct action is to cut the wire back to clean insulation and re-strip it to the correct length. Using a larger connector or heat shrink does not address the potential for short circuits.
Shielded Cable Splices: When splicing a shielded cable, a shielded splice must be used to maintain the integrity of the shield and prevent electromagnetic interference (EMI). Pigtails (a short length of wire connecting the shields) are not preferred because they increase inductance and reduce shielding effectiveness.
Connector Installation and Repair:
Crimping: Crimping is a critical process for creating reliable electrical connections. It requires the specific crimp tool and die specified for the connector type. Using a similar tool from another manufacturer is not permitted, as the crimp profile may be incorrect. The tool must also be calibrated.
Soldering: Proper soldering technique is essential. A temperature-controlled iron set between 300°C and 350°C is typically used. Solder should be applied to the heated joint, not to the iron tip, to ensure proper wetting and avoid cold joints. A cold solder joint (dull, rough surface) is unreliable and must be re-done by removing the old solder completely and re-soldering.
Connector Hardware: Any damage to connector hardware, such as a cracked backshell, is not acceptable. A cracked backshell can compromise shielding, strain relief, and environmental sealing and must be replaced.
Cable Ties and Lacing:
Cable Ties: Overtightened cable ties can compress and damage wire insulation. They must be cut and replaced with properly tensioned ties.
Lacing Tape: Lacing tape is cut with diagonal cutters, taking care not to damage the wire insulation. Reusing lacing tape is not allowed as it may be weakened.
Wire Routing and Separation:
Bend Radius: The minimum bend radius for a coaxial cable is typically 10 times the cable diameter for flexible cables. Exceeding this can cause impedance changes and damage the dielectric.
Separation from Fluid Lines: Wire bundles must be kept at least 2 inches (50.8 mm) away from hydraulic lines unless a protective sleeve or clamp is used. This prevents chafing and fluid contamination. The separation from fuel lines is typically 6 inches (152.4 mm).
Thermal Management: When installing an avionics unit on a heat sink, thermal grease is used to fill air gaps between the component and the heat sink, improving thermal conductivity. It is not an adhesive.

2.4 Test Equipment and Measurement (Module 7.6)

A B2 technician must be proficient in using various test instruments correctly.

Multimeter:
Continuity: A closed switch contact should have very low resistance, typically less than 0.5 ohms. Higher resistance indicates a poor contact. Infinite resistance indicates an open circuit.
Resistance Measurement: Resistance measurements must be taken on a dead circuit to avoid damaging the meter and to obtain a valid reading. Before measuring resistance in a circuit with capacitors, the capacitors must be discharged and verified to be safe.
Insulation Resistance Tester (Megger):
Voltage Setting: For a 28 V DC aircraft system, the correct test voltage is 250 V DC. Higher voltages (500 V or 1000 V) are for 115/200 V AC systems and may stress the insulation.
Acceptable Values: For a 28V DC system, a minimum insulation resistance of 1 megohm is generally acceptable. A measurement of 5 megohms would be considered satisfactory.
Oscilloscope:
Probe Selection: For floating signals, such as a tachometer generator, a differential probe is required to avoid ground loops and measure the true signal. Connecting to aircraft ground may introduce noise.
Frequency Counter:
Input Coupling: When measuring the frequency of an AC signal (e.g., 400 Hz power), the correct input coupling setting is AC coupling. This blocks any DC offset and measures only the AC component.
Specialised Test Equipment:
Synchro Test Set: Verifying the output of a synchro transmitter requires a specialised synchro test set to measure the phase and amplitude of the three stator voltages. A multimeter cannot measure phase relationships.
Pitot-Static Test Set: Used for functional testing and leak checks of the pitot-static system. The procedure involves applying a regulated pressure/vacuum and monitoring the decay over a specified period.
Ramp Test Set (e.g., for Transponder/TCAS): Used to interrogate the system and verify its response.

2.5 Functional Testing (Module 7.5)

A functional test verifies that a system operates correctly against the manufacturer's specified performance criteria after maintenance.

Purpose: The primary purpose is to confirm that the system, as installed, meets its design and performance specifications.
General Procedure:

Preparation: Before starting, ensure the aircraft is in a safe condition. This may involve coordinating with the flight crew, ensuring the aircraft is on jacks, or setting specific aircraft attitudes (e.g., level for AHRS tests).
Power-Up: Verify power is applied to the system (e.g., check circuit breakers).
Test Execution: Follow the AMM procedure, using the specified test equipment.
Observation: Observe the system's response and compare it to the specified limits.
Troubleshooting: If the test fails, follow a logical troubleshooting process. For example, if a system is unresponsive, the first check is to verify power is applied. If a transponder reply has incorrect pulse spacing, the fault is likely in the transponder's timing/encoder circuitry, not the antenna.

Specific Test Considerations:
Pitot-Static Leak Test: Before applying pressure, ensure all connections are secure and the system is properly grounded. If a reading is slowly drifting, pause, verify all physical connections, consider environmental effects (thermal expansion), and repeat the test to ensure stability.
Weather Radar: A constant return when the antenna is pointed at the sky suggests a receiver issue, often a faulty limiter causing saturation. A wet radome or antenna position would cause different effects.
Windshear System: These systems are often inhibited on the ground to prevent nuisance warnings. The test procedure may require overriding this inhibition.
ELT Test: Must be coordinated with ATC to avoid false alerts. Tests should be limited to the first 5 minutes of the hour and for no more than 3 seconds.
ILS Localizer: An off-center antenna installation causes a bias in the deviation indication.
VSWR: A high VSWR (e.g., 2.5:1 vs. a 1.5:1 limit) can be caused by cable damage, moisture, or poor connections. The first step is to inspect the cable and connectors before replacing the antenna.

3. Important Formulas and Regulations

3.1 Key Formulas

Peak Voltage (V_peak): For a sinusoidal AC voltage, \( V_{peak} = V_{rms} \times \sqrt{2} \).
Example: For a 115V RMS 400Hz bus, \( V_{peak} = 115 \times 1.414 \approx 163V \).
Minimum Bend Radius: For flexible coaxial cable, the minimum bend radius is typically 10 times the cable diameter.

3.2 Key Regulations and References

Regulation (EU) No 1321/2014, Annex III (Part-66): Defines the requirements for the certification of maintenance staff. Appendix I outlines the basic knowledge syllabus, including Module 7A.
Regulation (EU) No 1321/2014, Annex II (Part-145): Defines the requirements for maintenance organisations. Key clauses include:
Part-145.A.40: Equipment, tools, and material requirements. Tools must be controlled and calibrated.
Part-145.A.42: Acceptance of components and materials. Only approved parts may be installed.
Part-145.A.45: Maintenance data. Maintenance must be performed in accordance with approved data.
Part-145.A.50: Certification of maintenance. A certificate of release to service (CRS) is required after maintenance.
Part-145.A.55: Maintenance records. All maintenance must be recorded.
Aircraft Maintenance Manual (AMM): The primary source of approved data for specific maintenance tasks. It contains procedures, torque values, tolerances, and test limits.
Standard Wiring Practices Manual (SWPM): A manual (often ATA Chapter 20) that provides standard procedures for wiring, connectors, and cable repairs.
Minimum Equipment List (MEL): Specifies the equipment that may be inoperative for dispatch, along with any conditions or limitations.

4. Common Relationships Between Concepts

Safety and Procedure: The requirement to de-energise a circuit is directly linked to the safety of the technician. The requirement to use a specific crimp tool is linked to the reliability of the connection and the prevention of future failures.
Tool Accuracy and Data Compliance: The accuracy of a torque wrench or a test set must be equal to or better than the accuracy specified in the AMM. Using a less accurate tool means the task is not being performed in accordance with the approved data.
Bonding Resistance and Corrosion: Corrosion on a bonding braid increases its resistance. A visual inspection for corrosion is a precursor to an electrical measurement. The measurement determines if the bond is still within the AMM limit.
Test Failure and Diagnosis: A failed functional test (e.g., high VSWR, biased ILS, non-responsive TCAS) initiates a logical troubleshooting process. The symptom often points to a specific cause (e.g., cable damage, antenna misalignment, lack of power).
Defect Discovery and Documentation: Any defect found, whether it is chafed wiring, a missing static wick, or a cracked backshell, must be recorded and reported. The subsequent action (repair, replacement, or deferral) is a decision based on approved data and the defect's severity.

5. Typical Exam Focus Points

First Actions: Questions often ask for the "first" or "correct initial" action. The correct answer is almost always to de-energise/isolate the system (for electrical work) or to record and report the defect (for inspection findings).
Use of Approved Data: The concept that all work must be done per the AMM/SWPM is heavily tested. This includes using the correct tools, parts, and procedures. Substituting tools or parts without approval is always incorrect.
Specific Limits and Tolerances: Be prepared to recall common values:
Torque wrench general accuracy: ±10%.
Soldering iron temperature: 300°C to 350°C.
Closed switch contact resistance: < 0.5 ohms.
Minimum insulation resistance for 28V DC systems: 1 megohm.
Megger test voltage for 28V DC systems: 250V DC.
Minimum bend radius for coax: 10 times the diameter.
Wire bundle separation from hydraulic lines: 2 inches.
Bonding and Earthing: Understand the purpose of bonding, the typical resistance limits (e.g., 0.0025 ohms for critical bonds), and the correct procedure for testing (4-wire Kelvin method). Know that corrosion is a cause of high resistance and that the first step is to measure the resistance.
Electrical Wiring and Connectors: Understand the hazards of damaged insulation, overtightened cable ties, and improper crimping/soldering. Know the correct repair procedures.
Functional Testing: Understand the purpose of functional tests and the general principles of test execution. Be aware of specific safety precautions for tests like weather radar and ELT.
Safety: Questions on safety precautions are common, including ESD, radiation hazards, and the handling of hazardous materials like lithium batteries.
Measurement Principles: Know the correct way to use a multimeter (dead circuit), a megger (correct voltage), and an oscilloscope (differential probe for floating signals). Understand the relationship between RMS and peak voltage.