Aircraft Wiring, Bonding and Testing
Aircraft wiring, bonding, and testing
This domain tests the physical installation: wire selection and sizing, routing and support, connectors and terminations, bonding and grounding, insulation and continuity testing, and documentation of test results. The exam expects practical judgement about airworthiness and EWIS (electrical wiring interconnection system) care.
Core rules
- Select wire by current capacity, voltage drop, temperature rating, fluid resistance, and bundle derating. A wire must be protected by a breaker or fuse rated at or below its derated capacity.
- Route wiring away from heat, moving parts, fuel and hydraulic lines, and sharp edges. Use approved clamps and standoffs, respect bend radius, and protect against chafing. Cable ties are not a substitute for proper supports.
- Terminations must use the specified crimp tool and terminals. Pull-test and inspect crimps. Soldered joints are not a substitute for approved splices and can create stiff, fatigue-prone joints.
- Bonding provides a low-resistance return path and lightning or static protection. Bonding resistance limits are specified by the manufacturer or approved data; test with a bonded-point tester or calibrated milliohmmeter.
- Insulation resistance is measured with a megohmmeter at the specified test voltage. Isolate and disconnect sensitive avionics before testing. Continuity tests prove the path, not the insulation.
- Record test values, instrument calibration, and any defects.
Worked example
A 28 V DC feed runs 15 ft from the bus to a 20 A load and back, so the round-trip length is 30 ft. 10 AWG copper is about 1.0 mΩ/ft, giving 30 mΩ. At 20 A, voltage drop is 0.020 × 30 = 0.60 V, or 2.1% of 28 V. That is acceptable for many 28 V loads, but check the manufacturer's limit. If the wire were 14 AWG (about 2.5 mΩ/ft), drop would be 1.5 V (5.4%), likely excessive. The breaker still must not exceed the wire's derated rating.
Traps
Megohmmeter testing with avionics connected; using a general ohmmeter for bonding without calibration; over-tightening clamps; mixing wire types; and signing off a test without recorded values.