Maintenance Practices
What the module tests
Maintenance practices covers safety, workshop tools, engineering drawings, fits and clearances, torque, inspection, NDT, cleaning, bonding, rigging and release procedures. It tests whether you can work to approved data and record the result correctly.
Core rules
- Clearance = hole size minus shaft size; maximum clearance uses the largest hole and smallest shaft.
- Torque wrench readings change when an extension alters the effective lever length; use the approved correction formula.
- Calibrated tools must be in date; a torque wrench is not a breaker bar.
- Lockwire must be installed so that it tends to tighten the fastener, with the correct pigtail.
- NDT methods detect different defects: dye penetrant for surface-breaking cracks, eddy current for surface/near-surface in conductive material, ultrasonic for internal flaws.
- After maintenance, the aircraft is released only by authorised certifying staff against approved data.
Worked example
A hole is specified 25.00/25.02 mm and a shaft 24.97/24.99 mm. Find the minimum and maximum clearance. Minimum clearance = smallest hole minus largest shaft = 25.00 - 24.99 = 0.01 mm. Maximum clearance = largest hole minus smallest shaft = 25.02 - 24.97 = 0.05 mm. The fit ranges from 0.01 mm to 0.05 mm, so it is a clearance fit throughout.
Traps
Subtracting the wrong extremes; ignoring torque-wrench extension correction; using an out-of-calibration instrument; assuming lockwire direction is cosmetic; confusing surface and internal NDT methods; signing for work not performed.
Exam approach
For fits, write the four extreme combinations before subtracting. For procedures, ask what the approved data requires, then what the record must show.