APU borescope inspection: when it is due and what it finds

When it is due

• At the interval in the operator's program, or at a lease transition and pre-purchase.
• After an exceedance: overtemperature, hot or hung start, overspeed.
• After an event: ingestion, suspected foreign object damage, abnormal noise or vibration.
• When APU EGT or performance trends deteriorate.

What the inspector looks for

Compressor blades for tip rub, nicks, dents and erosion; the combustor for cracks, burn-through and carbon; turbine nozzles and blades for cracks, erosion, coating loss and missing material. Each finding is compared with the limit in the APU maintenance manual.

On wing, in the tail

Access is through the APU compartment doors; the inspection usually takes an hour or two once the unit has cooled. Findings beyond limits mean an APU removal, which on a narrowbody is a same-day job if a spare is available.

The report

Port-by-port images, each finding against its limit, serviceable or unserviceable status, the unit's identity and hours and cycles, and the inspector's signature. InspectView files the APU inspection like any engine inspection, with the same colour-coded status and the same sign-off rule.