Scheduled vs unscheduled borescope inspections: AOG, bird strike, EGT margin

Scheduled

The operator's maintenance program sets borescope tasks at intervals in hours or cycles, and the inspection is planned into a line or base check with the cowls already open. The scope is the AMM task; the outcome is a report and, occasionally, a repetitive inspection.

Unscheduled triggers

• Bird strike or other ingestion, or foreign object debris found on the ramp.
• Compressor surge or stall reported by the crew.
• EGT exceedance, hot start or hung start.
• Abnormal vibration, or a step change in trend data.
• Metal found in the oil or the chip detector, where the AMM calls for a borescope.
• A finding on the other engine that the manufacturer asks to be checked fleet-wide.

The AOG inspection

The question is narrow: can this aircraft be released, with or without limitations, or does the engine come off? The AMM troubleshooting task says what to inspect — usually the stages at risk from the event, not the whole engine — and the inspector reports against the limits for those stages. Done well, the aircraft is back in service the same day with the finding documented; done badly, the aircraft sits for a second inspection because the first one did not look in the right place.

What to have ready

The event description from the crew, the engine's identity and status, the last borescope report, access to the aircraft and the engine made safe, and a clear line to the engineering authority that will make the release decision.

How independent inspectors fit

Airlines use their own technicians for most unscheduled work. Independent inspectors are called when the in-house team is elsewhere, when a lessor wants its own eyes on an event, or when the aircraft is away from base.