8.6 Engine Inspection, Records, Compression, Borescope, and Airworthiness

The Differential Compression Test

The differential compression test is the standard cylinder-sealing check on reciprocating engines. The piston for the cylinder being tested is set at top dead center on the compression stroke, and a two-gauge tester applies shop air through a calibrated orifice so that a regulated 80 psi is supplied to the cylinder. The cylinder pressure gauge then shows how much of that pressure the cylinder holds, and the reading is judged against the minimum specified for the engine being tested in the manufacturer's procedure, rather than one fixed number for every engine.

Where the leaking air escapes localizes the fault:

Test a warm engine when the procedure calls for it, control the propeller throughout (the prop will try to rotate under 80 psi), and never condemn a cylinder on one number — staking the valves and retesting, plus borescope evidence, give the real picture.

Borescope, Records, and TBO

The borescope inspection provides direct visual evidence a compression test cannot: cylinder-wall scoring, burned or "tuliped" valves, detonation signatures, cracks, corrosion, deposits, and FOD. It complements compression data — a marginal compression number with clean valves and good walls reads differently than the same number with a burned valve. A borescope image, however, never overrides an approved-data rejection criterion: if the data requires removal for a given finding, the image supports that call.

Time between overhaul (TBO) is the manufacturer's recommended operating time before overhaul — typically 1,200 to 2,400 hours for piston engines and 3,500 hours or more for turbines (with a hot-section inspection at mid-life). TBO is a recommendation, not a regulatory limit, for Part 91 private operation; Part 135 and 121 operators must comply with their approved maintenance programs. "On-condition" maintenance can extend service when condition monitoring supports it.

Overhaul scope varies: a top overhaul addresses the cylinders (pistons, rings, valves) without splitting the case, while a major overhaul disassembles the entire engine, inspects/replaces parts to new or service limits, and re-establishes baseline time.

Records and Airworthiness Decisions

The ACS lists engine inspection under 14 CFR part 43 and part 91, covering life-limited parts, special inspections, approved data, service information, airworthiness directives (ADs), type certificate data sheets (TCDS), and maintenance records. A clean engine with missing required records can still fail the airworthiness decision. A powerplant records review confirms engine identity, total time/cycles, time since overhaul, life-limited-part status, AD compliance, required-inspection status, and proper entries.

Special inspections are event-driven: a propeller strike or sudden stoppage, overspeed, overtemperature, lightning strike, hard landing, or ingestion event requires the applicable event-specific instructions before the engine is serviceable — regardless of how it sounds. Engine mounts and hardware are part of the boundary because they carry loads into the airframe; cracked mounts, deteriorated isolators, elongated holes, missing safetying, or wrong hardware can make a healthy engine unsafe.

A complete inspection joins condition, records, and procedure into one return-to-service judgment.

Inspection Authority, Approved Data, and the Maintenance Record Entry

Under 14 CFR part 65, the holder of a mechanic certificate with the appropriate rating may approve a powerplant for return to service after maintenance, but a 100-hour or annual inspection has a higher bar: for an individual mechanic the annual must be signed by an Inspection Authorization (IA) holder, while a 100-hour inspection may be returned to service by an appropriately rated certificated mechanic.

Both inspections use the scope and detail of 14 CFR part 43, Appendix D, and the engine portion specifically calls for examining the engine and nacelle for cracks and security; studs and nuts for improper torque and obvious defects; the internal engine condition via the cylinder compression check (and the reason given for poor compression); and the engine mount for cracks and looseness.

It also covers flexible vibration dampeners; engine controls for defects, improper travel, and improper safetying; lines, hoses, and clamps for leaks, condition, and looseness; the exhaust stacks for cracks, defects, and improper attachment; and accessories for apparent defects in security of mounting.

The airworthiness decision is anchored in approved or acceptable data. Approved data includes the type certificate data sheet (TCDS), FAA-approved sections of manufacturer manuals, airworthiness directives (ADs), Supplemental Type Certificates, and Form 337 field approvals; manufacturer service bulletins and service letters are not required unless an AD refers to them or the authorized inspection program requires them, and only FAA-approved service bulletins count as approved data.

Life-limited parts (certain crankshafts, turbine disks, blades) must be retired at their published limit and cannot be "inspected past" it. The mechanic must verify AD compliance — recurring ADs in particular reset the clock and must be re-accomplished at their interval.

Use 14 CFR 43.9 for ordinary maintenance entries and 14 CFR 43.11 for required inspection entries. The entry states a description of the work (or reference to acceptable data), the date completed, and the signature, certificate number, and kind of certificate of the person approving the return to service. For inspections it also states the aircraft total time in service and whether it was found airworthy. An accurate, traceable record is itself part of airworthiness — an engine in perfect mechanical condition is not legally airworthy if the required entry, AD compliance, or life-limited-part tracking is missing.

This is why every chapter of powerplant study ends at the same place: condition plus records plus approved data equals one defensible return-to-service judgment.