Aviation Maintenance Glossary

The 100-hour inspection is required when an aircraft is operated for hire — including flight instruction for hire — every 100 hours of time in service. Its scope is identical to an annual inspection, but it may be performed and approved by an A&P mechanic without Inspection Authorization.

An A&P is an FAA-certificated mechanic holding both Airframe and Powerplant ratings, authorized to perform and approve maintenance, repairs, and 100-hour inspections on civil aircraft. Approving an annual inspection additionally requires Inspection Authorization (IA).

An Airworthiness Directive (AD) is a legally enforceable rule issued by the FAA to correct an unsafe condition found in an aircraft, engine, propeller, or appliance. Compliance is mandatory: each AD specifies which serial numbers or models it applies to, what must be done, and by when.

The annual inspection is a complete inspection of an aircraft required within the preceding 12 calendar months for most US-registered aircraft, performed to the scope of 14 CFR Part 43 Appendix D and signed off by a mechanic holding Inspection Authorization (IA).

An ELT is a crash-activated radio beacon required in most US civil aircraft that transmits a distress signal to aid search and rescue. US regulations require inspection every 12 calendar months and battery replacement after 1 hour of cumulative use or when 50% of the battery's useful life has expired.

FAA Form 337 documents a major repair or major alteration to an airframe, engine, propeller, or appliance. One copy goes to the FAA and one stays with the aircraft's permanent records, making 337s a key part of an aircraft's paper trail.

A ferry permit — formally a Special Flight Permit — is an FAA authorization to fly an aircraft that doesn't currently meet all applicable airworthiness requirements, for a specific purpose such as flying to a maintenance facility. It's issued for defined flights with conditions and limitations.

Hobbs time is elapsed clock time recorded by a Hobbs meter, which typically runs whenever the engine is running (often triggered by oil pressure or the master switch). It counts real hours and tenths — taxi, run-up, and flight all accumulate at the same rate.

Inspection Authorization (IA) is an additional FAA authorization held by experienced A&P mechanics that permits them to approve annual inspections and sign off major repairs and alterations on FAA Form 337. Every annual inspection must be approved by an IA or an authorized repair station.

An N-number is the registration mark of a US civil aircraft, always beginning with the letter N followed by 1–5 characters. Every N-number is unique: at any moment it is assigned to one aircraft, reserved by one holder, or available to claim.

A pre-buy (pre-purchase) inspection is an examination of an aircraft and its records performed for a prospective buyer before purchase. It has no FAA-defined scope — the buyer sets it — and typically combines a physical inspection with a records review covering logbooks, AD compliance, and damage history.

A progressive inspection breaks the complete aircraft inspection into scheduled segments performed throughout the year instead of one large annual event. It's an FAA-approved alternative under 14 CFR 91.409(d), used mainly by high-utilization operators to reduce downtime.

A service bulletin is a notice from an aircraft, engine, or component manufacturer describing recommended inspections, modifications, or product improvements. For most privately operated aircraft, service bulletins are advisory unless an Airworthiness Directive or the aircraft's own maintenance program makes them mandatory.

SMOH stands for Since Major Overhaul: the number of engine hours accumulated since the engine's last major overhaul. A listing that reads "1,200 SMOH" means the engine has run 1,200 hours since it was last completely overhauled.

A Supplemental Type Certificate (STC) is FAA approval for a modification to an aircraft, engine, or propeller that changes it from its original type design — for example an engine upgrade, avionics installation, or tip tanks. The STC is the approved data that makes such an alteration legal to install.

STOH stands for Since Top Overhaul: engine hours since the cylinders were overhauled or replaced without splitting the crankcase. A top overhaul addresses the cylinders, pistons, and valves — not the crankshaft, camshaft, or bearings — so it does not reset SMOH.

Tach time is engine time recorded by the tachometer, accumulating in proportion to engine RPM. It's calibrated to advance at one hour per clock hour only around cruise RPM — at lower power settings it accrues more slowly — which makes it the usual reference for engine maintenance intervals.

TBO (Time Between Overhauls) is the engine manufacturer's recommended interval, in flight hours and calendar years, between major overhauls — commonly 1,800–2,200 hours or 12 years for piston engines. For most privately operated (Part 91) aircraft, TBO is a recommendation, not a legal limit.

A Type Certificate Data Sheet (TCDS) is the FAA document that records the certified design data for an aircraft, engine, or propeller type: approved models, engine and propeller options, operating limitations, and required placards. It defines what the product's approved configuration is.

TTAF (Total Time Airframe) is the total number of flight hours accumulated by the airframe since it left the factory. Unlike engine times, TTAF never resets — it is the aircraft's lifetime odometer.