# Glossary

Terms that appear across the aircraft pages, written for someone new to the industry. Linked automatically from the first use on each page.

Airworthiness and regulation

Airworthiness Directive (AD)

A legally binding order from a national regulator requiring specific action on a specific set of aircraft. Not advice. If your aircraft is in the effectivity, the work happens within the compliance time or the aircraft does not fly.

Service Bulletin (SB)

A manufacturer's instruction describing a modification, inspection or repair. Advisory on its own. A regulator can make one mandatory by issuing an AD that references it, which is the usual route from SB to AD.

Safety Information Bulletin (SIB)

EASA's non mandatory alert. It flags something operators should know about without compelling action.

Effectivity

The exact list of aircraft a document applies to, usually by serial number, sometimes narrowed further by part number or configuration. The first thing to check on any AD.

Compliance time

The deadline attached to an AD, expressed in calendar time, flight hours or cycles, or whichever comes first.

Type Certificate (TC)

The regulator's approval of an aircraft design. A Supplemental Type Certificate (STC) approves a modification to an already certified design.

Entry into Service (EIS)

The date the first example starts flying revenue passengers or cargo with an operator.

Operations and maintenance

Cycle

One takeoff and landing. Structures and hot section engine parts wear by cycle more than by hour, so short haul aircraft accumulate damage faster per flight hour than long haul ones.

Flight hour (FH)

Time in the air. Paired with cycles as the two axes almost every maintenance interval is written against.

Check (A, C, D)

Scheduled heavy maintenance, increasing in depth. An A check is a light inspection every few hundred hours. A C check takes the aircraft out of service for one to two weeks. A D check is a near teardown every six to twelve years.

MRO

Maintenance, Repair and Overhaul. Both the activity and the companies that do it.

AOG

Aircraft on Ground. Unserviceable and not earning. The industry's highest urgency state.

Time on wing

How long an engine stays installed before it has to come off for shop visit. A headline number for engine reliability and the core complaint behind several current powerplant problems.

Shop visit

An engine removal for overhaul. Expensive, slow, and the bottleneck behind much of the current groundings story.

Programs and production

Rate

Aircraft produced per month. "Rate 38" means 38 per month. The single most watched number in commercial aviation manufacturing.

Backlog

Firm orders not yet delivered. Divided by rate, it gives the years of production already sold.

Book to bill

Orders taken divided by aircraft delivered over a period. Above one means the backlog is growing.

ceo and neo

Airbus naming. "Current Engine Option" and "New Engine Option". The neo family reengined the A320 series from 2016. The same idea drives Boeing's MAX versus NG split.

White tail

A completed aircraft with no customer, either because an order was cancelled or the customer cannot take it.

Powerplant

GTF (Geared Turbofan)

Pratt and Whitney's PW1000G family. A reduction gearbox lets the fan and the low pressure turbine each spin at their own best speed, which improves efficiency. Powers the A220, A320neo, E2 and others.

LEAP

CFM International's competing engine. LEAP-1A on the A320neo, LEAP-1B on the 737 MAX, LEAP-1C on the COMAC C919.

HPT and LPT

High Pressure Turbine and Low Pressure Turbine. The HPT sits immediately behind the combustor in the hottest gas path, so its blades are the most life limited parts in the engine.

Powder metal

A manufacturing method for high strength turbine discs. Contamination in powder metal batches drove the large GTF inspection campaign that grounded a significant part of the A320neo fleet.

Aircraft designations

Family versus variant

The A320 family includes the A319, A320 and A321. The 737 MAX family includes the MAX 7, 8, 9 and 10. A variant is a specific length or configuration within a family.

Freighter suffixes

An F in the designation is a factory built freighter, as in the A350F or 777-8F. A converted passenger aircraft is a P2F or BCF, as in the 737-800BCF.

Scope clause

A clause in US airline pilot contracts limiting how many regional jets a mainline carrier's partners may fly, and capping them by seat count and weight. The reason the 76 seat market exists in the shape it does, and the reason the E175-E2 has struggled in the United States.