Aviation Operations

An A Check is the lightest scheduled maintenance inspection performed on a commercial aircraft, typically every 400–600 flight hours or 1–2 months, whichever comes first. It takes 6–24 hours and is usually carried out overnight at the airline's base or at a line maintenance facility.

A B Check is a scheduled maintenance inspection historically performed every 6–8 months, taking 1–3 days and covering more detailed systems inspections than an A Check. Note: B Check as a distinct maintenance level has been phased out by many modern aircraft programmes and OEMs, with its tasks absorbed into enhanced A Check or C Check packages.

A C Check is a comprehensive scheduled base maintenance inspection performed approximately every 20–24 months or 6,000 flight hours, requiring 1–2 weeks in a maintenance hangar. It covers in-depth structural inspection, system overhaul and accumulated deferred maintenance.

A D Check is the most comprehensive and expensive scheduled aircraft maintenance event, performed every 6–12 years. The aircraft is essentially stripped to its airframe for complete structural inspection, taking 4–6 weeks and costing $4–6 million or more for a widebody aircraft.

A Heavy Maintenance Visit (HMV) is a scheduled base maintenance event in which an aircraft is withdrawn from service and taken to a hangar for deep structural inspection, system overhaul and modification work that cannot be performed during line maintenance. C Checks and D Checks are the primary heavy maintenance events.

Line maintenance covers all aircraft maintenance tasks performed between scheduled flights — pre-flight inspections, transit checks, overnight checks and minor defect rectification — to keep the aircraft in continuous service without hangar withdrawal.

MRO (Maintenance, Repair and Overhaul) is the industry term for the full spectrum of aircraft maintenance activities — from routine overnight checks to complete structural overhauls — carried out to keep aircraft airworthy and compliant with regulatory requirements.

A-CDM is an operational framework, developed by Eurocontrol, in which airport operators, airlines, ground handlers, and air traffic control share real-time data on each flight's turnaround so they can jointly optimize aircraft turnaround and departure sequencing.

GSE refers to the vehicles and equipment used to service an aircraft during turnaround, including pushback tugs, belt loaders, catering trucks, fuel bowsers, and de-icing vehicles.

An ADL (Additions and Deletions List) is the update message transmitted after the initial PNL to communicate last-minute passenger list changes — late check-ins, no-shows and additional passengers — to the ground handling agent, keeping the list current until gate closure.

A PNL (Passenger Name List) is the pre-departure message transmitted by the airline to the ground handling agent, handling company or partner airline, listing all passengers on the flight with their names, seat numbers and SSR information.

The AOCC is an airline's centralized, around-the-clock facility that monitors the day's flight operations in real time and makes the operational decisions needed to manage delays, diversions, and the reassignment of crews and aircraft.

A diversion is when a flight is redirected to land at an airport other than its originally planned destination, typically because of weather, a medical emergency, a mechanical issue, or another operational reason.

IROPS is airline shorthand for significant, unplanned disruptions to the flight schedule — caused by events such as severe weather, IT system outages, strikes, or widespread mechanical issues — along with the recovery processes airlines use to restore normal operations.

A BRS (Baggage Reconciliation System) is the security system that cross-references gate boarding data against checked-baggage records to identify passengers whose bags are on board but who have not presented at the gate. Mandatory for all international flights under ICAO and EU regulations.

Block time (also block-to-block time) is the total elapsed time from the moment an aircraft leaves the departure gate (off-block) to the moment it arrives at the destination gate (on-block). It includes taxi, flight time and any airborne holding, and is the basis for schedule planning, crew duty calculation and airline cost accounting.

ETA (Estimated Time of Arrival) is the current best estimate of when an aircraft will touch down or arrive on stand at the destination airport. It updates continuously based on actual departure time, airspeed, wind and routing, and is distinct from the scheduled arrival time (STA).

ETD (Estimated Time of Departure) is the current best estimate of the time at which an aircraft will push back from the gate or leave the stand. It is a dynamic value that updates as operational conditions change, and is distinct from the scheduled departure time (STD).

De-icing removes existing frost, ice or snow from an aircraft before departure, while anti-icing separately protects a cleaned surface from new contamination for a defined holdover time, using four standardised fluid types (I–IV) matched to weather conditions and aircraft type.

Ground handling encompasses all services provided to an aircraft between arrival and departure — passenger handling, baggage, ramp services (fuelling, towing, de-icing), cargo, catering, cleaning and technical support — typically provided by a specialised ground handling company contracted by the airline.

A jet bridge (also called a passenger boarding bridge or jetway) is the enclosed, moveable connector linking an airport terminal gate to the aircraft door, allowing passengers to board and disembark without exposure to the elements or the ramp environment.

An aircraft turnaround is the ground process between an aircraft's arrival at stand and its subsequent departure: the coordinated sequence of deplaning, cleaning, catering, fuelling, technical inspection, baggage offload/load and boarding that must be completed within a target time to maintain schedule.

A go-around is a routine, deliberate manoeuvre in which a crew discontinues an approach and climbs away instead of landing, triggered by anything from an unclear runway to an unstabilised approach — a planned, trained-for outcome rather than an emergency.

A NOTAM (Notice to Airmen, now officially Notice to Air Missions) is a notice distributed by official channels to alert pilots and flight operations personnel to information essential for safe flight — airspace restrictions, navigational aid outages, runway closures and hazardous conditions.

QNH is the altimeter setting that, when applied to the aircraft's barometric altimeter, causes it to indicate the aircraft's altitude above mean sea level (AMSL). It is derived from the local atmospheric pressure reduced to sea level and is the standard altimeter setting used below the transition altitude.

A waypoint is a defined geographical position — identified by a five-letter name code (e.g. LOGAN, TULIP) or a VOR/DME fix — used to define flight routes, airways, departure and arrival procedures, and to report position to ATC.

Zulu time is the aviation and military term for Coordinated Universal Time (UTC). All aviation operations — flight plans, ATC communications, NOTAMs, METARs and schedules — use Zulu time to eliminate confusion from time zones and daylight saving changes across global operations.

MCT (Minimum Connecting Time) is the shortest transfer time in which a passenger can safely make a connecting flight at a given airport without missing the onward departure. Values are set by IATA and airport operators and vary by transfer type (domestic-domestic, international-domestic, etc.).

The MEL is the airline-specific, regulator-approved list defining which aircraft equipment may be inoperative on a given flight and under what conditions, derived from and never more permissive than the manufacturer's Master Minimum Equipment List (MMEL).

MTOW is the certified maximum total weight — aircraft, fuel, passengers, baggage and cargo combined — at which an aircraft is permitted to attempt takeoff, distinct from the separate maximum ramp, landing and zero fuel weight limits that govern other phases of flight.

OTP (On-Time Performance) is the key performance indicator measuring the percentage of flights that depart or arrive within a defined threshold — typically 15 minutes — of their scheduled time. It is the primary punctuality metric used by airlines, airports and regulators.

An airport slot is the permission granted by a slot coordinator for an airline to use an airport's full infrastructure — runway, terminal and apron — to arrive or depart at a specific date and time. Slots are allocated at Level 3 (fully coordinated) airports where demand exceeds capacity.

A squawk code is a four-digit code entered into an aircraft's transponder that identifies it to air traffic control radar, including three universally reserved emergency codes — 7500 (hijacking), 7600 (radio failure) and 7700 (general emergency).

A ULD (Unit Load Device) is a standardised container or pallet used to consolidate cargo, mail and baggage into a single unit for efficient loading into the lower hold of wide-body aircraft. ULDs allow ground equipment to handle large quantities of freight as a single, forklift-compatible unit.

X-ray inspection is the primary security screening method for carry-on baggage and checked luggage at airports, using X-ray imaging to detect prohibited items — weapons, explosives, liquids above limits — without opening bags. Required at all commercial airports under ICAO Annex 17 and national regulations.