Learn what aircraft on ground (AOG) means, what causes AOG status, how the MEL affects dispatch and what must happen before the aircraft flies.
Aircraft on ground (AOG) means an aircraft cannot be released for flight because a technical defect, damage or unmet airworthiness requirement needs urgent attention. In aviation, AOG is primarily a maintenance and logistics status, not simply a description of an aircraft parked on the ground. It triggers priority engineering, parts and support.
In our Aviation & Real-World Flying coverage, the key distinction is dispatchability. A serviceable aircraft may be sitting on the apron, while an AOG aircraft is unavailable for its intended flight until the problem is rectified or an approved deferral permits dispatch. AOG is common industry terminology rather than one globally identical legal classification, so operators and maintenance organisations may define their internal thresholds differently.
When is an aircraft classed as AOG?
An aircraft is classed as AOG when its operator cannot safely and lawfully dispatch it in its present condition.
Typical causes include:
- No-go technical faults involving an engine, flight controls, brakes, avionics or systems such as those covered in our explanation of how landing-gear components operate.
- Aircraft damage from a bird strike, lightning strike, ground vehicle, hard landing or foreign object.
- Required maintenance or inspections that are overdue or cannot be completed before departure.
- Missing support, such as an unavailable replacement part, specialist tool or appropriately authorised engineer.
- Unresolved documentation that prevents the required maintenance release or proof of airworthiness.
The wording of a cockpit alert does not determine AOG status by itself. Our breakdown of ELAC 2 fault and NO GO indications shows why fault persistence, redundancy and approved dispatch conditions all matter.
Is every grounded or delayed aircraft AOG?
No; AOG has a narrower maintenance meaning than simply being grounded, delayed or out of service.
| Situation | Normally AOG? | Reason |
|---|---|---|
| Serviceable aircraft parked between flights | No | It remains available for dispatch. |
| Flight cancelled because of weather, crew or airport closure | Usually no | The disruption is operational rather than an aircraft defect. |
| Defect deferred in accordance with the MEL | Not necessarily | The aircraft may remain dispatchable if every stated condition is met. |
| Unplanned no-go defect | Yes | The aircraft cannot be released until maintenance acts. |
| Planned heavy maintenance | Usually no | It is unavailable, but this is scheduled rather than an urgent AOG event. |
An aircraft can become AOG at its home base as well as at a remote airport. The term does not require it to be stranded away from its normal maintenance facilities.
Can an aircraft fly with an inoperative component?
An aircraft can fly with certain equipment inoperative, but it must first satisfy an approved dispatch basis and cease to be AOG.
The operator's Minimum Equipment List (MEL) may permit a defect to be deferred subject to specified maintenance actions, crew procedures, placarding, operating restrictions and a repair interval. A Configuration Deviation List may provide similar relief for certain missing external parts within defined limits. Neither document is blanket permission to ignore a fault.
An inoperative APU, for example, does not automatically make every aircraft AOG. Dispatch may be permitted when ground power, air supply, engine-start arrangements, airport capability and the applicable MEL conditions are satisfactory; our guide to how an APU supports ground operations and engine starting explains why those conditions matter. If required equipment must be operative and no approved relief applies, the aircraft remains AOG.
Who decides that an aircraft is AOG?
The operator's maintenance control function normally classifies the event, while appropriately authorised maintenance personnel assess and certify the technical action under the applicable regulations.
Pilots report the defect and the captain retains final authority over accepting the aircraft. An aircraft dispatcher may re-plan the flight, assess operational restrictions and coordinate disruption, but does not sign the maintenance release; see our explanation of the operational role of an aircraft dispatcher.
What happens after an AOG report?
An AOG response moves from an accurate defect report to a documented return to service.
- Record the defect. Maintenance needs the aircraft registration, location, exact indications or logbook wording, circumstances, actions already taken and intended departure time.
- Establish dispatch status. Maintenance control checks approved technical data, the MEL or other permitted relief and any operational limitations.
- Mobilise support. The operator arranges engineers, parts, tooling and transport, often using an urgent AOG parts order.
- Rectify or defer correctly. Personnel follow approved troubleshooting and repair procedures, or complete every action required for a legal deferral.
- Test, document and release. Required inspections and functional checks are completed before an authorised person certifies the work and the crew accepts the aircraft.
A mistake we see often in simulation discussions is treating a successful system reset as proof of repair. A reset may form part of an approved procedure, but the disappearance of a cockpit message does not close the defect by itself; the required tests, records and certification still govern.
How long does an AOG event last?
An AOG event lasts until the aircraft is repaired or an approved deferral makes it dispatchable, with no fixed minimum or maximum duration.
A straightforward fault may be cleared after diagnosis, corrective action and testing. Structural damage, an uncommon component or a remote location can keep the aircraft grounded for days. AOG priority accelerates support but does not waive maintenance procedures, customs controls, parts-shipping restrictions or certification requirements.
Does AOG mean the aircraft is unsafe or written off?
AOG means the aircraft must not be dispatched in its present state; it does not mean the aircraft is permanently damaged or destined for retirement.
The underlying cause may be serious, but it can also be a failed sensor, a required inspection or a defect awaiting an ordinary replacement part. The status is deliberately conservative: the aircraft stays out of service until its airworthiness and dispatch requirements are satisfied.