What is an AOG situation?
AOG means aircraft on ground — but the two-word definition tells you almost nothing useful. This guide covers what actually happens: how an AOG is declared, who becomes involved, the competing clocks that decide the outcome, and why most AOG events turn out to be logistics problems rather than engineering ones.
- AOG describes an aircraft that cannot fly until a fault is rectified — usually because a part is not available locally
- It is not simply a broken aircraft: it is a fault that cannot be deferred, where the fix is not on the shelf
- Several clocks run at once — crew duty hours, slots, curfews, stand availability — and the first to expire decides the outcome
- There are five realistic resolutions, and "fix in place" is by far the cheapest
- Most AOGs are caused by small components, not dramatic damage
The definition, and what it leaves out
AOG stands for aircraft on ground. It describes an aircraft that cannot fly until a fault is rectified — most often because a part is unserviceable and no replacement is on site.
That definition is accurate and almost useless, because it makes an AOG sound like a maintenance category. In practice it is closer to an organisational event: a status that pulls in a dozen people across several companies, each working to a different clock, until the aircraft is released to service.
What follows is what actually happens between the fault being found and the aircraft flying again.
How an AOG gets declared
An AOG rarely announces itself dramatically. It usually begins with something small and routine.
| Where it starts | What happens next |
|---|---|
| Walk-round inspection | An engineer or crew member spots damage or a leak during a pre-flight check |
| Crew report | A defect entered in the technical log after a flight, sometimes hours before anyone acts on it |
| System indication | A fault message that requires investigation before the next departure |
| Ground damage | Contact with ground equipment, a bird strike, tyre damage or foreign object damage |
| Maintenance finding | A scheduled check uncovers work nobody planned for |
The moment that matters is the one where an engineer establishes that the fault cannot be deferred. Many defects can be carried for a defined period under the operator's approved procedures. When a defect cannot be carried, and the part to fix it is not on the shelf, the aircraft is AOG.
This is the distinction most explanations miss. An AOG is not simply a broken aircraft. It is a broken aircraft where the fix is unavailable locally — which makes it, more often than not, a supply and transport problem wearing an engineering label.
Who gets involved, and what each of them wants
Understanding an AOG means understanding that several parties are working the same problem from different directions, and their priorities do not perfectly align.
| Party | What they are trying to achieve |
|---|---|
| Line engineer | Diagnose accurately, then get the part and the approval to fix it |
| Maintenance control | Decide whether to repair here, defer, or move the aircraft elsewhere |
| The AOG desk | Locate the part anywhere in the network or the market, and get it moving |
| Operations | Protect the schedule — possibly by swapping aircraft rather than waiting |
| Crew scheduling | Watch duty hours, because a fixed aircraft with an out-of-hours crew still cannot depart |
| Handling agent or FBO | Manage the stand, the passengers and the ground arrangements |
| Supplier or MRO | Confirm the part is serviceable, certified and released |
| Transport provider | Get the part from where it is to where the aircraft is, by a known time |
Why this matters commercially: the person who calls a courier is usually the AOG desk or the engineer, but the decision they are supporting is being made by operations. That is why an honest arrival time is worth more than a fast-sounding one — operations is deciding whether to wait or swap the aircraft, and a wrong estimate sends that decision the wrong way.
The clocks that are running
People outside aviation tend to picture one clock: how long the repair takes. There are several, and they run at different speeds.
| Clock | Why it constrains the outcome |
|---|---|
| Crew duty hours | Fixed limits. Exhaust them and the flight cannot operate even with a serviceable aircraft |
| Departure slot | Time-bound. Miss it and the next may be hours away |
| Airport operating hours | At noise-restricted airports there is a hard curfew regardless of readiness |
| Stand availability | A grounded aircraft occupies a stand others are scheduled to use |
| Onward rotation | The aircraft is expected somewhere else, and that flight is already sold |
| Passenger obligations | Delay thresholds trigger welfare and compensation duties |
The practical effect is that an AOG is often lost not because the repair was slow but because one of the other clocks expired first. A part arriving thirty minutes after the crew ran out of hours is, operationally, a part that did not arrive.
How AOG events actually get resolved
There are only a handful of outcomes, and knowing them explains why speed of transport matters so much.
| Resolution | When it is used | Cost |
|---|---|---|
| Fix in place | Part available or obtainable in time; engineer and approval on site | Lowest — the aircraft stays in its programme |
| Defer and fly | The defect can be carried under approved procedures for a defined period | Low, but the clock is now running on the repair interval |
| Aircraft swap | Another airframe is available and can be repositioned | Moderate to high, and only large operators have this option |
| Ferry to maintenance | The aircraft is flown, usually empty and under specific approval, to a base with capability | High — a wasted sector plus the repair |
| Cancel | Nothing else is available in time | Highest — passengers, crew, positioning and reputation |
Almost every one of these is decided by a deadline. Fix in place is the cheapest outcome by a wide margin, and it is available only if the part arrives before one of the clocks above expires. That is the entire commercial case for dedicated transport in an AOG: it is not about being fast for its own sake, it is about keeping the cheapest resolution on the table.
Four things people get wrong about AOG
| Assumption | Reality |
|---|---|
| "AOG means the aircraft is badly damaged" | Most AOGs are caused by a small component. Aircraft are grounded by a sensor or a valve far more often than by anything dramatic |
| "The airline will have a spare" | Holding full spares at every station is uneconomic. Inventory is deliberately concentrated, so the part is usually elsewhere by design |
| "It is an engineering problem" | The diagnosis is engineering. The delay is usually logistics — locating, releasing and moving the part |
| "Flying the part is always fastest" | Frequently not, within the UK. Booking freight onto a flight involves cut-offs, acceptance, handling and collection at the far end. A vehicle that leaves immediately often wins |
Where transport sits in all this
Once the part is identified and released, the remaining question is purely one of movement — and this is the part of an AOG that is most within anyone's control.
| The part is nearly always elsewhere | A supplier's stores, another station, an MRO, or a cargo terminal awaiting release |
|---|---|
| Release can matter more than distance | For imported parts, clearance and handling agent release often account for more delay than the drive |
| Networks are built for a different job | Fixed cut-offs, depot sortation and multi-drop rounds all work against a single time-critical item |
| The value of a firm time | Operations can plan around "06:00". It cannot plan around "as soon as possible" |
How RushXO helps
RushXO provides dedicated urgent courier services for AOG situations. A vehicle is assigned to your consignment alone and driven directly from collection to the aircraft — no consolidation, no depot handling, no multi-drop route.
| 24/7 dispatch | A person answers at any hour, including nights, weekends and public holidays |
|---|---|
| Realistic timing on the first call | Rather than a headline response figure, so operations can make the right decision |
| Dedicated vehicle | Your part travels alone and the vehicle can be redirected if the aircraft moves |
| UK-wide | Collection from suppliers, MROs, stores and cargo terminals anywhere in the UK |
| Proof at both ends | Proof of collection and proof of delivery, confirmed back to you |
Full detail is on our AOG courier page, including airport-specific pages for Heathrow, Luton, Stansted, Farnborough, Biggin Hill and London City.
We do not advertise airside access, aviation security accreditations, temperature-controlled transport or guaranteed response times. We deliver to the facility and hand over to your nominated contact.
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Frequently asked questions
What does AOG stand for?
Aircraft on ground. It describes an aircraft that cannot be released to service until a fault is rectified, most commonly because a replacement part is not available at that location.
Is every technical fault an AOG?
No. Many defects can be carried for a defined period under the operator's approved procedures, allowing the aircraft to keep flying while the repair is scheduled. A fault becomes an AOG when it cannot be deferred and the means to fix it is not available locally.
Why is an AOG so expensive?
Because the aircraft is the smallest part of it. A grounded aircraft still carries its fixed costs while earning nothing, but the larger costs are downstream: the missed rotation, crew duty hours expiring, lost slots, passenger obligations and cargo missing connections.
Who decides what happens during an AOG?
Several parties at once. The engineer diagnoses, maintenance control decides whether to repair or move the aircraft, the AOG desk locates the part, and operations decides whether to wait or swap the aircraft. That last decision is why an accurate transport time matters so much.
What are the possible outcomes of an AOG?
Broadly five: fix in place, defer the defect and fly, swap the aircraft, ferry it to a maintenance base, or cancel. Fixing in place is by far the cheapest and is usually only possible if the part arrives before crew hours, slots or curfews expire.
Why not just fly the part to the aircraft?
Within the UK it is often slower than it sounds. Booking freight onto a flight means meeting an acceptance cut-off well before departure, then handling and collection at the far end. A dedicated vehicle that departs immediately frequently arrives sooner.
Are most AOGs caused by serious damage?
No. The majority are caused by small components — sensors, valves, avionics units, tyres. Aircraft are grounded by minor parts far more often than by anything dramatic.
How does a courier help with an AOG?
Once the part is located and released, the remaining problem is movement, and it is the part most within anyone's control. A dedicated vehicle carries the consignment alone, departs when the job is confirmed rather than at a cut-off, and gives operations a firm arrival time to plan around.