AOG Aircraft on Ground

AOG stands for aircraft on ground. It describes an aircraft that cannot be dispatched for flight because of a technical problem that must be fixed first. The term is used by airlines, cargo operators, business aviation and military operators, and it is also used as a priority level for parts orders and shipments. An AOG parts order signals that the part is needed immediately to return an aircraft to service, so suppliers and logistics providers treat it ahead of routine and critical orders. AOG events are among the most urgent situations in aviation maintenance and logistics.

An aircraft becomes AOG when a defect means it is not airworthy or cannot be legally dispatched. Some faults can be deferred under the aircraft's minimum equipment list, allowing it to fly with the item inoperative for a set period under specific conditions. When a fault is not covered by the list, when the conditions cannot be met, or when an existing deferral has expired, the aircraft must stay on the ground until it is repaired. Damage from bird strikes, ground handling incidents and failed inspections can also ground an aircraft until repairs and checks are complete.

A minimum equipment list, or MEL, is an approved document that lists the equipment on an aircraft that may be inoperative for a limited time while the aircraft still flies, along with the conditions and procedures that apply. It is based on the manufacturer's master minimum equipment list and tailored to the operator's aircraft and operations, then approved by the aviation authority. The MEL allows operators to keep flying safely with certain non-critical items deferred. Items not on the list, or items whose conditions cannot be met, ground the aircraft until they are repaired.

An MEL deferral is the formal postponement of a repair under the minimum equipment list. When an item on the list fails, maintenance can defer it, placard the inoperative equipment and follow any operating procedures, allowing the aircraft to keep flying. Each deferral has a repair interval, which sets how many days or flights the operator has to fix the item. When the interval runs out and the repair has not been made, the aircraft can no longer be dispatched and becomes AOG. That expiry date is often the real deadline when shipping the replacement part.

An on-board courier, often called an OBC or hand carry courier, is a person who travels with a shipment on a commercial flight and keeps it in their possession or under their control throughout the trip. The courier checks the item in as baggage or carries it on, collects it on arrival and delivers it in person. On-board couriers are widely used for small AOG parts, because they avoid cargo acceptance and handling times and keep the part with one person. The part must meet airline rules for passenger baggage, including size, weight and dangerous goods restrictions.

Next flight out is a service that puts a shipment on the first suitable commercial flight to its destination. A driver collects the part and tenders it at the airline's cargo counter before the cutoff, and another driver recovers it at the arrival airport and delivers it. For AOG parts, next flight out suits boxed parts on routes with frequent flights. Timing depends on cutoff times, flight schedules and how quickly the airline releases the shipment on arrival. When flights are infrequent or the part is too large for passenger aircraft holds, a charter or hand carry may be faster.

A serviceable tag is the document that certifies an aircraft part is airworthy and approved for installation. It records details such as the part number, serial number, the work performed and who approved the part's return to service. Without the correct documentation, maintenance cannot install the part, however quickly it arrives. Serviceable tags must travel with the part and should be checked against it at pickup and delivery. In the United States, the FAA's Authorized Release Certificate is a common form, and other authorities issue their own equivalents.

FAA Form 8130-3, the Authorized Release Certificate, is a document used in the United States to identify an aircraft part and certify its airworthiness status. It can be issued for new parts by approved production organizations and for repaired or overhauled parts by certificated repair stations. The form lists the part number, serial number, description and the work performed, and it is signed by an authorized person. Mechanics rely on it to confirm a part is eligible for installation. For AOG shipments, the 8130-3 or an equivalent release from another authority must travel with the part.

Some aircraft parts are classified as dangerous goods for transport because they contain substances or energy that could pose a risk in flight. Examples include batteries, chemical oxygen generators, fire extinguisher bottles, parts containing fuel, hydraulic fluid or oil residues, and certain magnetized or pressurized items. These parts can often still be shipped, but they must be properly classified, packed, marked and declared under air transport rules. Some are allowed only on cargo aircraft or must move by road. Declaring dangerous goods when the AOG request is logged prevents a refused shipment at the airport.

Aviation parts orders are often ranked by priority. AOG is the highest level, meaning the aircraft is grounded and cannot fly until the part arrives, so it is handled immediately at any hour. Critical priority, sometimes called work stoppage or imminent AOG, means the part is needed urgently to prevent an aircraft from becoming grounded soon or to finish a maintenance check on schedule. Routine priority covers planned stock replenishment. The priority affects how quickly suppliers ship and which transport option makes sense, with AOG usually justifying hand carry, next flight out or a charter.