AOG stands for Aircraft on the Ground and it is widely (although not desirable) used in aviation.
When an aircraft is in immediate need of a part or service on an AOG basis, this means that the aircraft is on the ground and unable to fly until the part is replaced or the service performed.
An AOG situation can be caused by a myriad of reasons, including conflicting flight schedules, the weather (blizzards, volcanic eruption or ashes), technical issues, regulatory factors or safety issues, when more than one aircraft is involved in accidents with the same root cause (see the recent 737MAX case) then the authority(ies) withhold its operation until the investigation is complete.
Having an aircraft in serviceable condition is a top priority for the airline, as well as for the manufacturer in order to maintaining its reputation.
That said, it is important to point out that the aircraft manufacturer, its entire supply chain, as well as the air operator have an efficient logistical and maintenance system to prevent AOG situations, or act quickly to restore the aircraft's airworthiness.
An AOG label affixed to a package is a widely used way to streamline processes, such as clearing parts at customs, as shipments of this type have high priority.
However, there is no regulatory definition of the acronym "AOG" when stamped on work orders or repair sheets (14CFR FARs). It is used by convention and indicates that the activity is urgent and the part or repair must be completed with priority.
The same convention is valid for requests for Technical Disposition, prepared by the Technical Support of the Operator, Service Center or Manufacturer.
Although "conventions" are treated as good practices, the contract must be observed, which normally contains very heavy fines.
In 2010, a study conducted by the FAA (Federal Aviation Agency of the USA) together with NEXTOR (National Center of Excellence for Aviation Operations Research) concluded that the delay in flights in the year of 2007 caused a loss to the airlines of around US$31 billion.
When a flight is delayed or "grounded" for an extended period of time, airlines are unable to recover these costs through ticket sales.
Boeing estimates that a 1- to 2-hour AOG situation can cost the operator between $10,000 and $20,000, rising to $150,000 in some cases.
Therefore, if you hear the message that your flight will be delayed for any of the reasons mentioned above, keep in mind that there will be a battalion of people concerned and interested in making this delay as short as possible.
And finally, and most importantly, everything will be done to the HIGHEST standard so that the aircraft, crew and passengers can SAFELY return to the skies.
Regards from Brazil!
Sources: