Any aircraft can fly with a set of predefined defects existing on that same aircraft without it affecting the airworthiness of the aircraft. This set of defects is laid down in the Minimum Equipment List (MEL) and is derived from the Manufactures Master Minimum Equipment List (MMEL). Within the MEL, defects are listed on aircraft systems which are allowed to exist, provided these defects are rectified on the aircraft within a predefined period of time.

An increasing number of deferred defects on both individual aircraft as well the fleet as a whole, can result in additional planning challenges due to further operational restrictions on affected aircraft, and potential additional aircraft downtime in order to rectify the deferred defect within its given MEL category rectification time. Once the rectification time of a deferred defect has been exceeded and the defect has not been rectified the aircraft is considered non-airworthy (AOG, aircraft-on-ground) until that same defect is rectified.