Unit
Any assembly (APU, control system component, etc.) that can readily be changed on an aircraft during line maintenance operations

APU
The Auxiliary Power Unit (APU) (core power section, gearbox, and if applicable, load compressor) plus all control system components. It is the demountable APU package, less generator and aircraft hardware

Control System Component
Any component which is a constituent of fuel, oil, electrical, electronic, or pneumatic subsystem that controls or maintains APU function

Removal
Any unit removed directly from the aircraft

Failure
Inability of a unit to perform within previously specified limits

Scheduled Removal
Any removal not due to operational impact. This category includes removals for preventative maintenance, precautionary, routine inspection, time limits, access to other equipment, modification or maintenance convenience

Unscheduled Removal
Any removal due to operational impact. This category includes removals due to problems evident to the flight crew, problems evident to the maintenance crew, personnel error, or maintenance errors

Confirmed Removal
An unscheduled removal where a failure, or a combination of failures is found which substantiates the reason for removal

Unconfirmed Removal
An unscheduled removal of a unit where no failure is found which substantiates the reason for removal, even though another failure may be found

MTBR Mean time between removals:
MTBUR Mean time between unscheduled removals (removed during a 12-month period):
MTBCR Mean time between confirmed removals (removed during a 12-month period):
Average TSR (Time Since Removal/ Repair/last shop visit)	Average of TSR (or TSN if the first removal) for APU removed during a 12-month period
First Run Reliability (MTTCUR)	Average of TSN for confirmed APU removals during a 12-month period (first removals only)
Time on Wing Time on Wing = TSN, for the first removal Time on Wing = TSR, for subsequent removals