In short a "TAC" or "Total Accumulated Cycle" is a form of life measurement that major engine components are tracked against. Mostly engine overhaul. (or in the PW Modular Concept, module overhauls) or other major components within the engine/modules.

In any given engine there are many MANY parts that are tracked by various methods. Engine Operating Time (EOT), Engine Flight Time (IFT), as well as other may be used. The method currently used by the USAF for fighter engines is TAC.

Think of an engine cycle as a throttle movement through the engine's operating range from OFF, to IDLE, through MID-Range, up to MIL and back again until it reaches OFF. Each time the engine accelerates or decelerates (or is started/stopped) wear/age is added to the components in the system at different rates. TAC is a complex calculation of engine RPM/Throttle usage.

Depending on how much a throttle is "used" during a flight, that flight may only accumulate a few "TACs" or many "TACs".

Example 1: A ferry-flight where the aircraft takes off, cruises, and lands with little throttle movement. On such a flight only 2 or 3 TACs will be used, even though the sortie may last for 6 hours.

Example 2: On a A/A combat mission, where the throttle is moved very often during ACM, an engine may gain 10-15 TACs even though the sortie is only 1 hour long.

The A/A mission's throttle movements and thrust demands put a lot more "stress" on the engine even though the flight time or operating time is short. It uses more of the engine's "life" than the first example where the aircraft flies "easily" from point to point without stressful demands from the engine.

There is no way to predict how many "TACs" will be used per hour or flight, as they are computed by actual engine RPM changes during actual flight operations. Even with a two-ship flight of the same duration/destination, one pilot may use more TACs than the other simply due to more/frequent throttle changes.

Commercial engines gain cycles very slowly compared to fighter engines. As a result they last a lot longer.