TAC (Total Accumulated Cycle)

I want to learn all about the "TAC". Could someone explain?

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.

Here is more of a text-book answer:

There are numerous parameters, by which the use of a component can be described. Flight time, engine running time, number of flights, number of engine runs, engine running time above certain spool speeds, time at certain temperatures. More appropriate for a description of the usually life limiting processes are parameters approximating the cyclic properties of engine operation. The best known method is the counting of so-called TACs (total accumulated cycles) mainly in use at the USAF.

Because of the somewhat arbitrary definition of the power ratings this procedure can be refined by admitting arbitrary spool speed values in the assessment of the contribution of a spool speed cycle. The method outlined below is an extension of counting TACs. If it is implemented in an on-board monitoring system or in a ground-based system for the assessment of recorded engine data, the results are directly comparable with specification values using TACs as a measure for cyclic engine or component usage.

If you have access to an Engine Management section, the official definition of TAC is in 00-25-254-1, while the actual "formula" for TAC is in the specific TO for the particular engine you are dealing with. For the F100 it would be 00-20-5-1-1, 00-20-5-1-9 for the T56, 00-20-5-1-8 for the TF33, just to name a few. (Consult your USAF TO Index for other engine models...)

Needless to say, I consider the actual formulas for TAC as "need to know"; but with the references I've given here, if you have the need, you know where to go...

Keep 'em flyin'
TEG

TEG,

Thrust cycles cause wear on the engine, as you say, and they also cause wear on both sides of the engine mounts (engine / fuselage), so fuselage structural tracking includes thrust cycles. Can you tell me if engine tracking also includes airplane maneuvering data? Such as, the engine side of the mounts, bearing loads, gyro effects, etc. due to vertical g, lateral g, pitch and yaw rate and acceleration. These would be engine structure effects, not thermal.

Engine usage also induces structural load on the inlet structure due to pressure from cruise conditions, acceleration, stall/stagnation, spillage, locked rotor, etc. So there is a lot of interaction between engine and airframe that is not obvious. That also explains why engine upgrades or alternate engines can give airframe folks a real headache.

johnwill wrote:Can you tell me if engine tracking also includes airplane maneuvering data? Such as, the engine side of the mounts, bearing loads, gyro effects, etc. due to vertical g, lateral g, pitch and yaw rate and acceleration. These would be engine structure effects, not thermal.

TAC is computed by using only engine performance data, most specifically RPM. To my knowledge the aircraft data you've mentioned is not used for engine life tracking in any way, nor is TAC used in aircraft tracking.

On a side note; It is implied that at certain RPMs the engine is receiving a given amount of stress. Don't forget at high RPM compression/airflow imparts a huge structural load on cases and supports. They tend to expand/contract due to thermal and compression loading. An F100 grows about 2" in length from what I can remember when it is "hot" versus when it is "cold" not to mention an increased diameter.

johnwill wrote:Engine usage also induces structural load on the inlet structure due to pressure from cruise conditions, acceleration, stall/stagnation, spillage, locked rotor, etc. So there is a lot of interaction between engine and airframe that is not obvious. That also explains why engine upgrades or alternate engines can give airframe folks a real headache.

True; but things such as stall/stagnation are tracked in other ways. During such an event temperatures spike and RPMs often drop. This will cause specific temperature tracking methods to increment, and can add TAC if RPM changes sufficiently. I should also mention these "events" cause fault codes to appear in the engine's monitoring system that require immediate maintenance. Rotor locking, or stall/stagnation can cause blade rub, and will reduce compressor efficiency. Specific values/ratios between RPM and pressure will indicate this performance loss and may set faults that require immediate maintenance actions. Even if the event doesn't cause short term damage, the resulting long-term health of the engine is continuously monitored to ensure the proper thrust is maintained.

And yes; engine upgrades can be a pain to integrate. Many things need to be checked even for what seems to be a "minor" change in engine performance. Engine weight, Center of Balance, Thrust Load, Fuel Flow, Bleed-air, Control Integration, Size.... just to name a few

Keep 'em flyin'
TEG

TEG, thanks for the information. The reason I asked is to satisfy my curiosity about how engine life is monitored as compared to how airframe structural life is monitored. One of my jobs back in 1976 was to develope the first F-16 engine mount structural design load spectrum, the structural equivalent of your TAC. I had to get a lot of information from our propulsion people of course to get a two-lifetime thrust spectrum, then combine that with a two lifetime maneuver spectrum for engine maneuver loads on the mounts.

One interesting thing I found was that engine thrust at the nozzle is never the same as engine thrust at the mounts, due to pressures on the engine face and drag on the engine case from nacelle ventilation air flow.

Thanks again, always enjoy reading your posts.