T/O Thrust time limited

[squawkident7700]

can some erudite tell me why on earth one engine is certified to produce T/O thrust for no more than 5' in normal condition and, magically, the same engine can work 5' more at that T/O thrust (with no objections) if one of its "collegues" has failed meantime.
What happens to that engine at the sixth minute (all engine operative) that doesnt happen if OEI. :confused:

[Empty Cruise]

Both engines operating - you want to use both engines again, without overhaul.

One engine operating - if you want to use remaining engine again (overhaul or not), you have to:

- take off with a lower MTOM (not good for revenue)
- fit a bigger engine or use less derate (not good for expediture)
- kick some seriosus a** with the remaining engine & accept the overhaul later.

Hope that's what you wer looking for - brgds fm
Empty:)

[Piltdown Man]

Presumably, the engine company have determined that five minutes at full tilt won't unreasonably shorten the life of the engine, but regular usage beyond that will. But when an engine fails, the future life of the remaining engine is immaterial - all it has to do is finish the flight it started.

[barit1]

FWIW:

I used to test-run a high-bypass commercial engine in performance tests on a 90-minute equivalent flight cycle; we always observed generous warmup-cooldown times and SLOW throttle excursions.

But we regularly held it at rated TO for 10 minutes or more, then followed that with a derated TO for a like time, then progressively lower settings.

We saw some deterioration in EGT margin, but not enough to be a real issue. There was certainly no safety concern with the engine.

I'm NOT recommending this in normal day-to-day practice, but anytime you NEED it, don't be hesitant to use it.

[Denti]

Depends probably what you get certified. We had that 5 minute limit as well and all of a sudden it was lifted to 10 minutes for either both engines or one engine operating. Probably means higher maintenance costs, but apparently its worth it otherwise the beancounters wouldnt pay for it.

[Old Smokey]

The 5 minute limit is typically established for the determination of engine guarantee, and for the scheduling of maintainance (Time between overhauls). For the Turbo-Jet engine, there is little, if any, concern for imminent failure if the 5 minute limit is exceeded, as barit1 says "anytime you NEED it, don't be hesitant to use it."

The guarantee, and engine life (TBO) is projected using this assumption, and, with the acceptance that failure is not a significant possibility if the 5 minute limit is exceeded, 10 minutes may be allowed in the exceptional case of engine failure. Similarly, in aircraft with APR (exceedance of normal Takeoff thrust limit following engine failure), a limited number of APR applications are factored into the guarantee/TBO as a normal state of affairs. Going in the reverse direction, where the operator employs De-Rate, this, too, is factored in determining an increased value of guarantee/TBO.

If the discussion is taken outside of guarantee and maintainance scheduling considerations and operational factors considered instead, sustained high thrust impacts upon engine life. For the Turbo-Jet engine, such engine life hinges upon cumulative engine stress (so long as no physical limits are exceeded which might cause immediate failure). In this sense, operations beyond 5 or 10 minutes at Takeoff thrust might reduce the remaining engine life from, say, 5000 hours, to 1000 hours - a severe economic consideration, but not an operational one (for the moment). From the operational perspective, this is in many ways a hang-over from the piston era, where prolonged operation of the highly rated 'big' engines at Takeoff power was a set-up for imminent engine failure.

Consider too, that there are a large number of engines which have been militarised, operating at time limits and physical limits somewhat above their civilian counterparts. The only real difference between the otherwise similar 'civil' and 'military' version of the same engine is a DRASTICALLY reduced engine life.

Regards,

Old Smokey