View Full Version : assumed temperature reduction

25th Apr 2008, 19:31

How does assumed temp actually result in less power from the engines - how can you fool an engine that the air going in the front is, say, 46c?

Is it something to do with combustion chamber temps? Ie. if we tell the engine it is ISA +30c that will result in a lower temp inside and make a difference to the chamber life?


26th Apr 2008, 01:22
This has been the subject of some lengthy exchanges such as over here (http://www.pprune.org/forums/showthread.php?t=313674&highlight=flex+thrust).

If you use the search function to look for "flex thrust" you might find more threads too.

Ultimately, when you have less than full MTOW on a loooong runway, it's a way of reducing TO thrust to a value that meets all legal requirements, but saves the engine hot parts from needless abuse.

Old Fella
26th Apr 2008, 06:32
Sometimes simply called what it is "De-rated Thrust" If the prevailing ambient conditions, Runway length available and BRW allow you can use a thrust setting of less than what is available to take-off. Purely and simply a means of increasing engine life.

26th Apr 2008, 10:59
Don't confuse derated thrust with assumed temperature method, at least on 737s.

Old Fella
26th Apr 2008, 11:45
Sorry Denti, in my day we used "de-rate" to come up with the same outcome as you guys today kidding your "electronic whiz-bang systems". Essentially the thrust required in both cases is less than that available up to the "flat rated" thrust. On the RB211-524's rated thrust is available up to 29 Degrees C OAT.

26th Apr 2008, 15:04
Old Fella is right to an extent - it's the same objective for derate vs FLEX thrust.

However the procedures, and accompanying limitations, are separate and distinct. Derate is accomplished in fixed increment(s), and may be applied whenever TOGW vs RW available permit.

FLEX (ATM) is continuously variable up to 25% reduction, but it's prohibited w/ contaminated runway. An engine condition monitoring program is also required with FLEX to assess engine health.

And both procedures have been around over 30 ++ years - long before FADEC and other magic boxes existed. :D

28th Apr 2008, 02:21
Sometimes simply called what it is "De-rated Thrust"

The terminology has varied a bit over the years .. however, one needs to be a bit careful here ... think, and plug in whatever particular words you prefer, the following ...

(a) "de-rate" functionally is the same as changing the installed engine on the wing. That is to say, the entire certification basis is reworked. Main risk for pilots is that this usually involves lower Vmcg/Vmca values for the lower rated takeoffs .. if you have the physical ability to push up the throttles, then you could run into handling problems in appropriate circumstances if you were to do so in the event of a failure

(b) "flex" functionally is using part throttle for the takeoff

(c) clearly you can have a derated takeoff and then apply a flex reduction on top of the derate

Qantas, back in Wal Stack's days as performance boss, was one of the original instigators of reduced thrust takeoff performance .. I can recall Wal, as a part time lecturer when I was a BE student at Sydney, telling us of the early experiments when the captain thought it a tad odd to see the FE PUSH the throttles UP on the 707 when he called for climb thrust to be set .. Wal then introduced a requirement that min reduced thrust was to be limited by the climb thrust setting to avoid such eye-catching things occurring.

28th Apr 2008, 03:41
the captain thought it a tad odd to see the FE PUSH the throttles UP on the 707 when he called for climb thrust to be set ..

Ironically, although it's counterintuitive for pilots, a powerplant engineer will tell you that's not a bad way to operate the machine: A minute or so at the reduced TO setting serves to warm up the hardware for (rated) climb thrust, and is less abusive to the hot section.