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Pseudo TAT on assumed temp derate?

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Moderator
Registered: May 2009
Posts: 5140
Hi,

as we know, the thrust references are based on TAT, not OAT.

The FMC THRUST LIMIT page has an OAT indication for reference. It allows this reference OAT to be replaced by an assumed temperature, i.e. by an assumed OAT.

During preflight, when the aircraft is stationary, OAT and TAT are equal. So at that stage the difference between OAT and TAT is irrelevant.

But what happens during the takeoff acceleration?

When the thrust limit refers to the real temperature sensed by the TAT probes, the thrust limit will decrease with the rising TAT (the rising airspeed increases the TAT).

But when an assumed "OAT" is entered, will this assumed "OAT" become an assumed "TAT"? Will the assumed temperature rise as well, according to the rising airspeed? Maybe even the green assumed temp value on the upper EICAS will increase during acceleration?

Does anybody know more about this subject?


Thanks!

|-|ardy
Member
Registered: Dec 2011
Posts: 497
Location: Portugal
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Jose Monteiro
Moderator
Registered: May 2009
Posts: 5140
140 knots is a lot. And it rises above 400 ft, with TO limit active until CLB activates.

The thrust limit decreases during acceleration, i.e. the TO thrust limit decreases more and more and reaches GA limit at approx V2. That's why you call "thrust set" at low speeds. And that's why the GA limit is lower than the TO limit.

Just watch PSX and monitor the TAT and thrust limit. The TAT is correctly modelled and the thrust limits are based on real-world databases.

Only when an assumed temp is entered, the temp remains static in the existing PSX versions, thus the assumed temp thrust limit remains static too. I'm not sure if it should stay static.
« Last edit by Hardy Heinlin on Sat, 06 Jun 2015 06:57:42 +0000. »
Member
Registered: Dec 2011
Posts: 497
Location: Portugal
Have you tried PPrune? They have a few Boeing engineers at the forum....
_______________
Jose Monteiro
Moderator
Registered: May 2009
Posts: 5140
I first ask the Boeing engineers here :-)

And the Boeing engineers here sometimes ask on PPrune :-)
Member
Registered: Jun 2009
Posts: 87
Location: KMEM
It should be static for takeoff thrust has to be set by 80 kts per the Boeing FCOM and the AFM. "Readjust thrust levers, if necessary, prior to 80 kts to obtain target EPR. Do not make further adjustments after 80 knots." B747-400 AFM D6U10001 Sec 4.1 p 11.
Moderator
Registered: May 2009
Posts: 5140
Yes, but does that mean the FMC's thrust limit value remains static until a CLB limit mode activates?

And if so, is it static for all TO limit modes, not just for assumed temp limit modes?


Thanks,

|-|ardy

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