PPRuNe Forums - As ambient temperature rises, full take-off power will necessitate ?

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As ambient temperature rises, full take-off power will necessitate ?

as ambient temperature rises, full take-off power will necessitate ?
as ambient temperature rises, full take-off power will necessitate ?

a- lower fuel flow with a low egt and a lower engine rpm
b-higher fuel flow with high egt and higher engine rpm
c-higher fuel flow with high egt and lower engine rpm

Which answer is correct and what site can i find info on such
thanks

Wow. Can we assume that the engine is flat rated with an RPM limit at the lower temperature and not at the higher? If so I'd guess that the egt is higher at the higher temperature and the fuel flow and rpm lower. I'd be fascinated to get an opinion and explanation from an engine specialist. Anyone work for RR?

Hell of a GOOD Question

I love it. Don't take any opinion on here as being correct. Work it from first principals.:E

I found a good explanation that might help. Have a look at this old thread and post 2 by Old Smokey, which might give you an idea.
Flat Rated Engines.

If it is flat rated I would go for B. As with increasing ambient temperature you would get a higher EGT, RPM (N1) and thus higher fuel flow until you reach max. EGT.

Quote:

Consider an aircraft at Sea Level on a very cold day. As the thrust levers are advanced, the maximum internal pressure would typically be first reached as the air is very cold and dense. The engine is well below the EGT and N1 limit. As ambient temperature increases, the air is less dense, and increasing EGT and N1 is required to achieve the pressure limit, although still below their limits. This is a situation where the engine is "Flat Rated", a rise or fall in ambient temperature will have negligible effect upon thrust, it is "Flat", with the thrust available being quite constant and governed by the internal pressure limit.

When you reach max. EGT, thrust and fuel flow will have to be reduced to keep the EGT below the max. limit. This will give you lower fuel flow, higher EGT and lower RPM, which is not an option in your question. I would go for B.

Where did you get this question from?

Answer b is correct for max allowable takeoff power (it works for B737 NG).

Easy one. My answer is A.

As ambient temperature rises, your air density decreases.
The gas turbine engine's Fadec or HMU will try to maintain an ideal mixture between air and fuel. If your air density decreases, the fuel flow needs to decrease to maintain this balance and squeeze the most juice out of your engine.

It doesn't say from where the ambient temperature rises so it's possible that the rise occurs within the temperature range where N1 is limited by OAT. If your N1 is already OAT temperature limited, an increased fuel flow will result in you going beyond the N1 and EGT limit for given temperature and cause damage to your turbine.
Result is, fuel flow for max thrust/power decreases in all cases of increasing ambient temperature so B and C are wrong.