ATPL systems question

[xxRED BARONxx]

Hey guys, got a question on something I read during my systems study. Its regarding entering a higher than ambient OAT on the CDUs TAKEOFF page... The book says if a higher than ambient OAT is entered then the thrust management computer will compute a max takeoff thrust based on the hotter temp which is LESS than the max takeoff thrust based on the original temp(ambient OAT i assume).

I dont understand why the max takeoff thrust will be LESS if we have entered a higher than ambient temperature... I would have thought it would have been MORE because increase temp=decrease density so wouldnt the computer calc a higher max t.o. thrust to compensate for the decreased density??

Any enlightenment would be appreciated!! thanks in advance

[Oktas8]

Warmer air, less dense. Less dense air means less mass flow of air through the combustion chamber, therefore less fuel can be burned, therefore less thrust. You could turn the compressors faster to recover air mass flow, but this would involve higher turbine temperatures, which are probably already limiting at take-off power. So accept the lower thrust.

Instead of turbine temperature, N1 or N2 might be limiting. Either way, you get the same result: warmer air --> less thrust.

Awaiting correction from those with more knowledge...

[kingRB]

isn't this relating to an assumed temperature / derated thrust performance take-off?

Entering a higher temperature allows a derated thrust for take-off - ie the computers only provide the thrust available for the assumed higher ambient temperature?

Thats my take on it, but I am by no means sure either. I am sitting systems next so I am also keen to know :)

[boltz]

The EPR setting will be less. Takeoff power is set using EPR, not N1 or N2 rpm. If N1/N2 remains the same, on a hotter day the EPR will be less, therefore the engine is producing less thrust.

[DraggieDriver]

boltz,
depends on the engine. Rollers use EPR, CFM's don't. CFM flex/ATM affect N1's.

[noclue]

The way it was explained to me (very simply, but enough to get the concept) is this:
1) First, you must consider that the thrust generated from a jet engine is a direct result of an increase in temp through that engine. i.e. the difference between the inlet temp (actual or assumed) and the MAX ITT.
2) Second, the engine has a max ITT, and it may be considered constant.
3) Therefore, at max power (read max ITT), the most thrust (the biggest difference between the temp into the engine and that expelled) will be generated at the lowest inlet temp.

By telling the thrust computer that the inlet temp at take off is hotter than actual, your really telling it to generate less thrust, as the difference between the max ITT (constant) and the intake temp is now less.

Because you'll be taking off with less power, your max take off weight may have to be limited to ensure adequate climb performance.

[xxRED BARONxx]

Ahhhh ok now we are on the same page, thanks for the clarification guys.

[DirectAnywhere]

ATM for Dummies::}

Can you explain the assumed temperature take off? (http://www.askcaptainlim.com/flying-the-plane-flying-90/166-can-you-explain-the-assumed-temperature-take-off.html)