PDA

View Full Version : Flex T/O

extricate
31st May 2011, 13:29
Hi,

Just hoping for some clarifications.

If i want a lower T/O thrust, an assumed high temperature is entered in the FMC to trick the computer.

Isn't it supposed to be the other way round? High temp = less dense = more power is needed for T/O. I know i'm wrong but can someone prove to me that my statement is wrong?

Rgds,
Trey

CI54
31st May 2011, 14:17
Basically, the engine is flat rated at sea level at certain temperature. For the sake of argument, lets say the engine is rated at 50000 ibs of thrust at sea level, standard atmospheric conditions to ISA + 15. That means, the manufacturer guarantees that the engine will produce 50000 lbs of thrust up to 30 deg C at sea level. Above 30 deg C, the manufacturer reduces the thrust accordingly to ensure reliability and maintenance schedules, under nominal use. Most of the time, the thrust limits are programmed in the FMC or FMGEC, which when coupled to the auto-thrust system, sets the engine N1 accordingly..

May not be the most comprehensive answer, I'm certainly not an engineer, but that's the basic logic...

barit1
31st May 2011, 14:33
For a given runway, the max aircraft TOGW decreases with increasing OAT. This is the result of two major factors:

1) Higher groundspeed required to reach V1, Vr etc. (Think a = F/m)

2) Reduced engine thrust available if OAT is above the flat rating limit.

Now if OAT = 10C and your performance-limited TOGW is 100t, but your actual TOGW is only 85t, you can look up the limiting OAT for a TOGW of 85t and call this your assumed or flex temperature (40C perhaps). At that OAT the engine power management would only permit say 90% of flat rated thrust. So it's within certified limits to use 90% thrust at your 85t weight.

But in fact you have MORE performance margin available, because the wing is flying in the 10C atmosphere, not 40C. :)

extricate
31st May 2011, 15:17
thanks for the in-dept explanation. I think i've got it.

Is my explanation correct?

The main idea of a Flex T/O is to have reduced power. So you bluff the engine to say that it's alot hotter outside and naturally the engines will perform with lesser thrust?? and not my original idea whereby at a higher temp,the engines will NATURALLY increase thrust?

Rgds

CI54
31st May 2011, 15:32
Yes, that's it...