Altitude and Temperature

[FMS Guide]

If an aircraft is flying along at 39000 feet and maintaining a set thrust setting. How does temperature affect speed in regards to increasing or decreasing your thrust setting to maintain your predetermined speed? If the temperature warms up what happens to speed and if the temperature cools down what happens to speed?

Thanks in advance,
FMS

[barit1]

The engine control unit operates differently on different engine models, and thus "maintaining a set thrust setting" (fixed throttle) will have different implications. Some models will maintain a constant corrected N1 and thus constant thrust, as temp. changes. Older engines may maintain a constant N2, and thus different implications for thrust.

So the answer is engine dependent.

[deltahotel]

75 operates in SPD mode in the cruise, so adjusts EPR to maintain Mach No which is either selected by FMC depending on operator pref, or manually input. thinking back - TAS/Mach No relationship is a function of OAT.

[CONF iture]

FL390
If OAT is -57C and IAS 230Kt then TAS 455Kt
If OAT is -45C and IAS 230KT then TAS 465Kt
If OAT is -70C and IAS 230Kt then TAS 440Kt
As temperature increases, TAS increases, but FF does also.
I think ...

[deltahotel]

Bear in mind we cruise at constant Mach no, not IAS. speed of sound (c) is directly proportional to square root of temp (in degrees kelvin). M=v/c, so TAS=Mc. so at constant M, TAS is directly proportional to square root of temp. Degrees kelvin are celsius + 273. You'd need quite a substantial change in OAT to get a big change in TAS; -60C is 213K, so a +/- 5C is only +/-2% and then there's a square root in there.

thnk have got all that right - long time since my engineering degree, or indeed CFS groundschool!!

Hope I'm adding to the sum of knowledge and not muddying the waters

DH