Minimum assumed temperature

[kilo]

Hi all,

Just wonderring why there is a minimum assumed temperature requirement when using this thrust reducing method,what kind of performance consideration is that based on?any replies and useful links will be appreciated.

[mutt]

What airplane/engine are you talking about?

We operate about a dozen different types without a minimum assumed temperature. The initial delta between the ambient temperature and assumed temperature is usually where you see the greatest savings. RR have quoted a 12% increase in blade life for a .01 EPR reduction in the RB211, therefore we would have a hard time justifying a minimum assumed temperture requirement.

Mutt.

[Popolama]

i believe there's a min assumed temp for the 737 you can find it in the QRH performance Chapter.
in my opinion ,the reason is there'll be no considerable gain when using an assumed temp lower than the min (ex min assumed temp at sea level is 30 deg C)

QUOTE (737.org.uk)
The normal range of assumed temperatures is from 30 to 55C. However the QRH shows that temperatures from 16 to 75C may be used. The maximum temperature is set by the maximum amount of thrust reduction allowable i.e. 25%; and the minimum is where the engine becomes "flat rated" and no further performance gain can be achieved.
UNQUOTE

[alatriste]

For MD 80´s powered by JTD9-219 engines, yes there is a minimum assumed temperature for flex TO.
This comes from an specific system call ART. When reducing thrust for TO, ART must be off and that means that the full power is available for the ATM ( assumed temperature).
When OAT and ATM are close, EPR max for ATM might be higher than EPR norm for OAT.

At sea level and ISA engine output is 21.000 lbs (ART auto) and 21.700 lbs for ART off.

At SL and OAT 26ºC EPR norm (TOGA/ ART auto) is 1.93.
If ATM is 35ºC, EPR max (flex TO/ART off) is 1.94.
Therefor ATM must be higher than 35ºC to perform a real thrust reduction.
REGARDS