Assumed temperature B738

[noox89]

Hi guys, I have one question. I was flying yesterday back from Malaga (14 deg OAT) with GW of 68 tons, 24k and according to our RTOW tables, we could assume up to 32 degrees. The FMC showed INVALID ENTRY when we tried to put the assumed temperature in. I know that the engines are flat rated to 30 degrees and there is basically no point to put something like 28 degrees in, but anything above should work, shouldn't it? We tried with Cpt to insert something higher just to try and it took 34 degrees minimum.

[ImbracableCrunk]

Isn't there a minimum difference (20?) between the OAT and the Assumed Temp?

[noox89]

FCOM 2 says nothing about minimum spread between OAT and Assumed Temp

[shaftsburn]

Remember that the engines are flat rated to ISA + 15°C, not just to a fixed +30°C.

According to the interweb, the QNH in Malaga was more than 1030, meaning a Pressure Altitude of around -500ft.

From the tables in the QRH, it looks like a minimum of 32°C.

It's still a bit shy of 34°C though.

[de facto]

There is a minimum and maximum temperature,have a look in your fcom.(perf).
If you cant insert an ass temp less than 32c,it means that for the rating,pressure altitude and outside temp ,your thrust is at maximum.
If you assume is too low,it would give you a thrust higher than your pressure limit and the fmc will not accept,as you increase the assume input,you will see it will accept when the thrust value is lower than your max thrust for rating.
Have a look at your climb limit weights,they will decrease by about 1.2T until a certain temp then remain kind of constant,it happens at your min temp for rating and temp and pressure alt.

I hope it helps.

[Dynamite1]

for the 737-800W/CFM56-7B24 version as per the FCOM; at 14 deg C for Malaga, the max Ass temp would be 61 and the min ass temp wd be 30.

[Flightaddict]

Maybe the temperature that you choose you were reducing the thrust more then 25% n1 ?

[de facto]

Flightaddict Maybe the temperature that you choose you were reducing the thrust more then 25% n1 ?

That would be the MAX assume temp limit.

[Capt Chambo]

I know that you said all the ATM input figures were done in C (Celsius), are you sure someone didn't muck around and change them to F (Fahrenheit) whilst you weren't looking!

[de facto]

Please people.......
If you dont understand the concept of MINIMUM Ass temp then open your manuals and come back here later....

[Flightaddict]

My bad.. Mimum assume temperature you can find it on the performance inflight / assume temperature reduce trust

For example on my 737 at sea level minimum is 30*

[BOAC]

de facto - out of interest, what is your opinion on min assumed? Can you in fact interpolate the tables, making AGP 31degrees, or are we restricted to block figures as with other performance detail?

[president]

Just to add something. A 25 % decrease in N1 is NOT a 25 % decrease in thrust. Remember CLB2 is 6 % less N1 but 20 % less thrust.

[BOAC]

Anyone answer #12?

[de facto]

de facto - out of interest, what is your opinion on min assumed? Can you in fact interpolate the tables, making AGP 31degrees, or are we restricted to block figures as with other performance detail?

Not sure what you mean by AGP.
But if you mean interpolate to use 1 c above min ass temp then not worth it anyways..your take off thrust will be basically the same as with no ass temp.
If you can not use assume temp higher than the minimum assume temp due to your climb limit weight then do not use assume temp at all.
I check the runway limit weight for runway lengh available and climb limit weight ,if thats ok i use speeds for the actual weight at the actual temperature.

[BOAC]

AGP is Malaga

[de facto]

In his example he was climb limited at his max rated thrust and an ass temp up to and not including 34 C would have given him aTO N1 higher than his max rated thrust (24k).
If 32c ass temp did not work for climb perf,a bleeds off ass temp 32c could have made his weight under climb limit....
Odd, a bleeds off with assume temp:-)