Diego
Topic Author
Posts: 135
Joined: Mon Apr 23, 2001 7:33 pm

### Derated To

Hi folks
Can anybody tell me why in the Derated Take off procedure you have to assume a higher temperature?
Regards Diego

JETPILOT
Posts: 3094
Joined: Tue May 18, 1999 6:40 am

### RE: Derated To

I think your talking about the assumed temperature of 49C for reduced power setings?

Your assuming a higher temperature in order to reduce the power output of the engine.

You only used the assumed temp as long as the runway or second segment climb limit don't dictate a higher power setting, or the max reducable numbers aren't exceeded.

If the runway limited weight is less than the assumed temp limit you must use the temp for the runway limited weight.

Same story with second segment limits as previously stated.

Why 49C is used I can't answer that. I don't know how the yarrived at that temp.

I don't know if my ramblings helped any.

JET

F-WWAI
Posts: 129
Joined: Thu Dec 30, 1999 7:41 am

### RE: Derated To

that's relatively simple.
imagine that your max take off thrust available depends on outside air temperature. it's flat rated (=constant) until a certain temp (e.g. ISA +15C) then decreases linearily with temperature to maintain constant turbine entry temperature.
given this, you may understand that you can take off a certain aircraft gross weight until a certain OAT. got it?
imagine now you are at low OAT but you have a GW which permits take-off at ISA + 40C. then you can tell you thrust control to compute the take-off thrust at the assumed temperature of ISA+40C, and you will obtain the thrust required for the take-off.
clear enough?
FW

JG
Posts: 165
Joined: Tue Jun 07, 2005 1:53 am

### RE: Derated To

Additionally, derated thrust and assumed thrust are two different beasts. The CFM powered 737's I used to fly could do both derate and assumed (also called balanced thrust) individually or combined. The 320 series I fly now simply uses assumed temperature. On our 320's that assumed temperature can be as high as ISA + 53C

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