How do you determine when to use a derated takeoff, and how do you determine the assumed temperature to use?

page 30 of the manual describes this. Here's what it looks like on a 737NG. I entered 40 as the assumed temp ,and reduced from 91.2% N1 at OAT ,to 88% N1 with 40 as the select temp. As you can see the NG also gives you preset reductions for TO and climb.

Nice pic Wallace, nice to have some real-world ones around here. :)

The manual says the number to input is based on a complex formula based on runway length, temp, etc. Does anyone have this forumla? This is what I originally wanted to know.As your pic illustrated, on a 737, this is not a problem since there are 2 preset derated takeoff settings. I'm using the 767 PIC though, so your pic doesn't help me. Anyone?

But that is not what you asked. You specifically asked "how to determine" when derated takeoffs are used. That's quite different from asking "what is the formula for calculating derated takeoffs?"I'm sure Wallace will apologize for trying to be helpful to you.

I do not believe you will ever be able to determine the formula used by the computer systems. Unless you completely decode it from one of an airline's dispatching computers.I can however tell you most of the factors and variables that go into calculating the assumed temperature as well as how the pilots request one of these via an ACARS transaction. This is called a WAT chart.Attached is an example of a request for a WAT chart. Also attached is the final WAT chart. As you can tell, this particular takeoff was executed with a packs on, dry runway surface and engine anti-ice on. Best Regards,

Ahh, wouldn't it be nice to have that kind of thing for the PC...

Well, you can, you just need muchos $$$:http://www.navtechinc.com/ProductsandServi...ispatch/aps.htm

Assumed Temperature MethodAs the assumed air tempterature is increased, the N1 Reference Decreases This flat rate 32* c is the maximum tempterature at which the engines can provide maxium takeoff thrustMaximum Reduction:25% of maximum Takeoff thrust ORPreselected Climb ThrustYou cannot select an assumed tempterature more than 53* c.AND you can use climb 1 or 2 in conjunction with D-TO. Simple here, if you need less takeoff thrust, use D-TO. Do you need to know all this very complicated stuff to use this method? No. Just be sure you get the "assummed Temperature" correct. Best Wishes,Randy J. [email protected]" A little learning is a dangerous thing"AMD XP 2100 |MUNCHKIN 512 DDR RAM |ECS[/b ] K7S5A MB |[b]GF3 64 MEG |WIN XP PRO |MITSUBISHI DIAMOND PLUS 91 19"

Sorry Trekkerj, I misunderstood you , yes each airline uses their own method, The big B uses Qrh's, Op's manuals, Flight Planning Performance Manuals (FPPM), and the associated Take off tables based on that particular aircraft . The maximum thrust reduction authorized by most regulatory agencies is 25% below any certified rating. Some versions of the 757/767 do have the same type of derate setup you see here on the NG's FMC.