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Calculating proper assumed temps/derates


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I've had the TOPER addon for a while now (it's a takeoff performance calculator), but I noticed that for the 300 it only has data for the 22k engines, whereas I believe the IXEG has 20K. So lets say TOPER says use an assumed temp of 67C, I imagine a 20K engine shouldn't derate quite as much as that since the engines aren't as powerful. I usually shave a few degrees off whatever temp TOPER gives me, but is there a more precise way of calculating it?

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I too bought TOPER and it is a waste of money, even for the aircraft with indicated engines (different from iXeg), too imprecise calcs.

To manually determine assumed temperature and corresponding N1 is a matter of looking up 3 tables in FCOM, done in under 2 minutes. Another 2 to determine V speeds. Can take more time if runway is contaminated (assumed temp method not allowed). All given you have determined TO maximum performance-limited weight (which you must always do).

Edited by NZWW
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Hi, I said this in another thread, I think. Derating a takeoff accurately is tremendously difficult - as every single runway needs a new calculation, dependent on runway length and obstacle path.

Before we used computers to calculate the TASS, we had two thick folders in the cockpit with "runway weight charts" that we would get out for every takeoff and enter them with all the necessary data...a cumbersome and error-prone process.

From experience I can tell you that most takeoffs that do not have an obstacle problem can be derated safely to 45 deg TASS, when runways get shorter (<2500m) you want to go to about 40.

I can count the "full power" takeoffs (excluding contaminated runway, they are mandatory for those)I did in flying the 737 for 10 years on one hand...it is very rare that you are so limited that you can´t even derate a few degrees (like going to 35 TASS).

Cheers, Jan

 

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Thanks for the answer @Litjan. I appreciate it but I like to fly more challenging places, among them Norway where you have strong winds and short runways with obstacles and very common with intersection takeoffs. Going out on a runway that is shorter than 2500 without calculating feels wrong. It would still be very nice to be able to see them, as boring as it sounds. Do you have any clue where they can be found? 

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No idea - one can find other (proprietary) material on the internet, but this stuff is so special. We pilots did not even have those as personal copies, they were on board the aircraft, only. They were issued by a department at my airline - the basis was performance calculation done with the charts from the Performance Engineers manual (climb gradients, etc.)

You could do some basic tests yourself were you do some accelerate - stop testing at different weights and different temperatures. Use the "distance traveled" dataref in X-Plane to help you with that.

You can also do the same (cut an engine at V1) to determine the net flight path profile. Now measure distance to obstacle to find the required profile and then you can do some ballpark guestimation at which temperature you still "could just make it" (add 0.8% to this for margin).

Of course all of this changes with runway state, intersection takeoffs, tailwind, anti-ice on, etc...

Cheers, Jan

 

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