View Full Version : Performance - Additional stop margin at RTO due to TAS effect ?

12th Jul 2007, 09:04

I first posted in the wrong forum...
now here is hopefully where the Performance Gurus are hidding. :)

I got the following question and I am still a little puzzled about it.

If you got a flatrated engine ( as most engines are nowadays ) and you are doing T/O calculations, your data will give you most probably a flex Temp and a V1 Vr and a V2 speed.
now there are two formulas:
1. Per each 10 degrees difference between OAT and Flex you got 3% more stopping distance then calculated out of the TAS ( True Airspeed Speed ) Effect.
( A high Flex Temp at 40 degrees and an OAT of actual 10 degrees brings an 9% margin on the Accelerate Stop distance or was it the TOD ?

That was the first question.

2. The other rule of thumb is: Per 1 degrees celsius Delta between Flex Temp and OAT, you got another 10m more stopping distance.
What if you got a high elevation (3000ft, engine is flatrated 32 degrees and the Flex Temp is 60 and the OAT at 3000ft is 9 degrees C )
That would mean another 510 meters stopping distance. Is this correct ?
From 60 to 29 is the Flex Range. ( At higher GW the Full T/O thrust has to be used )
Might it be only the 31 degrees times 10 meters ? so is it a plus of 310meters stopping distance ?

So is this correct ?

you FLEX on T/O and got an OAT of 10Celsius and your calculated FLEX is 60.

Now the stop margin shows 2 meters only.

But you got a margin actually from about 500meters due to the TAS effect.
So you brief for RTO we got a stop margin of 502 meters.
This is common practice in my company and I am not sure if this is o.k.

Any ideas ?

Old Smokey
12th Jul 2007, 17:44
Everything that you say is essentially correct. When Flex / Assumed Temperature is used, the calculation considers the engine only, and provides you with a Flex / Assumed Temperature to give the equivalent thrust as though the day were actually at the Flex Temperature, and produces the reduced thrust accordingly. The calculations DO NOT re-compute ASDR, TODR etc., thus, V1, Vr, and V2 being the same IAS, will be at a lower TAS than assumed.

From this, ACTUAL Accelerate-Stop Distance required will be less, OEI Distance to screen height will be less, and AEO margin to TODR will be greater than the nominal 15%. In short, you win in all cases. A fourth bonus is that for an OEI continued Takeoff, TAS during the 2nd segment will be less, leading to an increased climb gradient.

It's win, win, win, win all the way! :ok:


Old Smokey