Just a quick note. The stop margin displayed in your program (i suppose FOVE, right?) may not be the real stop margin if you did an RTO - this one may be far greater. Do a simple test.
1) calculate any tkof data that give you good flex increase. Say for OAT 25 you get FLEX /AssT of 45.
2) check stop margin, let's say 600 m
3) calculate again, this time for actual OAT at the Flex calcualted temperature - 45 degrees.
4) check stop margin, you should see the same number - 600m.
Now, in OAT 45 the engines will produce certain thrust T(45), your V1 may be 150 kt IAS, may be 170 TAS, with no wind GS 170 kt. That is the real speed you need to stop.
OTOH in OAT 25 the engines will produce same thrust T(45), your V1 will be indicated the same 150 kt IAS, but that will only be 158 TAS or G/S.
So there are two large margins not visible at first sight.
a) distance margin to reach the point on runway, where ASI reads V1 in kt IAS. All performance is calculated under assumed OAT of 45 (not only the thrust setting!) - not necessarily correct,There may be other inputs that are calculated under assumed temp. of 45° - hopefully not wrong, with actual OAT less, same V1 IAS will equal to lower TAS-G/S and this will be reached sooner during the tkof run. This in turn will give you more runway length to brake or continue on one engine than indicated by the computer tool.
b) energy margin. If the real life velocity is less than calculated by the computer tool (due to less pronounced IAS -> TAS rise) the total energy to be dissipated during braking is less. And velocity is squared ! E=1/2*mass*velocity^2. This will reduce the actual braking distance.
So not only you start braking sooner, but you also need less braking distance to stop in OAT 25 than in OAT 45 with same thrust and same V1.
The chances are high that the stop margins displayed by your computer tool for OAT 25 and Flex 45 are those that pertain to actual of OAT 45. The term used by Boeing "Assumed Temperature" is far more descriptive. But you really need to make the test suggested above at first, just to be sure.
FD (the un-real)
Last edited by FlightDetent; 6th December 2007 at 14:48.