(Airbus) How To Calculate The T/O Stop Margin (For Rejected T/O) ?

[kuwait340]

Hello...

in our airline we are using the computer to calculate the T/O speeds (L.P.C)...after the calculation comes up...plenty of info are avail such as the stop margin available.

if you didn't like the very few meters as a stop margin(Heavy Wt + short Rwy)...you simply can decrease the flex temp (more thrust)...and you will have more distance to add to the stop margin.

my question is:

before we had the computer as the main speeds calculation...we were calculating the speeds from the famous RTOW charts....but, did we have any clue of what is our stop margin ...i didn't have any idea

so is it possible to know the stop margin with out the computer ?

Thanks

[Old Smokey]

There is no simple way, but you could go to the AFM Field Length data, and, using your Actual Takeoff Weight, work the data in reverse to find the Field Length required. Any actual excess Runway Length available above the AFM Field Length required would then be the excess distance that you're seeking.

Regards,

Old Smokey

[kuwait340]

alright...

any idea on how is the computer programmed to calculate this distance ?

[FE Hoppy]

If you are JAA cert with low vis you should have accelerate stop distance tables in the AOM/AFM. you can use them.

[FlightDetent]

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)

[FE Hoppy]

All performance is calculated under assumed OAT of 45 (not only the thrust setting!)

Are you sure about this?

VMC!!

[kuwait340]

Hello...

we are not using the FOVE software..but something very similar.

i will try the test ....and i will let you know.

thanks

[FlightDetent]

Are you sure about this? VMC!!
No I am not! :ouch: Edited the above to indicate so. Thanks.

BTW, I'd like to ask: The Perf.Book provides minimum V2 speed sto observe Vmu/Vmca values based on pressure altitude(and weight). For a constant QFE, higher OAT means higer p.a. The higher the p.a. the higher the V2 limitaion expressed in kt IAS. If I used an actual higher temp to calculate V2, above the Vmu/Vmca limitation, and departed in colder air:
- The actual Vmu/Vmca limitation would be less so flying above the original limit would do no harm;
- If the original V2 value would be a constraining factor, I may be able to accept higher TOW once I recalculate to observe Vmu/Vmca value that corresponds to the actual OAT. (Wheater this is the case with a specific software tool is probably very hard to tell)

Correct?

Number two: FCOM also provides "Minimum Contol" values for V1,R,2. A different table, only pressure altitude (no weight). These are significantly lower and decrease with pressure altitude. What are they and why they lower with increasing p.a.:sad:

See, something has gone seriously wrong with my aerodynamics training. And memory perhaps.:(

thanks, FD (the un-real)

[kuwait340]

FD...

i am not an expert in this ...but i think the higher the pressure altitude the lower the speed is maybe because the density and the atmospheric pressure...may be :hmm:

may the experts shed some light please.

[Mad (Flt) Scientist]

FCOM also provides "Minimum Contol" values for V1,R,2. A different table, only pressure altitude (no weight). These are significantly lower and decrease with pressure altitude. What are they and why they lower with increasing p.a.

I'd guess that what these are are the minimum values of V1/Vr/V2 which respect the various limitations with respect to minimum control speed (V2 not less than 1.1 Vmca, etc., etc.)

They are reducing with pressure altitude because (I suspect) the minimum control speed is scheduled with pressure altitude, to reflect the fact that at higher altitudes the engine thrust is lower, therefore the VMCs are also lower. This is commonly done as a means of recovering some of the performance lost due to the thrust decrease.

[galahad]

any idea on how is the computer programmed to calculate this distance ?

S = V*dt, dt = dV/a then
S = VdV/a

a = g/W*[T-D-u(W-L)-Wsin runway slope

[kuwait340]

galahad can u please explain ...i did not understand