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N320278 - TOM and downhill slopes

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  • N320278 - TOM and downhill slopes

    The situation - I am taking off from a runway and I am V(BME) limited.

    This must mean that at my TOM V(MBE) intersects the 'too slow to go' line on the V vs Mass graph. I.e. for a safe take off run, my Minimum value of V1 is higher than V(MBE) which is not allowed because I would destroy the brakes before I stop.

    My TOM must therefore be reduced to a point where the Min V1 = V(MBE).

    If the runway now slopes down hill, I will accelerate faster. I will reach V1 earlier on the runway. This means I can take off slightly heavier since the acceleration is a bigger part of the ASDR than the deceleration. It's effectively the same as lowering the 'too slow to go line'?

    V(MBE) will also decrease because energy is mv^2.

    So I guess the V(MBE) line intersects the new 'too slow to go' line at about the same place as it did before, so no change in Max TOM?

    Obviously V(BME) is a product of the tyres and the mass. You have to get airborne and you can only do it at Vmu for a given mass in the TO config.

    So downhill doesn't fix anything there. But downhill does change the FLLTOM. And if the downhill slope bought the FLLTOM below the V(MBE) limit Mass then the slope could still affect the Max TOM.

    The answer given is slope has no effect.

  • #2
    I think you have analysed this quite well, except that a downhill slope increases FLLTOM and reduces the 'normal' V1 and the examiner was asking about the tyre speed limit, not VMBE. Too well, in fact, because the examiner only asks 'does slope affect the tyre speed limit?', to which the answer would be no. If it had been about VMBE you are right, the answer would have been much more complicated!
    Last edited by Alex; 02-10-2020, 16:25.


    • #3
      Thank you.