I use FSX and the PMDG-747-400 aircraft.How are the v speeds computed when the airport altitude is high,as for instance in Mexico City.I think the v speeds have to be higher then on sea level but how are they determined.In FSX and in the real world ?Any ideas ?Thanks

You use T/O performance charts or if your lucky the airline will provide a laptop onboard the aircraft with performance data.For FS I would suggest using a program called TOPCAT for the 744 and for the md11 try UTOPIA (here on Avsim)Rob

I could be wrong here, but I think the takeoff distance is significantly increased with higher density altitude but the Vr speed remain the same. The air is less dense resulting in a higher required forward velocity to obtain the equivalent indicated airspeed. If the V1 speed (decision speed) is based on distance to stop when runway length is a constraint then that is impacted by the less dense air. If the density altitude is high enough, you don't have enough runway to safely reach V1.

Robi77,Please sign your forum entries with your real name - it makes it so much more personal. Ta!There are 3 V speeds for takeoff - V1, Vr and V2. Airfield altitude will affect your aircraft performance quite considerably. The higher the airport, for a given ambient temperature, the thinner the air will be, and you will get less acceleration down the runway (less air to push against and less air for the engines to ingest). Of course higher airports will also tend to be colder - making the air more dense. So it is a balance.V1 is ONLY about your runway length for the given temperature and pressure. V1 is calculated as the point at which the aircraft will have accelerated down the runway at takeoff throttle setting (not necessarily full throttles) and achieved speed V1, giving you sufficient remaining runway length to abort the takeoff, apply brakes and come to a stop before the end of the runway. TOPCAT will calculated how much spare runway length you will have at the point you come to a stop. At V1, you are required to takeoff whatever the circumstances because to do otherwise would cause you to overrun the end of the runway (not a good idea as you will plough through all those nice ILS antennae, and may upset some vehicles on the perimeter road!)Vr is your rotation speed. Usually calculated as a speed a little higher than V1, it is the speed at which you raise the nose and expect the wings to lift you off the deck.V2 is your initial climout speed. Higher than Vr, this is the speed that will support the aircraft weight through the initial climb with takeoff flaps set.You can determine your V speeds from tables that are published for each aircraft and takeoff weight. TOPCAT does all the hard maths required to determine the V speeds very accurately, as in the real world.HTH,Cheers, Richard