I use TOPCAT for my takeoff calculations. I usually try to aim to have a margin of 2500 feet at Vr, but I was wondering what I should really be aiming for. I was just thinking that if I had an engine failure right after V1, would I have enough runway left to make it into the air? Should I be planning for more of a margin and how much should I be looking at?Ryan GamurotLucky to live Hawai'ihttp://www.virtualpilots.org/signatures/vpa296.png

I think that margin takes the remaining runway into account. So if you had a 5m margin, you have enough runway to stop + 5m.The minimum you should therefore have is 0 meters. At zero meters margin, you have exactly enough runway remaining.In other words, the margin is the 'excess' on top of what you need.That's how I've understood it anyway :()Subs

Ryan,TOPCAT does these calculations for you automatically. You tell it the takeoff runway, your TOW, QNH and water depth on runway. From these it calculates all the numbers you need including stopping distance at N1.I have found it a no-brainer as it does all my takeoff and landing planning for me.Cheers,

PS. It is also very useful in deciding which takeoff thrust setting to use. Starting with TO2, I work backwards until the thrust setting does not cause a warning from TOPCAT. Couldn't be easier!Cheers,

Thanks everyone for your replies. I'll stick with my 2500' margin. Ryan GamurotLucky to live Hawai'ihttp://www.virtualpilots.org/signatures/vpa296.png

>I think that margin takes the remaining runway into account. >So if you had a 5m margin, you have enough runway to stop +>5m.>>The minimum you should therefore have is 0 meters. At zero>meters margin, you have exactly enough runway remaining.>>In other words, the margin is the 'excess' on top of what you>need.>>That's how I've understood it anyway :()>>SubsThis is true in the literal sense. "0" is the absolute minimum. However, this is assuming that nothing goes wrong during your rejected takeoff. Any number of unexpected factors could increase your stopping distance during an RTO. Suppose the pilot is a little slow in pulling back the throttle? Suppose the brakes don't work quite as well as they should, or your tires are just a little on the slick side. Suppose your headwind suddenly dies down just as you start you takeoff roll. There is no harm in building in a nice big margin if you have it available. LIke the bumper sticker says, "#@$%@ happens."Andrew

>runway left to make it into the air? Should I be planning for>more of a margin and how much should I be looking at?Your real margin is having all engines operational. Your next margin comes from the fact that engine failure may happen way before the V1 point or much later. The further away engine failure occurs from the V1 point the better off you are, so this is your next big margin. No airline is adding yet another layer of margin beacuse the whole argument becomes nonsensical from the probability standpoint. There is so much less "margin" in other aspects of flying.Michael J.http://img142.imageshack.us/img142/9320/apollo17vf7.jpg