V1-Ground speed or airspeed?

[847NAS]

I was thinking that with a headwind (as in most takeoffs) groundspeed in relation to airspeed would be lower than without a headwind, surely when stopping an aircraft in an aborted takeoff the actual speed against the ground is more important than IAS?

So what is the normal procedure?

Thanks in advance

Jack

[wiggy]

V1 is IAS and Head/tailwind is taken into account when calculating the value as part of the performance calculations...thus the V1 call and the Stop/Continue decision are done reference the IAS (on everything I've flown).

However, having abandoned you will probably want to know how much energy has gone into the wheelbrakes to allow you to decide if they are of any further use, are the tyres about to bang, etc. That's where having knowledge of the Brakes on Groundspeed comes in really useful - and on some aircraft the Brakes on Groundspeed will be available from the maintainance computers or similar.

[847NAS]

Understood, that clears it up nicely.

Thanks wiggy!

[Northbeach]

The “V-speeds” are mostly flying speeds; meaning they reflect what the aircraft is capable of doing in flight. The lift the wing is capable of producing depends on the pressure differential between the upper surface and the lower surface of the wing, and that is a function of the speed of the air over those surfaces. Not the speed of the aircraft over the ground.

A simple illustration of how sometimes flying speed is unrelated with speed over the ground is the reason we tie down general aviation aircraft on the ramp. If you do not tie down your Piper Cub and along comes a squall blowing a 60 knot gust you may find your little Cub on her back. Her wheel blocks may have been in place and the wheels may not have even moved (perhaps the brakes were set-if she is equipped with parking brakes) yet she went airborne-why? Because with the gust of wind suddenly the wings were producing lift and she went flying just as her designers intended. So flying speeds and speed over the ground may or may not correlate.

The same aircraft at the same weight at sea level under standard temperatures will use less runway getting to V1, than it will at a different airport located at that aircraft’s maximum certified takeoff altitude under ISA+15C.

Landing at the same weight and at the same IAS (Indicated air speed) the ground speed will be much higher at the higher airport elevation under ISA+15 than it will be at the sea level ISA airport. Therefore the airplane will use more runway landing at the higher elevation hotter temperature airport, than the same airplane at the same weight flying the same indicated airspeed would at the cooler sea level airport (same weight and same IAS).

You are right on an aborted takeoff the stopping distance will be determined by the speed of the mass over the ground. But the V-speed of V1 is determined by IAS (Indicated air speed). So on one had it is ground speed that becomes important and on the other it is indicated airspeed. It is helpful to remember that V1, indicated airspeed, is adjusted for headwind/tailwind, temperature and runway slope to account for the differences those factors will have in stopping a given mass. My Boeing 737-800 performance handbook gives me V1/VR/V2 adjustments for altitude, temperature, wind and runway slope.

For example at a 77,000 kg takeoff weight (approximately 170,000 lbs+ 26K engine takeoff thrust rating) flaps 5 takeoff V1 is 147 KIAS. If everything else is standard except runway slope; and then we compare a 2% positive slope to a 2% negative slope we get a -3 knot adjustment to 147 KIAS resulting in a V1 of 144 KIAS for a 2% downhill slope. This compares to a +1 knot for a +2% positive slope giving us a V1 of 148 KIAS.

Unadjusted V1 for 77,000 kg takeoff weight = 147 KIAS
Takeoff downhill: negative slope -2%. (Subtract 3 knots) V1 = 144 KIAS.
Takeoff uphill: positive slope +2%. (Add one knot) V1 = 148 KIAS.

[xj8driver]

There, that simplifies matters. I hope you were paying attention...

:ugh:

[847NAS]

No the above makes perfect sense to me. I reasoned that there must obviously be adjustments to compensate for small but important differences in V speeds more relevant to speed along the ground.

Northbeach, I did understand how V speeds reference to the speed of the air over the airframe, but your explanation made that all the more clear.

Many thanks!