Very large aircraft, or aircraft that don't have the high thrust to weight ratio of most modern big twins accelerate relatively slowly, around three knots per second. If you were flying into a 30knot head wind at a Vref of say 140knots then of course your groundspeed would be only 110knots, your inertial speed. If as you descend on your approach the wind shears away to L&V then you faced with an IAS of only 110knots, 30 knots below your Vref. Very close to, if not at Vstall with some obvious consequences. Some operators use a wind additive i.e fly faster than Vref to counter this possibility, others fly what they call a 'reference groungspeed technique'. This technique involves calculating the minimum groundspeed you should fly in the prevailing conditions to arrive over the threshhold, after having passed through any shear, at or close to your planned Vref. Then you won't be faced with having to accelerate a heavy aircraft from a very low inertial speed to recover lost airspeed. That's a very basic version of it, if you want any more details e-mail me.
Why is it always easier to see the typo's after you've posted??
[This message has been edited by Bullethead (edited 09 April 2001).]