Originally Posted by
Mad (Flt) Scientist
For those advocating use of AOA for approach instead of airspeed. If the aircraft weight is correctly known, they amount to the same thing. Consider, though, what happens if the weight is wrong.
Assume the aircraft is 5% heavier than calculated (a pretty gross error, but its just a number...)
For an aircraft flying airspeed, it will be flying at a higher AOA than it should (effectively flying at 1.20 Vsr instead of 1.23Vsr) and it will also have 5% more energy/moment to bring to rest - say 5% more landing roll required (though in fact the brakes will work better, so it'll be less in practice)
If instead the aircraft flies AoA, it'll be at the 'correct' 1.23vsr, and consequently 2.5% faster than the book says for the weight. Therefore it'll now have 10% more enegry/moment to bring to a halt.
Flying AoA protects more in terms of stall speed margins, but puts all the error into the landing distance, unless you recalculate based on the actual speed flown (which would seem a bit of an imposition)
We used ADD (Airstream Direction Detector) to land Sea Vixen and Buccaneer aboard ships. ADD removed the need to adjust approach airspeed to cater for different landing weights. It was always sensible to check that the target approach ADD against the ASI/AUW was as expected. Both Vixen and Bucc had audio ADD which allowed the pilot to concentrate visually on the centre-line and mirror info, which were as vitally important as the speed. The audio signal was: short high interrupted tone for too fast, (peep peep peep) long, low, interrupted tone for too slow (burp, burp, burp) and a steady note for on target ADD. I only once met an ADD system that was sticky and therefore gave an innacurate reading,--and that caused loss of the aircraft.
So I remember being unimpressed later at Warton, with the recommended take off procedure for a heavy Jaguar, which was to rotate to, and climb out at, a high ADD reading, for guaranteed best performance. I remained uneasy about putting reliance on a simplex instrument that couldn't be properly checked on the ground before each flight. (You could twiddle the probe and see the gauge move, but you couldn't assess stiction).
In the military, returning to land with underwing stores, (in some cases with hangups which the pilot didn't know were there), use of ADD guaranteed stall margin on the approach.
When later, watching daughter flying 767s for BA, I was surprised that they didn't use AOA for the approach, with such a huge variation in possible landing weights. But she assured me that AOA was indeed measured and was contributing to the safety systems, such as stall warning.