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August 19, 2008
Subject: "What's my approach speed, Denver tower?"
Airplanes use 1.3 Vs plus half the wind and all of the gust for the final approach speed. Sailplanes use best L/D which is very close to 1.5 Vs plus the wind and half the gust. Airplanes and sailplanes, gliders etc. stall at the same indicated airspeed regardless of the actual altitude. Yes, Virginia, your ground speed will be higher at high altitude airports (you are going faster because more thin air must pass through the pitot tube to get the same indicated airspeed). But the wing stalls at the same IAS, so no correction is needed.
Note this discussion does not apply to jets flying at high altitudes and speeds. Mach effect causes the stall speed to increase and the Vmo/Vne to decrease until only a few knots separate stall from Mach overspeed buffet. This is known as the "coffin corner" of the flight envelope. Definitely a place to avoid at high gross weights in turbulent conditions. There have been more than a few Captains that have encountered these conditions and asked, "Number 1, get us a lower altitude now". See you at the airport. Steve.
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