Tuesday, January 26, 2010
When Slower Is Better
The whole point of most airplanes is speed—except during landings
Trouble is, we all slip into bad habits now and then, and recently, I did exactly that. I had been flying return-to-service check flights after maintenance and annual inspections on 300/400 Cessnas, Dukes and miscellaneous other twins, and I was so used to seeing 110-/120-knot approach speeds that I had forgotten how much easier it was to fly more slowly. I was sloppily using 85- to 90-knot approach speeds in my LoPresti Mooney.
Part of the good/bad news about flying a variety of airplanes on a regular basis is that you sometimes don’t have all the information you need on approach speeds. Though it’s true there are other contributors to approach speed besides dirty stall (i.e., stall characteristics, crosswinds, aircraft weight), maintaining a proper margin above stall is the primary concern.
Forty years ago, I flew a Super Cub in Alaska that stalled at less than 40 knots, and I remember droning around the pattern at 50 knots and feeling as if I could jump out and run alongside during the flare. Operationally, the difference between a 50- and 70-knot approach speed is far more than 20 knots. Indeed, it seems as if you’re covering ground at double the speed. The Cub’s wing was so large and the stall so slow that with any significant wind, the aircraft’s owner and I often saw takeoff and landing rolls shorter than 300 feet.
Today, the vast majority of general aviation singles sport stall speeds in the 50- to 60-knot range, with fixed-gear models flying more slowly, and retractables stalling more quickly. Perhaps sadly, there are few Cubs available these days. Discounting LSA, airplanes such as the Top Cub and Aviat Husky are among the few that can claim stall speeds below 40 knots.
Accordingly, I’ve adopted the universal, super-simple rule for jumping from airplane to airplane. According to the FARs, dirty stall speed on certified singles can’t exceed 61 knots, and the generic rule for approach speed is to use 1.2 to 1.3 Vso.
That means on a Bonanza, Mooney, Malibu or Centurion, I typically shoot for a VFR approach speed of about 1.25 Vso. That works out to about 75 knots on a high-performance retractable, and yes, I’m aware that’s a little sporty for a PA46, especially at gross. Piper recommends a minimum 78 knots down final on the Mirage, though Cessna allows 72 knots for short-field approaches in the P210.
For lighter, fixed-gear singles that I know enjoy slower stall speeds, I’ll stick with 1.2 Vso, or about 65 knots in a Skyhawk/Archer/Sundowner/Tiger.
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