AVIATION SAFETY NEWSLETTER
Thought for the month..... A little inaccuracy saves a world of explanation.
Quit stalling around.... I overheard a conversation recently where
one pilot was explaining to another that it was dangerous to turn downwind. His admonition was that extra care
was necessary because the loss of speed from flying into the wind to flying downwind could cause the aircraft to
stall. Specifically, if the airplane was flying at 70 knots into a 15-knot headwind and the pilot made a turn
downwind his airspeed would drop to 55 knots that was below the stall speed for the airplane in question. I thought
at first that he was just throwing this out to see if I would leap at the bait, but, unfortunately, he was serious.
There are a number of factors that affect a stall, but making a constant airspeed turn downwind isn't one of them. Exceeding the critical angle of attack of the airfoil causes all stalls. That fact by itself isn't particularly helpful, but knowing what contributes to excessive angle of attack is. An airplane can stall at any airspeed, in any attitude, and every airfoil is subject to stalling whether it is an airplane wing, a helicopter rotor blade, or the turbine blade in a jet engine.
Some of the things that affect the stall of an airplane are: weight, because the more an airplane weighs, the more angle of attack must be maintained to produce additional lift to support the weight; center of gravity, the farther forward the CG, the higher the stall speed, the farther aft, the lower the stall speed; load factor, as the load factor increases, stalling speed increases; and angle of bank, as the angle of bank increases in a constant altitude turn, the stalling speed increases.
Flaps affect stalling speed because they reduce it. Recall that the definition of angle of attack is the angle formed between the relative wind and the chord line of the airfoil. The chord line is measured from the leading edge to the trailing edge. When we lower flaps, the trailing edge is lowered so the chord line is changed, increasing the angle of attack. If the airspeed remained constant, this would create greater lift and the airplane would try to climb. Or, the same amount of lift can be created at a slower airspeed. Note that the lower limit of the white arc on the airspeed indicator (power-off stall speed with gear and flaps in the landing configuration), is lower than the bottom end of the green arc (power-off stall speed in a clean configuration). This is why it is important to maintain a higher indicated airspeed when making a no-flap landing.
In the case of a downwind turn, as long as the same indicated airspeed was maintained, the wing is oblivious to the fact that it has turned downwind. The air flowing around it remains the same. If a pilot were, for some reason, trying to maintain a constant groundspeed when turning downwind, the airplane could be put in a condition where a high angle of attack is required to maintain a constant altitude at a reduced indicated airspeed. This is often the case when a pilot turns base with a tailwind, ignores the airspeed indicator, and tries to slow down by raising the nose and reducing power. It gets particularly nasty if the pilot fails to compensate for the tailwind and overshoots final. With an already high angle of attack, more than usual bank to try to get back on the centerline, and maybe a little pedal to try to push the nose around faster, the aircraft is in a perfect set-up for a stall/spin accident.
The conversation concluded with the rather interesting rule of thumb that the pilot should accelerate to at least twice the stall speed before turning downwind. This might explain why some traffic patterns get pretty large at times, and perhaps justify a good topic for discussion during flight reviews. It's hard to know exactly where to start to correct the inaccuracy offered-up in that conversation.
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