Sunday, August 1, 2004
The Last Spin
Why do experienced and inexperienced pilots alike fall victim to this all-too-common traffic-pattern accident?
This is how it happens. The pilot turns base to final and notices a following wind is causing him to overshoot the centerline. He adds a little left uncoordinated rudder in an attempt to bring the nose of the aircraft back toward the runway. The aircraft rolls a bit to the left and he compensates by adding some right aileron to hold the 30-degree bank angle. The nose also drops slightly, so he compensates by pulling back a bit on the yoke or stick and adding a little power to maintain airspeed. Suddenly, the aircraft snap-rolls left to 150 degrees of bank. He instinctively pulls back on the yoke or stick to get the nose back to the horizon and, at the same time, uses aileron to turn the aircraft back to the right. Without warning, the airplane stalls, rolls inverted and spirals into the ground.
For pilots who are constantly drilled on recognizing characteristics of insipient stalls, why does this stall-and-spin scenario continue to happen frequently with fatal consequences? Why do pilots who recover from every other type of stall seem unable to recover from this one? The reason is simple. This stall occurs without warning.
A stall is nothing more than exceeding the critical angle of attack (AOA). The airflow over the top of the wing starts to separate (otherwise known as boundary layer separation), and we experience a loss of lift. Where on the wing the stall occurs, how the stall progresses and how we know in the cockpit that we’re approaching or actually in a stall can depend on many things, including wing shape, tapering, wing twist, boundary layer enhancers (BLEs), stall strips, vortex generators, washout, etc. Understanding how this stall in the traffic pattern can surprise even the most experienced pilots begins with a look at how wing designs stall differently.
General-Aviation Wing Stall Characteristics
Most GA aircraft today have wings designed to stall at or near the wing root first, then progress out toward the wingtips. This is a good thing because as the wing root area stalls, it generates turbulent air that flows over the elevator and around the fuselage. We feel it in the cockpit and on the yoke or stick as stall buffet—a warning to recover by reducing AOA.
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Labels: Accident Statistics, Flight Hazards, Flying Skills, Learning Center, Safety, Stalls and Spins