The Spin Debate
If spins can kill, why aren't pilots trained to handle them?
- By Joseph Bourque
- Air & Space magazine, November 2003
NASA Langley Research Center
(Page 2 of 5)
In the earliest days of flying, a spin was certain death, but beginning in 1912, a smattering of pilots somehow extricated themselves from spins. Mathematician F.A. Lindemann is most often named as the first to have developed an aerodynamic theory of spins and a procedure for recovery. He learned how to fly and tested both the theory and the procedure himself in 1916. The method of spin recovery now described in modern flight manuals, known as PARE, is no more than a mild refinement of Lindemann’s.
PARE is an acronym developed by flight instructor Rich Stowell to help pilots remember the sequence of control inputs necessary for a spin recovery: Power (close the throttle), Ailerons (neutralize), Rudder (full deflection in direction opposite the spin), Elevator (first, stick forward to un-stall the wing). An alternative is the Muller-Beggs method, which is similar to PARE with one dramatic exception: for PARE’s “neutralize ailerons” phase of recovery it substitutes “take your hands off the stick.” Neither of these methods is guaranteed to work for every airplane in every situation.
I needed to refresh my memory about PARE, so I sought out aerobatics instructor Adam Cope, who flies a Super Decathlon out of tiny Potomac Airfield, just outside of Washington, D.C. Cope is a born teacher. He may look young, but he knows his aerodynamics and excels at clear and sensible presentation. We first review the basics with his dog-eared charts. Figure 1 illustrates the most basic principle: As long as air flows smoothly over the wing, the wing will produce lift. As the wing’s angle of attack increases, air flowing over the top of the wing begins to detach from the wing, and with each increase in the angle of attack, the point of detachment moves closer to the wing’s leading edge. At some stage, which is different for each airplane, lift will no longer be sufficient to sustain the weight of the aircraft: That configuration of the wing with respect to the relative wind is called the critical angle of attack. Figure 2 demonstrates that a wing moving through the air produces both lift and drag. As the angle of attack increases, lift and drag both increase proportionately. At the critical angle of attack, however, lift drops dramatically while drag continues to increase. The result is a stall. A spin occurs when one wing stalls more sharply than the other, generally as a consequence of exceeding the critical angle of attack in connection with yaw (movement around the aircraft’s vertical axis).
In the air in his Super Decathlon, Cope has me do a few aerobatic maneuvers to get me used to employing the rudder, which will help control the airplane during spin recovery. Cope has prepared me intellectually, but no ground school can prepare you for the abruptness with which an airplane flips over, nor did I retain a memory of that shock from my training years ago. We do only two turns on the first couple of tries and I am able to recover adequately, but then we graduate to four- and then six-turn spins. As the number of turns mounts, the spins get tighter and faster. At four turns, I’m experiencing nystagmus—the eyes rapidly oscillate from side to side as they attempt to establish a point of reference. Consequently, when Cope tells me to recover, I perform the PARE sequence, but I’m unable to distinguish when the rotation stops, so we nearly fall into a spin in the opposite direction. Though I am too busy to detect it, I suspect that Cope is nudging the stick to keep me from getting into too much trouble. It becomes clear to me that without regular practice, I would not likely survive a spin if I allowed it to develop into more than three or four turns.
But the debate centers on whether spin training would improve pilots’ chances of avoiding inadvertent spins in the first place, and, if a spin should occur, would spin recovery training improve the pilots’ chances of survival. To understand that debate, some background is necessary. When the FAA eliminated the requirement for spin training in 1949, it shifted the burden of responsibility to the aircraft manufacturers, to some degree, by stating that airplanes should be made more spin-resistant. But while the FAA wanted more spin resistance, customers demanded airplanes that could perform, and at the time, those were somewhat incompatible goals: Increasing spin resistance necessitated a loss in aircraft performance—generally losses in speed and fuel efficiency.
Whatever spin resistance was built into general aviation aircraft manufactured before the 1990s was largely an accidental byproduct of other design considerations. A few craft were quite spin-resistant, but most were not, and some had unrecoverable spin modes, even after the FAA began requiring manufacturers to demonstrate that their craft could recover from a one-turn spin. Additionally, we now know (though it was not common knowledge until the late 1980s) that anything you do to improve spin resistance has the unfortunate side effect of increasing the difficulty of recovering from a spin. Unless you could produce a spin-proof aircraft, the tradeoff was a risky one.
So while the general trend over the years was for aircraft to be somewhat more spin-resistant because engineers were learning to design better airplanes, the high accident rate from stall/spin accidents clearly demonstrated that aircraft built before the 1990s did not meet the FAA’s goal for spin resistance.
At this point, it must be said that nearly everyone is in favor of spin training, even the FAA, as long as it’s not required. Says the FAA’s Wensel: “The FAA doesn’t prohibit spin training. That’s why we require CFIs [certified flight instructors] to undergo spin training, so they can pass that on to their students if either the instructor or the students wish that training to take place.” Warren Morningstar, vice president of communications for the Aircraft Owners and Pilots Association, says his organization agrees with the FAA’s position.