Unrecoverable Spins

In December 2000 a tower controller in southern Florida received a single radio transmission:

"Mayday mayday mayday Pitts 260DB in an unrecoverable flat spin at 3,500 feet."

The airplane crashed in the Everglades, coming to rest partially inverted and nearly vertical in several feet of water. The canopy, which had been jettisoned in flight, was several hundred feet away. The bodies of the pilots were closer by the wreckage; both had bailed out, but there had not been time for their parachutes to open.

A motorist on Interstate 75 had seen the crash and alerted the local sheriff's office, but subsequent efforts by the National Transportation Safety Board to locate the witness were unsuccessful. He or she might have provided useful information about the appearance of the spinning airplane and the altitude at which its occupants emerged. In the absence of that information, the NTSB determined that there was no evidence of any airframe, control system or engine malfunction, and that the airplane had entered an inverted flat spin while maneuvering. Investigators also determined that the airplane had been 120 pounds over its maximum acrobatic-category weight at the time of the accident and that its center of gravity had been slightly-about three-eighths of an inch-aft of the rear limit.

Associates of the two pilots mentioned that the owner had been having trouble with landings and with hammerhead stalls. He would normally sit in the rear seat, as he would to solo the airplane, and the instructor, who was preparing him for his biennial flight review and teaching him some basic acrobatics as well, would be in the front. Although the NTSB report does not provide the weights of the pilots, the aft CG condition was most likely due to the weight of the rear-seat pilot.

The maximum acrobatic weight is normally a function of structural strength-or rather it is the other way around, with the size of structural members being determined by the selected weight and load factor. The weight has a secondary effect, however, in relation to spinning. It affects the airplane's moment of inertia, which is a measure of the amount of force needed to make it rotate or stop rotating. Only one aft limit for the CG is specified, but it assumes the maximum acrobatic weight; excessive weight exaggerates the effect of an aft CG position on spin recovery.

The mention of the hammerhead is suggestive, because this is a maneuver from which, if it is badly executed, it is possible to enter an inverted spin. In the hammerhead, or stall turn, the pilot pulls the airplane into a vertical climb and, when a little forward speed still remains, kicks full rudder while adding a little opposite aileron. The airplane pivots about what would normally be its vertical axis (which is now horizontal) into a vertical dive, from which the pilot recovers along the same track as he entered the maneuver, but in the opposite direction. Despite the names "hammerhead stall" and "stall turn," a correctly executed hammerhead does not involve stalling; the airplane has little airspeed at the top of the climb, but its angle of attack is zero.

Various things can go wrong with a hammerhead, including a tail slide if the pilot is late with rudder. If the airplane rotates about its spanwise axis, a startled pilot might suddenly apply forward stick together with the rudder and unwittingly set up the conditions for an inverted spin entry. But we know nothing about what the airplane was doing before it spun, and in fact it is not even clear from the accident report why investigators concluded that the spin was an inverted one. The mayday call merely mentioned an unrecoverable spin, and the fact that the airplane came to rest in the water in a slightly inverted attitude after a violent crash does not mean that it was in that attitude as it descended. Furthermore, the mayday call reports a flat spin, and the airplane appeared to have struck in a vertical attitude. On the other hand, once the pilots had gotten out, the CG would have moved forward and the airplane would have tended to drop into a vertical dive on its own.

The fact that the pilots were late bailing out could be taken to suggest an upright spin. They evidently recognized that they were in trouble while they were still at a high altitude-3,500 feet. An inverted spin is a negative-G maneuver, tending to pull the pilots out of the cockpit. They may have continued to try to arrest the spin and save the airplane-normally a Pitts recovers promptly, even if the pilot merely lets go of the controls-or they may have found it difficult to get out of the cockpit against the G-loads induced by an upright spin. Any developed spin-especially an unintentional one-can be quite disorienting, and pilots have been known to step on the wrong rudder pedal to recover. But the instructor pilot had a good deal of acrobatic experience, and it is unlikely that he would have used, and persisted in, a faulty recovery technique; it is more likely that the airplane, because of its weight and CG position, did not respond as expected.

A similar accident occurred four years later in Oklahoma. In this case the airplane was a kit-built Christen Eagle II, a sport biplane nearly identical to the Pitts. A witness about a mile from the accident site described the airplane climbing vertically, then falling over into a flat spin at an altitude of 5,000 feet above the ground.

In this case as well, weight and CG location were suspect. The acrobatic flight weight and CG limits of the Eagle are 1,520 pounds and 99.60 inches. The NTSB estimated the weight at the time of the accident as 1,589 pounds. The 268-pound pilot was in the back seat and his 225-pound passenger in the front, putting the CG 1.75 inches aft of the aft limit. The airplane was light on fuel-fuel burn moves the CG aft-but the effect of fuel weight on the CG position, compared to that of the pilots, was minor.

The Eagle II flight manual is emphatic about the importance of CG location, warning that spin recovery may be "extremely difficult or impossible" if the CG is out of limits. "Weight and balance considerations must be taken seriously," the manual warns, "and pilots must be absolutely certain that the flight CG of their aircraft is within design limits." There was, in fact, no way that these two big men could have flown the Eagle and been within acrobatic CG limits, even if they had switched seats.

In a normal spin, the downgoing wing (the right wing in a spin to the right) is at a higher angle of attack than the other, but because both wings are stalled, the wing with the greater angle of attack has less lift. It is this difference that keeps the airplane spinning. If the center of gravity is not too far aft, the nose remains down and the flow of air over the airplane, and particularly over the rudder, has a significant front-to-back component. If the CG is too far aft, however, or the fuselage moment of inertia is too large, centrifugal force, pulling the CG away from the axis of the spin, tends to raise the nose and flatten the spin. In a fully-developed flat spin the airflow is predominantly from below, and the chordwise component of flow over the vertical tail is slight. The rudder, normally the primary spin recovery control, becomes ineffective. Depending on the airplane type-spinning behavior can be highly idiosyncratic-unexpected combinations and timing of control movements and engine power (used for its gyroscopic effects, as well as to blast the tail surfaces with slipstream) may be needed to recover. In many airplanes, the speed and stability of the spin increases with time, and it is more difficult to recover late in the spin than early.

On occasion, an airplane fails to recover from a flat spin for no apparent reason. In April 2004 a pilot rehearsing for an airshow in a Sukhoi SU-31, a competition acrobatic single-seater, initiated a flat spin at 5,000 feet over the ocean off the Florida coast at Fort Lauderdale. The spin continued to the water, killing the pilot, who did not bail out, evidently believing until it was too late that the airplane would recover from the spin. The NTSB found no indication of a mechanical malfunction.

Many airplanes will recover from a spin if the pilot simply releases the controls-this is the so-called "Muller-Beggs recovery technique"-but the method is not universally successful. In November 2004 the pilot of an amateur-built Skybolt biplane entered a left spin at 3,000 feet and after two turns neutralized the controls. The airplane continued to spin for some 6 to 8 revolutions before the pilot applied out of spin controls-right rudder and, presumably, forward stick (the NTSB report is unclear about the stick)-without effect. He then tried engine power for several seconds, but the Skybolt continued to spin. The pilot then neutralized the controls again, and this time the biplane stopped spinning, but too late. It struck trees as he tried to pull out and was substantially damaged; the fortunate pilot emerged with minor injuries and a good story to tell.

There are a couple of things that all pilots ought to know about spins. One is that the classic and almost inevitably fatal stall-spin accident occurs at low altitude, usually in the traffic pattern, when there is insufficient height for recovery. It is consequently argued-this is the FAA's position-that the proper antidote is to train pilots to identify and avoid stalls, not to recover from spins (which ceased to be a required part of the training syllabus in 1949).

Another is that basic spin recovery technique is the same for all airplanes. In the extremely unlikely event of an unintentional spin occurring at a sufficiently high altitude to allow recovery, the acronym PARE (not CIGAR, ARROW or GUMP) summarizes the proper actions. Power to idle. Ailerons neutral. Rudder hard against the spin. Elevator sharply nose-down (or nose-up for an inverted spin). It should be recognized, however, that the CG range for good spin recovery behavior may be much more restricted than that for normal flight operations, and that there is no certification requirement other than a normal or utility category airplane be able to recover from a developed-that is, more than one turn-spin.

This article is based solely on the National Transportation Safety Board's reports of the accidents and is intended to bring the issues raised to the attention of our readers. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory.