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.
