By David Jack Kenny
Those of us who spend a lot of time following NTSB reports often marvel at how much information the NTSB’s investigators can extract from even the most catastrophic accident sites. To a sufficiently expert eye, the charred fragments remaining after an aircraft hit the ground and was consumed by fire can still say a great deal about what went wrong, or at least what didn’t: The fire proves fuel was on board, scoring and twisting of propeller blades show how much power the engine was producing, and continuity of control cables between separations where fracture signatures suggest overload largely rules out flight-control failures. Even the pattern of soot marks on any unburned surfaces can help determine whether the fire began in flight or only after impact.
Yet sometimes the circumstances are so odd and the physical evidence so unusual that the most plausible explanation investigators can construct still rests on some degree of conjecture. Crucial details simply can’t be confirmed.
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One example was the crash of a two-seat PiperSport in Spring Hill, Fla., on the night of May 6, 2011. The airplane, flown by a 23-year-old CFI, had departed from Fort Myers, Fla., about 10:15 p.m. on the first leg of a positioning flight to Joliet, Ill. Though young, the pilot had already earned his instrument and multiengine instructor ratings, and had given the owners of the PiperSport dual in it. A medical application filed two months earlier claimed 2,350 hours of total flight experience.
The weather was fine, if dark, with clear skies, calm winds, 7 miles visibility, and a crescent moon setting in the west. The instructor knew the airplane and the route, having flown from Illinois to Florida with one of the owners the previous fall. Two factors, though, were not in his favor. The airplane was not approved for night flight due to its lack of both interior and panel lighting. And youth notwithstanding, the pilot had reason to be tired; he had started traveling around noon, when one of the owners drove him from Joliet to Chicago Midway to catch an airline flight to Fort Myers. According to that owner, the pilot had declined several different offers of lodging for the night, choosing instead to begin a 950-nautical-mile cross-country after 10 p.m.
The first hour and a quarter were uneventful. The flight proceeded northbound at 6,500 feet, receiving traffic advisories from Tampa Approach. A few seconds before 11:28 p.m. it was handed off to Jacksonville Approach; the pilot’s readback of the new frequency was the last transmission received from him.
Radar track data shows that in the next four seconds the PiperSport lost 100 feet, then 400 feet in the five seconds after that, and then 2,500 feet in nine more seconds. Its calculated rate of descent went from 1,500 feet per minute to 4,800 fpm to almost 16,700 fpm in the space of 18 seconds. Transponder returns ended after that, though one or two primary echoes were observed in the vicinity of the eventual crash site.
The wreckage was found with extensive crush damage everywhere except the tail. A post-crash fire caused the onboard ballistic parachute to discharge; the lack of soot marks on the tail seemed to rule out a fire in flight. The pattern of crushing suggested that it hit in an “almost vertical, nose-down, slightly inverted” attitude. Parts of the frame of the bubble canopy were found on the scene, but very little of the transparent plastic, and the one canopy latch recovered showed fire damage but no distortion from impact, suggesting the canopy had opened in flight. The few fragments of the shoulder harnesses in the wreckage showed some evidence of stretching, but the only remnant of a lap belt, which included one half of the buckle, did not.
The pilot’s body was found about a third of a mile away. A trail of debris consisting mostly of items carried inside the cockpit lay in between, with the heavier objects closer to the airplane’s flight path. No shards of the canopy were among them.
Testing on the ground with an identical airplane and an NTSB investigator about the pilot’s size showed several things: The abrasions found on the pilot’s body could only be matched if the shoulder straps were loosened, the lap belt unbuckled, and the pilot leaned forward and pulled himself up—and undoing the lap belt and loosening the shoulder straps was the only way the investigator could reach the baggage shelf behind the seats. In doing so, he found himself unable to avoid kicking the left rudder while pushing the stick right. In that position, either the loosened shoulder harness or the pilot’s headset cords could wrap around the T-handle for the canopy latch, pulling it open.
The NTSB’s best guess as to the accident sequence runs along those lines: As he read back the frequency change, the pilot tried to retrieve something from the shelf behind him. In the process he inadvertently shoved the stick and kicked in opposite rudder—just at the moment the canopy popped open. In the sudden noise and wind, in the dark, with the airplane rolling inverted, he was unable to regain control before he slipped from his restraints and fell. But as closely as this description matches all available evidence, the board had to acknowledge that uncertainty remains. The last paragraph before the official statement of probable cause is just four short sentences. It includes three occurrences of the word “likely” plus one use of “possibly.”