A question of emphasis

By David Jack Kenny

Make a regular habit of reading NTSB reports, and you’ll likely be struck with admiration at the amount of information their investigators can distill from the most catastrophic accident scenes. They can extract physical evidence making clear how hard (and whether) the engines were running, whether the airframe failed during or prior to impact, and how the fuel selectors were set. The board’s factual reports generally impart a forceful sense of the weight of the evidence, but occasionally those narratives don’t quite seem to align with the eventual finding of probable cause.

During the afternoon of Nov. 23, 2012, a 68-year-old private pilot with more than 5,000 hours of flight experience undertook some flight testing in an amateur-built Coot-A Amphibian. The Coot is a tandem two-seater somewhat reminiscent of the Lake Amphibian, a low-wing flying boat with a pylon-mounted pusher engine. On this occasion, its water-handling qualities were not under evaluation; the test flights were made from the Calaveras County Airport on the western flank of California’s Sierra Nevada. 

After two flights with other pilots in the back seat—one with the canopy off, one with it installed, and both with the gear down—the pilot took off solo and retracted the gear. According to both the pilots who’d accompanied him on the first two flights, the airplane’s speed increased impressively after the gear came up. Along with a third witness, they saw him fly an apparently normal downwind leg, then overshoot the base-to-final turn. One estimated the bank angle at 60 degrees when the airplane stalled and began to spin, the other at 75 to 80 degrees. Along with a third witness, they saw pieces begin to depart the airplane when or shortly after the spin began. Both wings ultimately separated from the fuselage, and the pilot was killed.

The NTSB attributed the accident to “The pilot's excessive flight control inputs, which led to flight that exceeded the structural limits of the airplane and resulted in structural failure of both wings.” The interpretation that the pilot’s “excessive” control inputs precipitated the accident rests largely on minute examination of the physical evidence:

The right wing’s front spar fracture face showed areas of tension and compression failures of the wood fibers that were consistent with the wing failing in a downward direction. Both the leading edge and front spar of the left wing had diagonal cuts through them that were consistent with having been struck by the airplane’s propeller...the propeller strike on the leading edge and front spar could only occur if the left wing failed upward into the propeller. There were no obvious signs of rot or preexisting conditions in the wood spars examined, and none of the wing attachment bolts failed.

The investigators likewise note “… witness reports indicating that the airplane was in a continuous steepening turn from the downwind leg to final approach immediately before the accident” and conclude that the pilot overstressed the airframe. The witness accounts leave little doubt that the base-to-final turn reached an unusually steep angle of bank, but the implication that the pilot did this deliberately is hard to reconcile with the same witnesses’ descriptions of the first two flights of the day.

The one who’d accompanied him on the first flight recalled the pilot commenting that turning the airplane required more rudder pressure than he’d expected, and that it handled poorly with the landing gear down; according to that witness, he was “anxious to see how it handled with the gear up.” The same man watched the second flight, which he described by saying that the “pattern was wider with an extended downwind, the turns appeared to be of 20 to 30 degrees [of bank], smooth, and there was no visible overshoot [of the runway] on final.” The pilot in the back seat on that second flight described an “uneventful” right pattern.

Neither account leads easily to the conclusion that a highly experienced pilot flying an unfamiliar airplane deliberately twisted it into an extreme bank angle close to the ground rather than going around after overshooting the turn to final. (Curiously, the NTSB report doesn’t say how long the pilot had owned the airplane, but the mention that he “wanted to see how it handled with the gear up” and the fact that the FAA registry still lists the airplane as “Sale Reported” suggest it was a new acquisition.) It seems at least equally plausible that on both his first solo flight and first flight with the gear retracted—going much faster than on previous test flights, with the center of gravity shifted forward and less rudder authority than expected—he found himself unable to overcome an overbanking tendency in the limited altitude available.

It’s also possible that this particular airframe, built from plans nearly 30 years earlier and subjected to unknown shocks and stresses ever since, couldn’t sustain the same G-loads required of certified airplanes. Whatever the truth, this accident also points up the wisdom of the advice offered by the FAA’s Advisory Circulars on test-flying and transition training: As safe and familiar as the traffic patterns seem, the safest place to learn the quirks of an unfamiliar aircraft is at altitude, not within 1,000 feet of the ground.