Aftermath: Three to Get Ready

Considering how rare single-seat twins are outside the military, it’s remarkable how little information about N72TZ can be found on the Internet. The NTSB’s report on the fatal outcome of its first flight provides scant detail about the airplane itself. It seems, from bits and pieces mined here and there, to have been one of a kind, designed by a homebuilder, to have had a gross weight of 800 pounds and to have been powered by two two-stroke Hirth engines, each developing 28 hp at 6,500 rpm and driving fixed-pitch propellers, possibly through cog-belt speed-reduction systems.

Two photographs of N72TZ can be found, seemingly taken moments apart. It was a sleek, attractive airplane, painted a deep claret color, with sinuous lines — the sort of airplane amateur designers love to sketch. Its general arrangement resembled that of the three-surface Piaggio Avanti, with a T-tail, a small canard and pusher engines set inboard on the wings. Its ground attitude appears to have been a bit nose-low, like that of many jets. From its shadow — the sun stood more or less at the zenith when the photographs were taken — one can see that the areas of its three surfaces, relative to one another, were roughly similar to those of the Avanti. It was shorter-coupled, however, and the mainplane was located approximately amidships, whereas that of the Avanti is farther from the canard.

The creator of N72TZ had previously built a Lancair Legacy and was a commercial multi- and instrument-rated pilot with more than 2,000 hours. It is unclear whether he had any formal training or experience in aeronautical engineering — but many successful airplanes have come from self-taught amateur designers.

The airplane had been issued a special airworthiness certificate on July 2, 2009. Initial taxi testing had revealed a lack of pitch authority; even at 55 knots, the pilot could not get the nosewheel off the ground. Accordingly, two changes were made: The canard elevator was enlarged by an unspecified amount, and ballast was added to move the CG “aft of 30 percent of mean aerodynamic chord.” These changes had the desired result: It was now possible to rotate the airplane at 45 to 50 knots. During a later series of high-speed taxi tests, the pilot “routinely raised the nose of the airplane.”

N72TZ was equipped with a Dynon electronic flight information system that recorded a number of parameters, including airspeed, altitude and pitch attitude, at one-second intervals and made it possible for accident investigators to reconstruct its first and final flight in some detail. The accident, which took place on Sept. 24, was preceded by a high-speed taxi test during which the EFIS recorded a maximum speed of 67 knots and a pitch angle of 9 degrees — not necessarily at the same moment.

On the next run, the airplane became airborne — whether intentionally or not we don’t know. It rapidly pitched up 45 degrees, zoomed more than 120 feet above the runway, turned to the right, stalled, lost airspeed, pitched down and fell, hitting the ground in a roughly level attitude. The pilot-designer-builder, 62, died after the crash.

The airplane was not fragmented. Investigators established continuity in the flight and engine controls, and witnesses reported that the engines had run throughout the brief flight. The NTSB accordingly concluded that the probable cause of the accident was “the pilot’s failure to maintain pitch control of the airplane.” It did not speculate about the reason.

Mistrim suggests itself, but this was an extremely small and light airplane, and even if it were severely mistrimmed it could not have overpowered its pilot. A more likely symptom of mistrim would have been some overcontrol and porpoising, not an apparently unmanageable pitch-up.

“Failure to maintain pitch control” could be interpreted to mean “inept piloting.” But this pilot was experienced, had previously built and flown a high-performance homebuilt, and to all appearances was taking a reasoned, cautious and incremental approach to testing. So it is unlikely that he lost control of the airplane because he didn’t know any better.

A more probable explanation is suggested by the changes that had been made after initial taxi testing — the canard surface had been enlarged and the CG moved aft — in order to facilitate rotation.

It appears from photographs that the main landing gear was located slightly ahead of the trailing edge of the wing. Main landing gear axles are normally placed along a line sloping back at an angle of about 15 degrees from the center of gravity. The vertical axle-to-CG distance appears to be about two feet, and so the CG would be expected to be at least six or seven inches forward of the gear. This would roughly coincide with the quarter-chord point on the wing. In other words, the relationship of wing, landing gear and center of gravity seems to be the one that would be expected for a conventionally configured airplane.

But this airplane was not conventionally configured. Since it so strongly resembled the three-surface Piaggio Avanti, it is instructive to note that the landing gear of the Avanti is located approximately at the leading edge of the wing, implying that the Avanti’s CG is well ahead of the wing. This is typically the case with canard-equipped airplanes, unless, as on the B-1 bomber and some highly maneuverable fighters, the canard area is so small as to be almost negligible.

Air does not make the same distinctions as we humans do between wings, canards and tails. They are all lift-producing surfaces, all flying along together, and must be taken as a whole. The lifting canard surface behaves like a part of the wing and moves the airplane’s center of lift forward. The amount of forward shift depends on the relative areas of the two surfaces, on how far apart they are, and on their lift coefficients. For stability, however, the canard must be more heavily loaded than the wing, and so its contribution to the total lift is disproportionately large.

The conventional placement of N72TZ’s landing gear with respect to the wing, and the mention of ballasting the CG to 30 percent of the mean aerodynamic chord — presumably the mean aerodynamic chord of the main wing — suggest that allowance may not have been made for the forward shift of the aft CG limit due to the canard, especially after the latter’s area had been increased. Thirty percent of wing chord is pretty far aft, even for a conventional airplane; flight testing usually begins with the CG forward of 25 percent to ensure good static stability. When a canard surface is present, however, the usable CG range moves forward significantly, and 30 percent becomes very far aft indeed.

As the CG moves farther and farther aft relative to the aerodynamic center of the lifting surfaces, longitudinal stability diminishes, disappears and then becomes negative or “divergent,” meaning that rather than seek a trimmed attitude the airplane actively flees from one. The sudden extreme pitch-up is consistent with longitudinal divergence, and longitudinal divergence is consistent with a three-surface airplane whose CG has been positioned as though the canard were not there. The airplane would seem to behave normally as long as its wheels were on the ground, but once airborne it would be fatally tail-heavy. Because of the rapid loss of airspeed in a steep climb, the elevators would lose effectiveness — assuming that they were effective in the first place — and the pilot’s efforts to force the nose down might be in vain.

Like many other NTSB findings, the conclusion that this accident was due to “the pilot’s failure to maintain pitch control” does not so much answer the question as restate it. Read it as a concession that they — and we — can’t say precisely what happened. But the appearance of longitudinal instability is clear enough that the issue is at least worth discussing, if only to alert future amateur designers to the complexities lurking in configurations other than the conventional one.

This article is based on the NTSB's report of the accident and is intended to bring the issues raised to our readers' attention. 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.

Peter Garrison taught himself to use a slide rule and tin snips, built an airplane in his backyard, and flew it to Japan. He began contributing to FLYING in 1968, and he continues to share his columns, "Technicalities" and "Aftermath," with FLYING readers.

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