Aftermath: Three Blades, Minus One

Loss of a prop blade is startling and ­disorienting.

Aftermath

Aftermath

The velocity is a kit-built composite four-seater. Similar in configuration to Burt Rutan's VariEze and Long-EZ, it has a swept wing with tall upturned tips, a pusher propeller and a rectangular foreplane of high aspect ratio. The landing gear may be fixed or retractable. Triangular leading-edge extensions, or "strakes," along the sides of the fuselage carry fuel. Typical engines are Lycomings of 180 to 300 hp.

In canard airplanes like this, both the wing and the foreplane contribute lift. In terms of pounds carried per square foot, the foreplane, though the smaller, is the harder-working of the two. The center of gravity is not located near the quarter-chord point of the wing, as it would be on a conventional airplane; instead, it lies between the two lifting surfaces, like a patient between two litter-bearers.

The reason the foreplane is more heavily loaded than the wing is that, in order to avoid driving the wing to its stalling angle of attack, the foreplane, which is the airplane’s pitch control, must stall first. Once the foreplane stalls, it cannot pitch the nose up further, and so in principle the wing is prevented from stalling. Airplanes of this type behave innocuously at full aft stick: The nose rhythmically rises and falls a few degrees as the foreplane stalls and unstalls, while the wing retains lift and perfect roll control. This is a great safety feature; it is the most powerful argument in favor of the canard configuration, and has probably saved a number of lives.

Still, every silver lining has its cloud. If the wing does manage to stall somehow or other, the airplane may enter a stable, flat, parachute-like descent with little or no forward speed. Recovery may be difficult or impossible. Wing stalls can occur if the CG is too far aft, or under certain conditions of violent maneuvering.

In the late 1980s, three well-publicized accidents of this type befell Velocities. Remarkably, only one was fatal. After those accidents, the Velocity’s wing was modified to increase its stall margin.

At the time, it was widely believed that the stable stall produced a very low rate of descent. This implied an unexpectedly high, in fact unprecedented, drag coefficient, and novel, semimagical vortices were posited to explain the new phenomenon; but ground tests, conducted by Rutan with an airplane set up vertically on the back of a truck, failed to produce them. In the end, it appeared most likely that the two pilots had survived because their airplanes fell into water, and the rounded undersides of the fuselages provided some shock absorption.

On a Saturday in May 2009, a pilot, who had bought his 200 hp Lycoming IO-360-powered ­Velocity from its builder and had flown it himself for perhaps 250 hours with a two-blade fixed-pitch propeller, was test-flying a newly installed three-blade constant-speed prop. He proceeded incrementally, beginning with a fast taxi down the runway. He then flew a circuit and landed, then flew two circuits and landed. After each step he returned to his hangar, presumably to make adjustments. On the third flight he circled the field once, outside the traffic pattern but within gliding distance. Just as he was turning onto the downwind leg for the second time, at about 2,000 feet agl, one blade separated from the brand-new propeller.

The loss of a prop blade, or part of one, is startling and ­disorienting. The engine vibrates so violently that the instrument panel becomes a blur. There is a danger that the engine may break away from the airplane — although that is less likely to happen when the blades are of the lightweight composite type, as these were, than with heavier aluminum blades. There is nothing to do but shut down the engine immediately, hope it stays attached to the airplane long enough to stop turning, and look for a place to land. So long as the engine does not fall free, the airplane should remain flyable.

It didn’t.

The only witness to the ­accident was a nonhuman one: a Garmin GPS, which recorded the groundspeed and altitude of the Velocity at intervals of approximately one second. The story it told was bizarre. It appeared that within three or four seconds of the failure — to judge from the location of the separated prop blade relative to the wreckage — the airplane, without climbing, decelerated very rapidly. Within 10 seconds, its groundspeed fell to zero, a 0.75-G deceleration that an airbrake would be hard put to match. Almost immediately after beginning to slow down, it settled into a steady descent at nearly 7,000 fpm and maintained that rate to the ground.

The “probable cause” of the accident, the NTSB decided, was “the pilot’s loss of control following an in-flight failure of the propeller.” A detailed examination of the errant propeller blade revealed that it had failed as a result of improper surface preparation where metal parts were bonded to composite ones. The manufacturer of the uncertified propeller, the NTSB observed, “states that the propeller is not deemed airworthy and that it is the pilot’s responsibility to ensure that the propeller is properly tested prior to flight.”

The airplane was found the next day, two miles from the runway, at the edge of a small wooded area. It was lying upside-down. The wings had broken at midspan in the negative-lift direction, evidently from the impact upon the upturned tips, but they had remained attached to the fuselage, and the fuselage itself was in one piece and looked practically repairable. One would have thought the airplane had fallen inverted, except that the right landing gear leg, a spring of glass fibers and epoxy as big as your arm, was broken where it entered the fuselage, suggesting a violent impact. Possibly the airplane descended upright, bounced and then came to rest inverted. A striking feature of the wreckage, however, was the absence of longitudinal crushing. The airplane was evidently in a more or less level attitude when it struck the ground.

The NTSB’s accident report (CEN09LA288) did not comment on the surprising fact of a fast-moving airplane practically halting in midair and coming more or less straight down at an unvarying vertical speed of 67 knots. Nor did it detail the pilot’s injuries — beyond the coroner’s standard formula of “blunt force trauma” — in a way that might have hinted at the airplane’s attitude on impact.

When a conventional airplane descends vertically, it is either in an inverted dive — in which case it crashes at a very high speed and is completely demolished — or in a spin, in which case the nose is usually down and at least some longitudinal crushing occurs on impact. Only after a flat spin, in which the nose is close to the horizon, would the wreckage exhibit no longitudinal crushing. A flat spin has occurred (during experiments by a professional test pilot) in a modified Long-EZ; it took 15 turns, an altitude loss of 4,000 feet and extreme persistence and self-possession on the pilot’s part to find the means to recover. Canard airplanes are said to be spin-proof, but “practically spin-proof” might be a more accurate description.

The constant rate of descent is noteworthy. It is near the stalling speed of the airplane, and, since the drag coefficient of the airframe in a level, vertical descent is similar to its lift coefficient in a normal landing, it encourages the hypothesis of a stable, “locked-in” stall.

But if such a stall occurred, how did it happen? And what did the failure of the propeller have to do with it?

The usual precursors of wing stall in a canard airplane are low speed and an aft CG. The heavier propeller would have moved the CG aft somewhat, and with only the pilot in the airplane the CG would have been in the aft part of its range. But the Velocity’s indicated airspeed at the time of the prop failure

was quite high — probably around 128 knots.

One plausible theory is that the pilot, startled by the violent vibration of the unbalanced propeller (and possibly even thinking that he had encountered flutter), abruptly applied full aft stick in order to slow down or gain altitude, or both. This could have resulted in a 4-G pull-up — not enough to break anything, but enough to pitch the airplane sufficiently rapidly that its rotational momentum might have allowed the wing to stall.

It’s just a theory — but it goes further than mere “loss of control” to account for the strange tale told by the GPS.

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.