Bad Vibrations

Parts widely scattered along a line indicate an inflight breakup. The first items to separate from the airframe are at the beginning of the wreckage path; the fuselage and engine are usually found at the end. In this case, the first item in line was the top of the rudder. Various other pieces of empennage debris, including the stabilizer and both elevators, were found 900 feet from the main wreckage, as was a portion of the left wing and flap. Fragments of the acrylic canopy were scattered along the wreckage path, but although both pilots were wearing parachutes, there was no evidence of their having jettisoned the canopy. The lower portion of the rudder was never found.

The only part of the airframe left intact was the left aileron, and it was carefully examined by accident investigators. They knew from the maintenance logs that the airplane had been repainted 96 flight hours before the accident, but the flight control surfaces had not been rebalanced. They found that the aileron, at 7.15 pounds, was both three-quarters of a pound heavier than when originally installed (according to the manufacturer's records) and considerably more tail heavy.

The nose- or tail-heaviness of a control surface is expressed as a "residual hinge moment." "Residual" simply means what unbalance remains after the surface balance weights have been installed. "Hinge moment" is what you get when you support the surface on its hinges, put a weighing scale under the trailing edge and multiply the scale reading by the distance from the hinge line to the point where the surface rests on the scale. For example, a scale reading of four ounces with the scale contact 12 inches behind the hinge line would be described as a residual hinge moment of 0.25 foot-pounds (one-quarter pound times one foot).

Grob specifications for aileron balance permitted a hinge moment range of -0.22 foot-pounds (a negative hinge moment means the surface is leading-edge heavy) to +0.074 foot-pounds. The measured hinge moment of the recovered aileron was "between +0.138 and +0.200 foot-pounds"-considerably out of factory limits in a tail-heavy direction.

It's hard to understand how a coat of paint can add three-quarters of a pound to a surface area of only around 15 square feet, but the National Transportation Safety Board's (NTSB) report on the accident passes over this oddity without comment. Apparently, however, investigators hypothesized that the other control surfaces, including the mostly vanished rudder, might have been similarly overweight and underbalanced, and Grob's analysts affirmed that rudder flutter was possible under these conditions. The probable cause of the accident, the NTSB concluded, was "failure of maintenance personnel to rebalance the flight controls after the airplane had been repainted, which resulted in rudder flutter and inflight breakup of the airplane."

Flutter is often associated with excessively high speed. In one accident, a Cessna 195 experienced elevator flutter, followed by failure of both wings under negative loading, after entering a 19,000-fpm dive with a true airspeed of 280 knots-more than 100 knots above the airframe never-exceed speed. The NTSB attributed the accident to "loss of control due to pilot incapacitation," implying that the pilot had lost consciousness and had slumped forward against the yoke, forcing the airplane into a steep dive.

In another case, a Mooney M20K modified with a turbocharged 305-hp engine disintegrated in flight, leaving, again, a wreckage path half a mile long. The first items to come to earth were the outboard portions of the elevators. (Note that in this case, as with the Grob, the outboard portion of the fluttering surface was the first to go.) They were followed by the empennage and wings. A structures expert from Mooney attributed the failure to flutter "initiated after buckling of the horizontal stabilizers due to excessive loading." The airplane was in a 3,500-fpm descent in IMC at the time, with the pitch trim set fully nose down; the pilot was hustling to keep up with a rapid series of descent clearances from his cruising altitude of 19,000 feet. The airplane's groundspeed shortly before the breakup was 240 knots. The speed for onset of elevator flutter in this airframe, however, is above 241 KIAS.

Flutter is usually associated with high speed, but low-speed aircraft can also flutter if they are sufficiently flexible. In 1991 a homebuilt amphibian crashed during an early test flight after one of its external-airfoil flaperons fluttered, causing the left wing to separate from the airplane. The builder-pilot was wearing a parachute, but he bailed out at such a low altitude that there was insufficient time for it to open. In this case, the builder had elected not to install the "strongly recommended" balance weights on the flaperons because they would make the airplane heavier, and he "felt that they were not needed."

Felt is a word that seems out of place in discussions of flutter. Susceptibility to flutter is determined by a large number of variables that are not visible to the naked eye and are, in fact, difficult to measure even with sophisticated equipment. Much depends on accidental coincidences of the natural resonant frequencies of various substructures. In general, stiff structures are less likely to flutter than limber ones are, and balanced control surfaces are preferable to unbalanced ones. Certified aircraft must demonstrate freedom from flutter up to a speed well above their never-exceed speed.

Kits and homebuilts are a different story. When the rules for them were written, speeds above 170 knots were rare. Today, with the increasing penetration of the homebuilt market by powerful but comparatively inexpensive engines such as the 640-hp Walter turbine (manufactured in the Czech Republic) and the turbojet conversion of the General Electric T58 turboshaft, airplanes capable of more than 300 knots are becoming more common. The ambitious homebuilders who take on this kind of project are wandering into unknown territory-and flutter may await them there.

This article is based solely on the National Transportation Safety Board's report of the accident and is intended to bring the issues raised to the attention of our readers. 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.