Deficient Mach research caused SJ30-2 accident

 - November 1, 2006, 7:19 AM

In its determination of probable cause, the NTSB implicated a history of stability issues at high Mach speeds in the April 26, 2003, crash of Sino Swearingen’s number-one SJ30-2 prototype. Company test pilot Carroll Beeler was killed in the accident.

In its final report, released March 30, the Safety Board concluded that the probable cause of the accident was the manufacturer’s “incomplete high-Mach design research, which resulted in the airplane becoming unstable and diverging into a lateral upset” apparently from which there was no ability to recover.

The jet was in a descent to attain a Mach 0.884 target speed during an airplane type certification flutter test. The airplane, a unique testbed, “had a known speed-dependent tendency to roll right, which was attributed to wing and aileron twist deviations,” the NTSB said. “As the speed increased during the accident flight, the pilot had to apply full left aileron to be able to maintain airplane control. The airplane completed the test point about 30-degrees right-wing-low and subsequently began to roll to the right, ‘like a barrel roll…not real fast,’ that the pilot reported he could not stop.”

Although the manufacturer’s engineering analysis (which did not include any high-speed wind tunnel testing) predicted positive lateral stability up to Mach 0.90, the pilot lost lateral control during the accident flight at about Mach 0.884, and the airplane rolled about seven times during a 49-second descent from about 30,500 feet until a near-vertical ground impact.

According to the NTSB, nearly a year before the fatal crash, during high-speed flutter testing, speed restrictions were temporarily put in place as company engineers attempted to deal with the twinjet’s lateral stability issues. It was discovered that the prototype SJ30-2 had a right-wing-down (RWD) rolling tendency that required a “significant amount of roll-trim adjustment and that roll-trim requirements were speed-dependent.” No design changes were made at the time, and the RWD tendency was accepted as a “known airplane-specific characteristic."

Gurney Flap Helped, but Not Enough

Because the known speed-dependent tendency to roll right had created significant control problems on a previous flight, the ailerons were removed, modified and replaced, and a Gurney flap, a device intended to balance the airplane laterally, independent of airspeed, and restore lateral trim margin, was added to the right wing.

After the addition of the Gurney flap, the lateral trim margin improved to about 40 percent required (where 50 percent was neutral) up to 305 kcas. Sino Swearingen then decided that flutter testing could continue to higher airspeeds if the pilot needed to apply a “small” wheel force to augment the trim. Beeler had been instructed to reduce airspeed if there was a problem during the flutter testing and had done so during an uncommanded roll to the left on the flight before the accident flight.

“Telemetry data from the accident flight revealed that at initiation of the upset, the pilot attempted to level the wings and raise the nose, but the airplane continued to diverge from stable flight, and it continued to accelerate beyond the airplane’s demonstrated flight diving speed,” the Safety Board said. The NTSB was unable to determine whether Beeler could have reduced the speed of the airplane during the initiation of the upset so that the airplane would not diverge.

Wind-tunnel tests after the crash revealed that lateral stability decreased with increasing Mach and angle of attack. Lateral stability “became negative (unstable) above Mach 0.83,” and any rudder or elevator input to augment the lateral trim and raise a low wing above a certain Mach “could instead actually aggravate the situation.” Post-crash high-speed wind tunnel data also revealed that roll authority deteriorated above Mach 0.86 and that by Mach 0.88 “the aileron upper and lower surfaces were both in separated flow regions.”

The follow-on flutter test airplane, which last August successfully completed the certification requirements, was equipped with vortex generators and thicker-trailing-edge ailerons. This test aircraft also did not require the Gurney flap installed on the accident airplane due to improvements in manufacturing tolerances.

Since the accident, Sino Swearingen has made numerous other design changes to the SJ30-2, and the third and final conforming prototype joined the flight-test program just six days before the NTSB released its final report. The company hopes to receive FAA certification in the fall.

A company spokesman did not respond to a request for comments.