NTSB Reveals History of SJ30-2 Stability Issues
In the year before April 26, 2003, when Sino Swearingen’s number-one SJ30-2 prototype crashed after entering an uncommanded and unrecoverable right roll during high-speed flutter testing, company engineers were attempting to deal with lateral stability issues with the twinjet, according to the NTSB’s recently released factual report on the accident.
The lateral control history was a key focus of the NTSB report. The lateral trim system on the accident aircraft used an adjustable trim spring to apply a constant force to the control wheel. “The constant-force design dictated that the amount
of trim required to balance an aerodynamic force asymmetry was speed-dependent,” the Safety Board said.
On May 7, 2002, the company issued a so-called temporary test aircraft limitation (TTAL) that restricted the pilot’s use of aileron trim to the 20- to 80-percent range because the aileron trim motor bogged down at approximately 13.8 percent and 92 percent of travel.
A speed restriction of 250 kcas was imposed for test flight 114 on June 1, 2002. In addition, engineers discovered that the airplane required a “significant amount of roll trim adjustment, and that roll trim requirements were speed-dependent,” the report said. The ailerons were removed, measured and replaced, to attempt to correct twist deviations from the aileron surface design.
During flight 114, the roll trim requirement was consistently left-wing-down (LWD) and increased with airspeed, but the airplane could be trimmed in the lateral direction “with much less roll trim adjustment” within the 250-kcas speed restriction. Sino Swearingen subsequently concluded “that the airplane’s tendency to roll right-wing-down (RWD) could be attributed to wing and remaining aileron twist deviations from their respective surface designs.”
After October 2002, the airspeed restriction was increased to 320 kcas, or Mach 0.83, after completion of Phase One flutter testing. No design changes were made and the RWD tendency was accepted as a “known airplane-specific characteristic, which required nearly full left-wing-down lateral trim at 320 kcas.”
For flight tests 199 and 200 on December 16 and 17, 2002, respectively, tuft testing on the upper wing surfaces confirmed the presence of “large regions of shock-induced separation above Mach 0.81.”
On April 14, 2003, the airplane’s speedbrake design travel of 35 degrees was limited to 17.5 degrees to “reduce undesirable speedbrake deployment pitch characteristics.”
Because of the airplane’s lateral trim issue and the flutter-test plan airspeeds exceeding 320 kcas for the second phase of high-speed flutter tests, Sino Swearingen expected that “full LWD trim and pilot hand pressure on the yoke would be required.” As a result of this expectation, engineers equipped the right wingtip of the accident airplane with a Gurney flap, an aerodynamic device intended to balance the airplane in the lateral axis, independent of airspeed, and restore lateral trim margin.
On April 24, 2003, flight 229 was conducted to quantify the Gurney flap’s effectiveness, flight-test the flutter instrumentation and perform a telemetry range check. According to the Safety Board, the Gurney flap improved the lateral trim margin, and for airspeeds up to 305 kcas, approximately 40-percent lateral trim was required to balance the airplane.
Despite the effectiveness of the Gurney flap in flight 229, Sino Swearingen engineers considered the fact that the airplane would “likely require additional LWD control input to trim laterally as airspeed increased beyond Vmo (320 kcas).” Nevertheless, for flight 230 on the day before the accident flight, Sino Swearingen decided to continue with the flutter testing if the pilot needed to apply a “small” wheel force to trim laterally as the airspeed increased beyond 320 kcas.
During flight 230, all available aileron trim was required at Mach 0.84 at altitudes between 31,000 and 30,000 feet. Rudder pedal was used to augment aileron trim as the airplane descended from 33,000 to 31,000 feet. “Data revealed that all of the earlier TTAL lateral trim margin (20 to 80 percent) was required to trim the airplane between Mach 0.84 and 0.86.”
Then, as the airplane approached the next test point, it went into an uncommanded LWD roll. The roll was corrected by pilot wheel input over a period of about 20 seconds as the airplane decelerated below Mach 0.85. The pilot also used rudder to augment aileron during the recovery.
Sino Swearingen engineers concluded that the LWD roll resembled “a wing drop, likely caused by the presence of shock-induced separation.” The pilot was briefed to expect “increased vibration, buffeting and possible wing drops as the airplane passed the 1g buffet boundary at Mach 0.86.
SJ30-2 S/N 002 took off on April 26 for test flight 231. The airplane was at 39,000 feet indicating Mach 0.884. When test pilot Carroll Beeler was cleared to the next test point, Mach 0.894, he did not acknowledge the clearance and instead reported that the airplane “was rolling to the right, and he couldn’t stop it.” During the rolling descent, Beeler, 59, said that he couldn’t get out of the airplane, because “there were too many gs.”
No High-Speed Tunnel before Crash
Before the accident that killed Beeler, Sino Swearingen had conducted low-speed wind tunnel tests that revealed “that separation, due to either speedbrake deployment or high (post-stall) angles of attack, tended to reduce wing lateral stability.”
After the accident, the company developed a test plan to define the high-speed stability and control characteristics of the SJ30-2. Data from wind tunnel tests in January last year indicated that “lateral stability on the SJ30-2 deteriorated with increasing Mach number and angle of attack.”
Lateral stability, measured in terms of rolling moment due to sideslip, became unstable above Mach 0.83, according to the wind tunnel data. “Because of this, a rudder input intended to augment the lateral trim and raise a low wing could instead, beyond a certain Mach number, actually aggravate the situation. Similarly, any elevator input would tend to increase the angle of attack, also resulting in deteriorated lateral stability.”
Wind tunnel test data also provided evidence that roll authority deteriorated above Mach 0.86, although the report did not describe the severity of the deterioration.
As a result of data from the wind tunnel tests and other post-accident research, Sino Swearingen has made aerodynamic improvements to the SJ30-2. It added vortex generators to the wings to delay the onset of shock-induced separation and installed thicker trailing edges on the ailerons to improve aileron effectiveness at high Mach numbers. In addition, a high-Mach-number roll spoiler system will augment roll control above Mach 0.835.
Design rework that the company said had actually been started before the accident includes relocating a speedbrake panel on each wing farther outboard to minimize the large pitch-down effects caused by tail lift interference. Also, the panels became operational at all airspeeds within the design deployment range.
SJ30-2 flight tests resumed within three months of the accident, and the aforementioned modifications had been incorporated into S/N 004 when it passed all critical high-speed tests in August last year. Interestingly, S/N 004 was fitted with a high-speed drag chute. Sino Swearingen decided not to install one on S/N 002 “due to pilot concerns about the possibility of an inadvertent chute deployment,” according to the report. The NTSB has yet to issue a final report that is expected to point to the causal factors of the accident.
Meanwhile, the program continues toward FAA certification, possibly as soon as the third quarter (see page 46). Before the crash of S/N 002, the company estimated that the light jet would receive certification in the fourth quarter of last year.