The NTSB has blamed the April 2, 2011, flight-test crash of a Gulfstream G650 on an aerodynamic stall and an uncommanded rolling moment that the pilots were unable to control.
However, the Board also notes that it was the manufacturer’s rush to complete its aggressive flight-test schedule for obtaining certification that set the stage for the stall and uncontrollable roll. The stall and roll to the right occurred in close proximity to the runway after one engine had been cut to idle at 107 kcas and the initiation of rotation for takeoff at 127 kcas.
In a report released on October 10, the NTSB determined that the fatal accident at Roswell (N.M.) International Air Center on the morning of April 2, 2011, was the result of Gulfstream’s failure to properly develop and validate takeoff speeds; failure to recognize and correct errors in the takeoff safety speed manifested during prior G650 test flights; the flight-test team’s persistent and aggressive attempts to achieve a takeoff speed that was erroneously low; and Gulfstream’s inadequate investigation of the uncommanded roll events that occurred during previous test flights, “which should have revealed incorrect assumptions about the airplane’s stall angle of attack in ground effect.”
“Deadlines are essential motivators, but safety must always trump schedule,” said NTSB chairman Deborah Hersman. “Flight-test should not be rushed or compromised.”
At about 9:34 a.m. MDT on April 2, 2011, during a one-engine-inoperative takeoff, the outboard section of the right wing stalled, causing the G650 to roll to the right far enough for the right wingtip to strike the runway. The jet then veered off the right side of the runway, where it hit a concrete structure and an airport weather station, causing extensive structural damage to the airplane and a post-crash fire that consumed the fuselage and cabin interior.
The two pilots and two flight engineers on board died in the accident. The NTSB said that although the impact was survivable the accident “became unsurvivable because of the large amount of fuel, fuel vapor, smoke and fire entering the cabin through the breaches in the fuselage.”
The Board determined that the probable cause of the crash was an aerodynamic stall and an uncommanded rolling moment that the flight crew was unable to control.
Contributing to the accident, the NTSB found, was Gulfstream’s pursuit of an aggressive flight-test schedule without ensuring that the roles and responsibilities of team members were appropriately defined, or that sufficient technical planning and oversight was performed and that hazards had been fully identified and addressed with appropriate, effective risk controls.
The NTSB’s probe found that the aircraft stalled while lifting off the ground. As a result, the Safety Board examined the role of “ground effect” on the airplane’s performance. Ground effect results in increased lift and reduced drag at a given angle of attack (AOA) as well as reduction in the stall AOA.
In preparing for the G650 field performance flight tests, the NTSB said, Gulfstream considered ground effect when predicting the airplane’s takeoff performance capability but overestimated the in-ground-effect stall on AOA. Consequently, the airplane’s AOA threshold for stick-shaker activation and the corresponding pitch limit indicator were set too high, and the flight crew received no tactile or visual warning before the actual stall occurred.
The accident flight was the third occurrence of a right outboard wing stall during G650 flight-testing. Gulfstream did not determine (until after the accident) that the cause of the two previous uncommanded roll events was a stall of the right outboard wing at lower-than-expected AOA.
The NTSB said that if Gulfstream had performed an in-depth aerodynamic analysis of these events shortly after they occurred, the company could have recognized before the accident that the actual in-ground-effect stall AOA was lower than predicted.
V Speed Exceedances
During field performance testing before the accident, the G650 consistently exceeded target takeoff safety speeds (V2). V2 is the speed that an airplane attains at or before the 35 feet above the ground with one engine inoperative. Gulfstream needed to resolve these V2 exceedances because achieving the planned V2 speeds was necessary to maintain the airplane’s 6,000-foot takeoff performance guarantee (at standard sea-level conditions). If the G650 did not meet this takeoff performance guarantee, then the airplane could operate only on longer runways.
However, the NTSB determined that a key assumption that Gulfstream used to develop takeoff speeds was flawed and resulted in V2 speeds that were too low and takeoff distances that were longer than anticipated. The Board wrote in its executive summary:
“Rather than determining the root cause for the V2 exceedance problem, Gulfstream attempted to reduce the V2 speeds and the takeoff distances by modifying the piloting technique used to rotate the airplane for takeoff. Further, Gulfstream did not validate the speeds using a simulation or physics-based dynamic analysis before or during field performance testing.
“If the company had done so, then it could have recognized that the target V2 speeds could not be achieved even with the modified piloting technique. In addition, the difficulties in achieving the target V2 speed were exacerbated in March 2011 when the company reduced target pitch angle for some takeoff tests without an accompanying increase in the takeoff speeds.”
According to the NTSB, Gulfstream maintained an aggressive schedule for the G650 flight-test program so that the company could obtain FAA type certification by the third quarter of 2011. The schedule pressure, combined with inadequately developed organizational processes for technical oversight and safety management, led to a strong focus on keeping the program moving and reluctance to challenge key assumptions and highlight anomalous airplane behavior during tests that could slow the pace of the program.
The Safety Board said these factors likely contributed to key errors, including the calculations for takeoff speeds, as well as the superficial review of the two previous uncommanded roll events, which allowed the company’s overestimation of the in-ground-effect stall AOA to remain undetected.
Board member Earl Weener, who was a chief engineer at Boeing, said that in flight testing, “anything that differs from what was predicted should raise red flags.”
Hersman told AIN video recorders in the cockpits of the test G650s proved invaluable in the investigation. They enabled investigators to see exactly what the pilots were doing during the roll incidents. In the two earlier incidents, the non-flying (monitor) pilot had “jumped on the controls” at the onset of the uncommanded roll.
“Safety is Gulfstream’s first priority,” said the manufacturer in a statement. “Since this accident we have redoubled our efforts to strengthen the safety culture in flight test and throughout the rest of the company. We are committed to continuous safety improvement.”