Mishandling of physics and a lack of management controls by were two key factors in the fatal crash of a Gulfstream G650 test aircraft on April 2, 2011 in Roswell, N.M., according to the 's final report on the accident.
Alarmed that the flaws may not be limited to flight-testing at Gulfstream, the NTSB is calling on the industry to take note.
Along with determining a probable cause for the accident, which killed two test pilots and two flight-test engineers, the Board issued 10 recommendations to theand others, many of which called for the dissemination of information about the underlying reasons for the accident and program countermeasures that may catch similar errors in the future.
According to the NTSB's analysis, the $65 million twinjet, Gulfstream's largest, longest-range aircraft to date, experienced an uncontrollable, asymmetric aerodynamic stall during a 10-deg. flaps simulated single-engine takeoff during the ninth and final takeoff of the day. Dragging its right wing on the ground and catching fire after exiting the right side of the runway, test aircraft 6002 came to a stop near the air traffic control tower. NTSB ascertained that the initial crash was survivable, but not the post-impact fire that consumed the entire aircraft. It could not be determined if the crew had attempted to escape.
The investigation found that Gulfstream had experienced two similar “roll-off” events during prior takeoff testing of the 6002 at Roswell, though neither had been positively identified as wing stalls and no independent analysis was carried out. Due to a convergence of factors, including faulty assumptions and insufficient analyses, key target takeoff speeds and pitch angles for the takeoff tests were set too low and too high, respectively, and a stickshaker warning system for the pilots was programmed to activate after the stall. The accident occurred during developmental flight tests for the G650, before official certification tests with the FAA had begun. Gulfstream received FAA type certification for the twinjet in September.
As revealed by an extensive analysis by Gulfstream following the accident, the errors in part were linked to widely held assumptions about stall aerodynamics when an aircraft is flying within one wingspan's height above the ground, also known as “inside ground effect” (IGE), which for the G650 is within a height of 100 ft.
In its analysis, Gulfstream found the IGE stall angle of attack (AOA) for the accident scenario to be approximately 3 deg. less than the out-of-ground effect (OGE) stall AOA due to compression effects between the wing and the runway. While downwash and drag are reduced near the ground, stall angle of attack is also reduced as is the maximum coefficient of lift (CLmax), the measure of how much lift a wing produces at a given airspeed.
The conventional thinking before the accident, at least at Gulfstream, was that CLmax would not change due to ground effect, and that an AOA reduction for IGE operations of 1.6 deg. compared to the stall AOA for OGE operations was adequate. As a result, the wing was physically stalling at an AOA as low as 11.2 deg. though the stickshaker was not programmed to activate until above 12 deg. AOA. Without increasing takeoff speed, a reduced CLmax would require a higher pitch angle to achieve the same lift, further eroding the stall margin.
Of the 10 NTSB recommendations, five were issued to the FAA and three to the Flight Test Safety Committee (FTSC), an international flight-test safety advocacy organization operating under the auspices of the Society of Experimental Test Pilots.
The NTSB is asking the FAA to inform domestic and foreign airframers “about the circumstances of this accident,” and to advise them to consider the possibility that an aircraft's maximum lift coefficient in ground effect could be, in contrast to common belief and practice, lower than its maximum lift coefficient at altitude, a finding made by Gulfstream in its post-crash analysis.
The board is also asking the FAA to work with the FTSC to issue updated operating flight-test guidance to manufacturers, based on the report, and develop flight-test safety guidelines based on best practices in aviation safety management.
Only two recommendations were addressed to Gulfstream, one of which is to commission an audit by qualified safety experts before its next flight-test campaign is launched, focusing on weaknesses brought out in the NTSB report.
“We recognize the safety actions they took [after] the accident,” says Deborah Hersman, NTSB chairman. “They recognized that many changes needed to be made, and that is why we don't have as many actions to them as we could have.”
The airframer, in a May 2012 submission to the NTSB, accepted responsibility for the accident and detailed a host of changes being made to address the issues raised, including the creation of an Aviation Safety Officer position, reporting directly to the company's president.
Gulfstream, as of July, continued to report a five-year backlog for the G650, with plans to deliver 24 green and 17 completed aircraft by year-end.