Lessons learned from the fatal April 2, 2011, crash of a Gulfstream G650 test aircraft in Roswell, N.M., could bolster safety for airframers and flight test departments globally if recommendations handed down yesterday by the U.S. (NTSB) are adopted.
Along with determining a probable cause for the accident, which killed two test pilots and two flight test engineers, the safety board issued 10 recommendations, many of which called for the dissemination of information about the underlying reasons for the accident and program countermeasures that may prevent repetition of errors made during the G650 test program.
The twin-jet, Gulfstream’s largest, longest-range aircraft to date, experienced an uncontrollable, asymm etric aerodynamic stall during a simulated single-engine takeoff that morning, dragging its right wing on the ground and catching fire after exiting the right side of the runway and coming to a stop near the air traffic control tower. NTSB determined that the initial crash was survivable, but not the post-impact fire that consumed the aircraft.
The investigation revealed the G650 had experienced two similar “roll-off” anomalies during takeoff testing in the months before the accident, though neither had been positively identified as wing stall events. Due to a convergence of errors and incorrect assumptions, key target takeoff speeds and pitch angles for the tests were set too low and too high, respectively, and a stick shaker warning system for the pilots did not activate before the stall occurred.
As revealed by an extensive analysis Gulfstream conducted after 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—the “inside ground effect.”
Regardless, the NTSB, in its probable cause finds that Gulfstream failed to “properly develop and validate takeoff speeds and recognize and correct errors in the takeoff safety speed that manifested [itself] during previous G650 flight tests.”
Further, the NTSB says the crash was caused by the flight test team’s “persistent and aggressive attempts to achieve a takeoff speed that was erroneously low; and Gulfstream’s inadequate investigation of uncommanded roll events that occurred during previous flight tests, which should have revealed incorrect assumptions about the airplane’s stall angle of attack in ground effect.”
The accident occurred during developmental flight tests for the G650 before official certification tests with thehad begun. Gulfstream ultimately received its FAA type certification for the twin-jet in September.
Of the 10 NTSB recommendations, five were issued to the FAA and three to the Flight Test Safety Committee (FTSC), an international flight test 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 also is 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 the weaknesses brought out in the NTSB report.
“We recognize the safety action they took [after] the accident,” says Deborah Hersman, NTSB chairman.