The accident occurred at Roswell (New Mexico, U.S.) International Air Center the morning of April 2, 2011. The flight crew was conducting a simulated one-engine-inoperative (OEI) takeoff and was not able to correct an uncommanded roll that occurred when the right wing stalled on liftoff. The wing tip struck the runway, and the experimental ultra-long-range, fly-by-wire business airplane veered right, struck a concrete platform housing electrical equipment and was consumed by a fuel-fed fire. The pilots and both flight test engineers succumbed to smoke inhalation and thermal injuries.

Flight testing that day, as well as during several previous tests, had focused on achieving the manufacturer’s target for V₂, basically the minimum speed that a transport category airplane must attain at a height of 35 ft to meet the required OEI climb gradient to 400 ft. The V₂ speeds achieved during the tests had consistently been too high to meet the manufacturer’s goal of providing a balanced field length of 6,000 ft (1,829 m).

Various rotation techniques and angle-of-attack (AOA) targets had been tried by Gulfstream Aerospace’s flight-test crews, but none had succeeded in meeting the target V₂ without exceeding the 20-degree pitch angle that had been set to assure passenger comfort. The most recently developed technique included an abrupt and rapid rotation, using the maximum allowed 75 lb (34 kg) of pull force on the control column, to an initial 9-degree pitch attitude, then a further increase in pitch attitude to achieve V₂. This technique had produced the best results, exceeding the target by only 3 kt.

However, in the course of about a dozen test flights earlier the morning of April 2, the pilot-in-command (PIC) had decided that a smooth rotation, pausing only briefly at 9 degrees before increasing pitch to about 16 degrees, might be a better and more repeatable technique. “I’m not doing that jerk stuff,” he told a flight test engineer. “It doesn’t work … and I don’t think the FAA [U.S. Federal Aviation Administration] is going to like it, either. It’s such a great-flying airplane, you shouldn’t have to abuse it to get [it] flying.”

The NTSB report noted that both flight crewmembers had extensive experience as test pilots. The PIC had 11,237 flight hours, including 263 hours in G650 certification testing. The second-in-command (SIC) had 3,940 flight hours, including 140 hours in type.

During some of the earlier test flights that morning, the use of a smooth rotation with a brief pause at 9 degrees produced V₂ speeds within 4 kt of the target (135 kt). The pilots agreed to try an even briefer pause at 9 degrees during the next takeoff, which was conducted on Roswell’s Runway 21 with flaps extended 10 degrees. As planned, the SIC moved the right thrust lever to idle at 105 kt, simulating an engine failure. At 127 kt, the PIC began rotating the airplane for takeoff. Recorded flight data indicated that there was no pause when the pitch attitude reached 9 degrees. AOA quickly exceeded 11 degrees, the outboard section of the right wing stalled, and the airplane rolled right. The PIC attempted to level the wings, but the bank angle increased beyond 16 degrees.

The pilots had received no warning of the asymmetric stall. Because of a miscalculation of the G650’s stall AOA in ground effect, the threshold for activation of the stick shaker and the primary flight display pitch-limit indicators had been set too high. “Ground effect refers to changes in the airflow over the airplane resulting from the proximity of the airplane to the ground,” the report explained. “Ground effect results in increased lift and reduced drag at a given [AOA], as well as a reduction in the stall AOA.”

The stick shaker activated and the indicated pitch attitude reached the limit shown on the primary flight displays only after the stall occurred. The PIC pushed the control column forward, applied full left control wheel and rudder, and called for “power.” The SIC already had moved the right thrust lever full forward. Despite these inputs, the airplane remained in a stalled condition. The sound of an automatic warning when the right bank angle exceeded 30 degrees was captured by the cockpit voice recorder shortly before the recording ceased about 24 seconds after the takeoff was initiated.

The report noted that uncommanded rolls resulting from right outboard wing stalls had been encountered during two previous test flights. In both cases, the pilots recovered by reducing AOA. The events subsequently were attributed to stalls induced by over-rotation. “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,” the report said.

Investigators found that the stall precipitating the accident had occurred at an AOA of 11.2 degrees, or about 2 degrees lower than the predicted stall AOA in ground effect.

The report said that contributing to the accident was the manufacturer’s “aggressive” flight-test-program schedule, which was designed to achieve certification of the G650 by the third quarter of 2011. “The schedule pressure … led to a strong focus on keeping the program moving and a reluctance to challenge key assumptions.”

The report noted that after the accident, Gulfstream suspended performance flight testing and implemented several corrective actions. The target V₂ was increased by 15 kt, while maximum takeoff thrust was increased by 5 percent to meet the takeoff performance goals. Certification of the new airplane eventually was achieved in September 2012.

The accident investigation generated 10 safety recommendations, including the NTSB’s call for the FAA to work with the independent Flight Test Safety Committee to develop detailed guidance for aircraft manufacturers on flight test operations (ASW, 11/12, p. 15).

 Source Document

NTSB Faults Gulfstream for G650 Test Crash

In the race to certify what is, for now, the world’s fastest business jet, Gulfstream Aerospace Corp. ignored warning signs that might have signaled a fatal stall well in advance of an April 2, 2011, crash in Roswell, N.M., the NTSB found.

Board members, meeting Oct. 10 to review findings of the investigation, faulted the company for failure to analyze stalls that occurred on two prior test flights, and consider that stall speed in ground effect is lower than engineers had predicted.

Four people—two pilots and two engineers—lost their lives during a test of single-engine takeoff performance. The jet stalled just after liftoff, at an angle of attack lower than expected, with no activation of the stick shaker and audible warnings, investigators found. The stick shaker and warning systems had been calibrated to activate at higher angles of attack. The stricken jet rolled to the right, dug a wingtip into the ground, and veered off the runway at Roswell International Air Center. It struck a concrete building on the field and burst into flames.

NTSB Chairman Deborah A.P. Hersman, among other board members, said Gulfstream’s test program crew should have acted on warning signs from two previous flights that resulted in uncommanded rolls just after liftoff, which were attributed, incorrectly, it turns out, to causes other than a stall.

“I think the event itself should have been a trigger for people to stand back and understand what happened,” Hersman said. “They got a very significant sign and it didn’t seem to be addressed.”

NTSB staff noted that Gulfstream has made changes to its flight test program procedures in response to recommendations. The G650 test program concluded with no further incidents, and Gulfstream announced certification of the bizjet in September.

Source Document

Have a good week, thanks for letting the 3DB be a part of your week, and fly safe/be safe.

Robert Novell

September 1, 2014