Photo: G650 N652GD, 3 Apr 2011, from NTSB.

Eddie Sez:

This was a mishap of an aircraft on its road to initial type certification not caused by the pilot at the controls, but by the management of the company pushing performance numbers and the flight test team's lack of integrity when bending to the will of the salesmen even when they knew the end result would be wrong. There is a lot to critique here about management at Gulfstream but I will leave that to you. The NTSB Aircraft Accident Report does a good job of it.

We are here for an aerodynamics lesson about ground-effect-stall and a pilot lesson about decision making and integrity. If you keep within your aircraft's limitations you should never have to be familiar with ground-effect-stall, but learning about it will become useful to you when dealing with takeoff and landing performance. The lesson about decision making and integrity boils down to this: you sometimes have to step forward and say no. Doing so can move those doing the asking into reconsidering an unsafe act. And if it doesn't, it gets you out of harm's way.

Oh yes, there is another lesson. If you have ever been asked to perform a functional check flight that borders on becoming a real test flight, consider the minute details and the high level of skill required by this flight test team. Have you been adequately trained? More about this: Normal Procedures & Techniques / Functional Check Flights.

What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.


Accident Report


Narrative

Figure: G650 pitch limit indicator, from NTSB AAR-12/02, figure 2.

[NTSB AAR-12/03, ¶1.1]

  • Gulfstream was performing field performance flight testing to (1) gather data to support type certification of the G650 under 14 CFR Part 25, “Airworthiness Standards for Transport Category Airplanes,” and (2) develop takeoff and landing speed schedules and distances for the G650 airplane flight manual.

  • The takeoff performance flight tests were being conducted with an angle-of-attack (AOA) limiter function disabled. The AOA limiter function was intended to be the primary stall protection system for the G650 (instead of a traditional stick pusher) once the airplane was certificated. The development of the AOA limiter software was not completed, so Gulfstream intended that the stick shaker would provide test pilots with a tactile warning of an impending stall. In addition, the pitch limit indicator (PLI) on the primary flight display would provide the pilots with a visual indication of an impending stall. [The figure] shows a representative PLI (as depicted in Gulfstream‟s draft G650 aircraft operating manual).

  • Note: The PLI is displayed when the normalized AOA is greater than 0.7. Normalized AOA is a measure of the usable AOA range of an airplane, with a normalized AOA of 1.0 corresponding to the reference stall AOA in free air and a normalized AOA of 0.0 corresponding to the zero-lift AOA in free air. This figure is presented for information purposes only and is not intended to depict the flight conditions on the day of the accident. More about this: Basic Aerodynamics / Angle of Attack.

Figure: G650 flaps 10, one engine inoperative takeoff test card, from NTSB Accident Docket (N652GD, DCA11MA076).

[NTSB AAR-12/03, ¶1.1] According to test team members, [flight test engineer 1] FTE1 briefed the team members on the day before the accident. During this briefing, FTE1 indicated that the target pitch attitude for continued takeoff tests with the flaps set to 10° (flaps 10) would be reduced from 10° to 9°‚ (± 1°). FTE1 also indicated that they should discontinue a test if pitch reached 11° during the initial takeoff and then decrease pitch and add engine power. Gulfstream‟s principal engineer for airplane performance (who discussed the change in target pitch with FTE1 at an informal meeting in late March 2011) and [airplane performance group head engineer] APG1 stated that FTE1 made the change in target pitch to be consistent with the procedure for takeoff tests conducted with flaps set to 20° (flaps 20) and ensure that the AOA would remain below the range at which two previous uncommanded roll events (as discussed in section 1.3.2) had occurred.

[NTSB AAR-12/03, ¶1.1] The takeoff speed schedules to be used by the flight crew consisted of tabulated values for the decision speed (V1), rotation speed (VR), and takeoff safety speed (V2) as a function of flap setting and airplane gross weight; liftoff speed (VLOF) values were included in the speed schedules for one-engine-inoperative (OEI) continued takeoffs. The speed schedules were based on the free-air (out-of-ground-effect) stall speeds of the airplane, as determined by previous flight testing, and the Part 25 takeoff speed requirements.

The back of the test card notes the following under "Preventative Actions / Minimizing Procedures" item 11.

Stick shaker and PLI-intercept are set to maintain an in-ground-effect stall AOA margin of at least one (1) degree.

The wing's induced drag increases out of ground effect because the aerodynamic force is pulled aft and less lift is pointed perpendicular to the relative wind. The wing will required a greater angle of attack to produce the same amount of lift. This mishap occurred while in ground effect so the issue was not escaping ground effect, rather it was the problem of computing the stall angle of attack while in ground effect. More about this: Basic Aerodynamics / Ground Effect.

[NTSB AAR-12/03, ¶1.1] The G650 program's takeoff performance guarantee target was 6,000 feet ± 8 percent at standard sea level conditions. Gulfstream indicated that achieving the target V2 speeds was necessary to maintain the takeoff distance within the guaranteed target, or the operation of the airplane would be limited to longer runways. (For the G650, the runway length required for takeoff is minimized if the V2 speed is minimized. Thus, there is a performance advantage to keeping the V2 speed as close as possible to the minimum required.) The test team completed its first OEI continued takeoff test run (7A1), but the airspeed reached 145 knots at 35 feet and exceeded the target V2 value (136 knots) by 9 knots. The OEI continued takeoff (with the same flap configuration) was being repeated during the accident test run to reduce V2 to the target value for that run (135 knots).

Falling short of a 136 knot target V2 by 9 knots is not insignificant. As pilots we strive to get the job done but there comes a time you have to say no. If the planned procedures result in performance that is off by such a wide margin, somebody needed to raise the flag and say the planned numbers were wrong.

Photo: Aerial view of wreckage path, from NTSB AAR 12/02, figure 4.

[NTSB AAR-12/03, ¶1.1]

  • Test run 7A2 began at 0933:00. According to the on-board video recording, at that time, the PIC advanced the thrust levers for takeoff. Recorded flight data showed that, between 0933:36 and 0933:37, when the airspeed was about 105 knots, the SIC moved the right thrust lever to the idle position. About that time, the CVR recorded the SIC stating “chop” to confirm this action.

  • At 0933:46, the CVR recorded the SIC stating, “standby, rotate.” About 1 second later, when the airspeed was about 127 knots, the video recording showed the PIC pulling on the control column for rotation. At 0933:50, the pitch attitude and AOA reached about 10°, and then the PLI appeared. About that time, cockpit displays showed that the airplane's wings were about level and that the slip indicator was displaced slightly to the left. At 0933:50.4, the airplane's pitch and AOA exceeded 11°. Immediately afterward, the CVR recorded the PIC stating, “[unintelligible] going on,” and the video recording showed that the bank angle was increasing to the right and that the PIC was making a slight left wheel input.24 The video recording ended at this point.

  • Between 0933:52 and 0933:53, the CVR recorded the SIC and the PIC repeating, “whoa.” Recorded flight data showed that the stick shaker activated at 0933:52.2 for 0.6 second (with the pitch at 12.7° and the AOA at 12.4°) and at 0933:53.5 for 6.4 seconds (with the pitch at 11.8° and the AOA at 12.2°). At 0933:53.6, the CVR recorded the electronic annunciation “bank angle”; recorded flight data showed that the bank angle at that time was about 16.2°. The PIC then stated, “power, power, power,” and the SIC responded, “power's up”; flight data showed that the right thrust lever had been advanced all of the way forward about that time. At 0933:58.5, the CVR recorded the electronic annunciation “bank angle,” which was 30.5° at that time. The last communication recorded on the CVR (which was unintelligible) was at 0934:05, and the CVR recording ended at 0934:10.

  • The National Transportation Safety Board‟s (NTSB) aircraft performance study for this accident found that, when the airplane's AOA reached 11.2° during the accident takeoff, the AOA exceeded the stall AOA for the combination of flap setting, height above the ground, Mach number, and roll angle present at the time, resulting in a loss of roll control. This finding was based, in part, on the results of Gulfstream‟s simulation residual analysis, which indicated that, at 0933:50.5, as pitch angle and AOA were increasing through 11.2°, large aerodynamic rolling and yawing moments to the right were acting on the airplane. These aerodynamic moments were indicators of flow separation on the right outboard wing and an asymmetric stall of the airplane. Before the accident, Gulfstream estimated that the in-ground-effect stall AOA would be 13.1° and set the AOA threshold for the activation of the stick shaker stall warning at 12.3°.

  • Recorded flight data and ground scars and markings on the runway and airport property indicated that, shortly after rotation, the airplane's right wing contacted the runway. The airplane subsequently yawed to the right, departed the right side of the runway, traveled along about 3,000 feet of airport property, and came to rest about 8,404 feet from the runway 21 threshold and 1,949 feet to the right of the runway centerline.

Analysis

[NTSB AAR-12/03, ¶1.1]

This isn't the first time Gulfstream has played these games to meet field performance numbers:

[G450 Aircraft Operating Manual, §12-01-10, ¶1] The takeoff performance presented in this section is predicated on employing the following techniques during rotation and climb out following an engine failure. After reaching the target rotation speed, a rapid and aggressive column pull shall be applied. Adjust pitch following liftoff to achieve V2 at the 35 foot height.

[G450 Aircraft Operating Manual, §12-01-10, ¶1] The takeoff performance presented in this section is predicated on employing the following techniques during rotation and climb out following an engine failure. After reaching the target rotation speed, a column pull force of approximately 75 pounds should be applied in an aggressive manner (less than 1 second). The following table presents recommended pitch attitudes (q) that should be targeted from liftoff to the 35 foot height. These pitch attitudes should allow the aircraft to accelerate with one engine inoperative to approximately V2 at the 35 foot height. Airspeed control is critical during a continued takeoff with an engine failure, therefore, the pitch attitude should be adjusted as required to capture and maintain V2. Pitch attitudes during a normal takeoff (all engines operating) may be increased to obtain the desired climb out speed and flight path, but should be limited to a maximum of 20 degrees.

These statements do not appear in the earlier GIV and GV manuals, though Gulfstream may have changed that by now. This "rapid and aggressive" (G450) or "aggressive manner (less than 1 second)" (G550) runs contrary to normal practice and basic airmanship, all for the sake of reaching an arbitrary speed after lift off. If you don't have any obstacles, you will be well advised to use a more sane rate of rotation to keep the wing flying. If you do have obstacles, you might want to pad the aircraft flight manual performance numbers. The project test pilot did the program no favors and, if it were not for this mishap, may have backed future G650 pilots into a performance corner. This is so alarming that I have modified my approach to Gulfstream performance data. I add 10% to everything. That may be too conservative, but if their certification team is willing to bend this much to the sales team, I can no longer trust them.

What is a more "sane rate of rotation?" Boeing long ago preached 2 to 3 degrees per second. I've used that ever since. If you do lose an engine, it allows the aircraft to accelerate. If you need to clear an obstacle, once the climb is established you can fine tune V2 to V2 + 10 on your terms. An aggressive rate of rotation risks overshooting V2 and an asymmetric out of ground effect stall, as we shall see with this incident.

[NTSB AAR-12/03, ¶1.1] The test team found that an abrupt column pull force of about 70 to 75 pounds was the most successful in reducing the magnitude of the V2 overshoot. (The maximum column pull force permitted by FAA regulations was 75 pounds.) The test team also found that, if the flying pilot rotated rapidly (at peak pitch rates between 6.1° and 8.5° per second) to the 9° target pitch attitude and then exceeded 9° shortly afterward, V2 overshoots (and V2 + 10 knot overshoots for all-engines-operating [AEO] takeoffs) could be reduced to within a few knots of the target speeds. The takeoff rotation technique that produced the best results during flight 111 resulted in a V2 speed that was still about 3 knots high. According to Gulfstream‟s GVI Field Performance Certification Flight Test Plan (revision A, dated October 2010), the required tolerance for the target V2 speed was ± 2 knots.

[NTSB AAR-12/03, ¶1.3.2] The two previous uncommanded roll events during field performance testing at ROW occurred on November 16, 2010 (during flight 88), and on March 14, 2011 (during flight 132).

[NTSB AAR-12/03, ¶2.2]

NTSB AAR 12/02, figure 5.

[NTSB AAR-12/03, ¶2.3] As stated in section 1.1, ground effect refers to changes in the airflow over the airplane resulting from the proximity of the airplane to the ground. Ground effect results in increased lift and reduced drag at a given AOA as well as a reduction in the stall AOA; thus, the stall AOA is lower for airplanes in ground effect compared with the stall AOA for airplanes in free air (out of ground effect). Ground effect decreases as the distance from the ground increases and is generally negligible above a height equivalent to the wing span of the airplane (which is about 100 feet for the G650). [The figure] depicts the changes in the airplane‟s lift and stall AOA due to ground effect.

Of course this seems to run contrary to an analysis of this mishap. The wing produces more lift at a given angle of attack when in ground effect than when out. But it will stall at a lower angle of attack.

[NTSB AAR-12/03, ¶2.3]

[NTSB AAR-12/03, ¶2.4.1]

The "without exceptional piloting skill" should have been a primary thought in all those pushing for the abrupt and aggressive rotation technique.


Probable Cause

[NTSB AAR-12/03, ¶4.2] The National Transportation Safety Board determines that the probable cause of this accident was an aerodynamic stall and subsequent uncommanded roll during a one-engine-inoperative takeoff flight test, which were the result of (1) Gulfstream‟s failure to properly develop and validate takeoff speeds for the flight tests and recognize and correct the takeoff safety speed (V2) error during previous G650 flight tests, (2) the G650 flight test team's persistent and increasingly aggressive attempts to achieve V2 speeds that were erroneously low, and (3) Gulfstream‟s inadequate investigation of previous G650 uncommanded roll events, which indicated that the company's estimated stall angle of attack while the airplane was in ground effect was too high. Contributing to the accident was Gulfstream‟s failure to effectively manage the G650 flight test program by pursuing an aggressive program schedule without ensuring that the roles and responsibilities of team members had been appropriately defined and implemented, engineering processes had received sufficient technical planning and oversight, potential hazards had been fully identified, and appropriate risk controls had been implemented and were functioning as intended.


Post Accident Changes

NTSB AAR 12/02, table 4.

[NTSB AAR-12/03, ¶2.4.2]

  • After the accident, Gulfstream revised its takeoff airspeed development and testing methods. According to Gulfstream, airspeeds are now generated using a desktop computer simulation that represents the dynamics of the maneuver, the aerodynamics of the airplane in and out of ground effect, and the “control effectiveness” of the airplane. Gulfstream indicated that the desktop computer simulation was developed to more precisely model the takeoff maneuver and predict V2 speeds that ensure, among other things, (1) an achievable and repeatable initial pitch attitude at rotation and (2) a suitable margin between the operating AOA and the stall AOA during in-ground-effect operations and the climb to the obstacle clearance height. The revised takeoff airspeed development and testing methods were used to determine updated G650 takeoff speeds for the accident flight test conditions, as shown in table 4. As indicated in the table, the updated V2 speed was 15 knots (11 percent) faster than the V2 speed provided to the accident test team.

  • In addition, Gulfstream indicated that it implemented actions (besides increased takeoff speeds) to ensure the safe operation of the G650 in achieving the 6,000-foot takeoff performance guarantee. First, Gulfstream modified the takeoff technique to reduce the necessary column pull forces so that a pilot could reliably attain the initial pitch attitude within 3 to 4 seconds. Second, Gulfstream changed the stall warning system (for both flight test and production airplanes) so that the in-ground-effect stall AOA would be continuously computed in the flight control computer using the height above the ground and Mach number. Gulfstream believed that this change would provide pilots with increased situational awareness and would help ensure a timely reaction if an over-rotation were to occur. Last, the maximum takeoff thrust was increased by 5 percent to minimize the performance penalties associated with higher takeoff speeds.
  • Of course these are all good changes, but one must note that of these actions the only thing that would drive takeoff performance into the 6,000-foot target was the increase in thrust. Had someone on the test team had the integrity to speak up when the V2 performance targets were not met, Gulfstream may have been prompted to seek the thrust solution before losing an airplane and four people.


See Also

Basic Aerodynamics / Angle of Attack

Basic Aerodynamics / Ground Effect

Normal Procedures & Techniques / Functional Check Flights


References

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G550 Aircraft Operating Manual, Revision 27, July 17, 2008

NTSB Aircraft Accident Report, AAR-12/03, Crash During Experimental Test Flight, Gulfstream Aerospace Corporation GVI (G650), N652GD, Roswell, New Mexico, April 2, 2011