Eddie Sez:

I used to do functional check flights in our Boeing 747 squadron where one of the required items was to intentionally stall the aircraft to validate stall warning indications, either from the airplane itself or from its warning systems. One of the flight engineers in my squadron trusted with this duty retired from the Air Force and ended up with the same job flying for Airborne Express. He died during this mishap through no fault of his own.

The NTSB blames the pilot flying for inappropriate control inputs during the stall recovery and the company's lack of formal functional check evaluation flight procedures. All of that is certainly true but it misses a few points that can be instructive to any pilot flying a large aircraft:

  1. If you are tasked with doing a functional check evaluation flight, you need to think about the purpose of every maneuver. The purpose of stalling an aircraft intentionally during an FEF is to prove it provides adequate warning of the stall. You compute the numbers, if they don't pan out you abandon the maneuver.

  2. The old school thought process of a stall recovery losing minimum altitude misses the point of the exercise: to get the airplane out of danger. To do that you need to break the stall. To do that, you need to lower the angle of attack aggressively.

  3. If you are in charge of a cadre of "elite" pilots who take on these kinds of tasks, you must instill in them the idea that you must learn from previous mistakes and be willing to adopt techniques to mitigate risks.

  4. If you are an instructor pilot you must always remember that you are the pilot in command and there may come a time when you must drop the "mister nice guy" facade and take control.

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


Accident Report


Narrative

[NTSB AAR-97/05, ¶1.1]


Analysis

[NTSB AAR-97/05, ¶1.5.4] According to ABX records, the PNF on the accident flight had a total of 1.1 hours of flying experience as PIC (logged the previous day) of a DC-8 post-modification FEF. The PNF had 12.6 hours of experience as a nonflying second-in-command (SIC) on post-modification DC-8 FEFs and had conducted other, less-extensive FEFs in the DC-8 that did not involve a stall series. Between 1991 and 1993, the accident PNF had flown 15 FEFs as PIC in DC-9s, according to ABX records. Some of these DC-9 flights may have involved approach to stall. The accident flight PF had no experience as a pilot on a DC-8 post-modification FEF before the abbreviated December 21 FEF.

The purpose of the maneuver was to validate stall warning speeds, not to demonstrate stall recovery techniques. There is a difference. In a stall recovery demo, the throttles are generally brought way back, the shaker, pusher, nudger, or the wings themselves are used to provide the warning and the recovery is begun. Back in the 90's most operations graded these by measuring altitude loss. Of course this is wrong, you need to break the stall. But back then, the minimum altitude loss recovery was what a check airman wanted to see. But even that is beside the point. The point of this maneuver was to validate that the warning came up at the proper speed. How I've always done this is to pull only enough thrust so that the airspeed rolls back 2 knots per second. Once I got the warning, I pushed the power forward to gently recover.

[NTSB AAR-97/05, ¶1.11.1] FDR Data During Stall Entry and Subsequent Loss of Control

[NTSB AAR-97/05, ¶1.16.1] According to Douglas DC-8-63 performance certification data, an aerodynamic stall is typically preceded by aerodynamic buffet about 15 knots above stall speed. Based on the flightcrew's calculation of 122 knots for the stall speed, the expected buffet airspeed would be 137 knots. Douglas data also indicated that the stall warning (stick shaker) activation point had a tolerance band of plus or minus 5 knots.

I did a fair number of these tests under Air Force rules where we briefed the stall number and the expected buffet speed so as to be able to know when the buffet should happen and when to abandon the maneuver.

[NTSB AAR-97/05, ¶1.17.3] According to the director of flight technical programs, he was the FEF PIC for most of the DC-8s that had earlier undergone major modification. From 1991 through 1994, the DC-8 flight standards manager was PIC for nine of these DC-8 post-modification FEFs, while the director of flight technical programs performed two of them. Beginning in October 1994, the director of flight technical programs instructed the accident PNF in FEF procedures on several flights during which the accident PNF served as SIC and nonflying pilot. The director of flight technical programs also told Safety Board investigators that he had provided the accident PNF with a simulator training session in which stall recoveries and unusual attitude recoveries were practiced. The manager said that the stall recovery procedure he taught the accident PNF was identical to the procedure taught to line pilots for their proficiency checks.

Herein lies a very big problem: the person leading the FEF program did not understand the reason they intentionally stalled the aircraft after modification and instilled in his pilots the stall recovery procedure was no different than what was used in a proficiency check.

[NTSB AAR-97/05, ¶1.18.1]

[NTSB AAR-97/05, ¶2.2.1]


Probable Cause

[NTSB AAR-97/05, ¶3.2] The National Transportation Safety Board determines that the probable causes of this accident were the inappropriate control inputs applied by the flying pilot during a stall recovery attempt, the failure of the nonflying pilot-in-command to recognize, address, and correct these inappropriate control inputs, and the failure of ABX to establish a formal functional evaluation flight program that included adequate program guidelines, requirements and pilot training for performance of these flights. Contributing to the causes of the accident were the inoperative stick shaker stall warning system and the ABX DC-8 flight training simulator's inadequate fidelity in reproducing the airplane's stall characteristics.


See Also:


References

NTSB Aircraft Accident Report, AAR-97/05, Airborne Express N827AX, Douglas DC-8-63, Narrows, Virginia, December 22, 1996