On December 22, 1996, an Airborne Express DC-8 N827AX with 6 crewmembers on board (3 flight crew and 3 maintenance/avionics technicians) crashed in mountainous terrain in the vicinity of Narrows, Virginia. The crash was the result of the crew’s failure to fully recover from a stall that they had intentionally initiated as part of a Functional Evaluation Flight (FEF), which was required after modifications had been performed on the aircraft. Although this accident involved a modern jet airliner, there are valuable lessons to be learned for pilots of any aircraft.
The crew of this DC-8, N827AX, departed from Piedmont Triad airport in Greensboro, North Carolina late in the afternoon of December 22. They had planned to depart earlier, but maintenance delayed their takeoff time. ATC assigned them a block altitude of 13,000 to 15,000 feet mean sea level for the checks they needed to perform. About 13 minutes before the crew initiated the stall, they had reported flying in and out of some clouds and had reported some ice build-up, but cockpit voice recordings indicated that they were clear of icing conditions before the stall was initiated. The extent of any ice build up on the aircraft at the time of the stall is unclear. It was also dark outside when the crew initiated the stall, so outside visual attitude references were limited. Guidance on FEF profiles at the time recommended that maneuvers such as stall series be performed during the daylight hours.
After other checks had been completed, the crew decided to initiate the stall series. The plan was to record the airspeed at which the stick shaker activated and the airspeed of the first stall indication. At that point, they would recover from the stall. They expected a stick shaker at 128 knots and first indication of the stall at 122 knots. The approach to stall was uneventful, as the pilot slowed the aircraft down at about 1 knot per second. The PF (pilot flying) noted “some buffet” at 151 knots at 18.08:06, and the crew commented that this was early for buffet (Whether this was due to some ice build-up or the fact that the control surfaces had been re-rigged during maintenance is unclear). At 18.08:09, the sound of rattling was heard on the cockpit voice recorder. At 18.08:11 the flight engineer stated “that’s a stall right there…ain’t no [stick] shaker.” The PF called “set max power” at 18.08:13. Perhaps confusing to the crew was the fact not only that the stall occurred at a higher than expected airspeed, but also that the stick shaker failed to activate. For the next 8 seconds, the PF continued to hold the nose up, maintaining a relatively constant pitch attitude. Popping sounds were also heard coming from one or more of the engines, and engine indications of surging were present, which can happen when airflow into the engine intakes is at an excessive angle. Airspeed continued to decay, and the aircraft began descending as the stall progressed. At 18.08:30 the PNF (pilot not flying) stated “You can take a little altitude down…” He was implying to the PF to push forward on the yoke. But at 18.08:42, he added, “Start bringing the nose back up.” For the next 56 seconds, the DC-8 continued descending and began a series of roll reversals. At times, the PF did move the control column forward somewhat, but the data recorder indicated several instances where the control column was full aft, which corresponded with un-commanded aircraft downward pitches. The crew failed to recover from the stall and impacted terrain at approximately 3400 feet mean sea level in a 52-degree, left wing low and 26-degree nose-down attitude.
Complicating successful recovery was the lack of outside visual references. Further, once the aircraft began to descend, it entered IMC conditions and remained there until shortly before impact. Most civilian aircraft do not have any on-board angle-of-attack visual reference, despite the fact that most military aircraft have such references. Angle-of-attack indicators and their associated equipment are not complicated devices, and many official agencies have recommended repeatedly that they be installed in airliners. It is possible that, without adequate visual references of pitch attitude or angle-of-attack, and with an inoperative stick shaker, that the crew of this DC-8 may not have realized in the descent that the aircraft was still stalled. It appeared as though their priority may have shifted from a stall recovery to a nose-low unusual attitude recovery as they retarded the engines toward idle, pulled the yoke well aft as the nose pitched down, and attempted to roll wings level at one point with full left rudder deflection as the aircraft rolled to over 100 degrees of right bank.
It is worth noting the stall characteristics of an aircraft such as the DC-8, and how that differs from the stall characteristics of other airplanes. Most straight wing aircraft have favorable stall characteristics. Their wings stall at the root first providing ample warning that the stall is progressing (fuselage buffet as the turbulent air flows off the wing roots and past the fuselage). They have fairly good lateral stability since, at least in the early stages of a stall, the outer portions of the wing are un-stalled, and the ailerons are somewhat effective. A modern swept wing airliner such as the DC-8 has somewhat different stall characteristics. Since aft-swept wings tend to stall at the tips first, the Center-of-lift may move ahead of the Center-of-gravity (CG) at the stall causing a pitch-up moment. The aircraft may begin descending in a nose up attitude unless positive forward pressure on the yoke is applied (Straight-wing aircraft will usually pitch down on their own accord when the stall occurs, as long as they are within aft CG limits.). That is why swept-wing aircraft have stick shakers that give an artificial warning of impending wing stall. Some even have stick pushers to force the aircraft to a lower angle-of-attack before the stall progresses too far. Many types of aircraft will tend to roll or yaw if recovery from the stall is delayed, and swept wing aircraft are particularly prone to becoming laterally unstable as the stall progresses. (Of note is the fact that the simulator that this crew had recently trained in did not exhibit lateral instability if held in a stall). To further complicate the problem, those aircraft with engines mounted underneath the wings can cause a further pitch up, since the engines’ thrust lines are below the aircraft’s CG. The pitch-up associated with adding power can cause the stall to worsen, if the yoke is not moved forward to counter this tendency, or if emphasis is not placed on lowering angle-of-attack first with forward pressure on the yoke. Swept-wing aircraft do not normally have the luxury of engine or propeller wash over the horizontal tail and elevator, or stabilator, to aid in pitch control. In the past, if a pilot encounters an impending stall in such an aircraft, he has been taught to hold the pitch attitude and apply maximum power to minimize altitude loss and to “fly” out of the stall. The success of this recovery lies in the fact that a stall has not yet occurred (the stick shaker will typically activate at an airspeed 5-10% above the stall speed.). It is not really a stall recovery that most of us would use in a typical general aviation aircraft for instance. It is not really a stall recovery at all, since a stall has not occurred. Guidance did exist at the time of this accident both for the DC-8 and other similar aircraft that recommended pushing forward on the yoke to lower angle-of-attack first, then adding maximum power.
The stall characteristics of the DC-8 are relatively good for a swept wing aircraft. The crew of N827AX obviously did not anticipate any problems. The crew noted early buffet, but the flight data recorder indicated that the actual stall occurred within a few knots of planned stall speed. Published guidance on the DC-8 warned that, when approaching a stall, aircraft buffet does not always precede the stick shaker, and may occur simultaneously with the shaker. But on this night, the stick shaker did not activate at all. The pilot’s decision to hold the DC-8’s pitch attitude constant as maximum power had been applied would have been uneventful if the stall had not progressed as far as it did. This leads to an important consideration involving pitch attitude and angle-of-attack.
The pitch attitude is typically defined as the angle between an aircraft reference such as its longitudinal axis and the horizon. Angle-of-attack, conversely, is the angle between the wing’s chord line and the relative wind, which could be coming from anywhere. We can approximately see the aircraft’s pitch attitude by referencing the angle that the wings or nose makes with the horizon, or by referencing the attitude indicator or other attitude reference. Certainly if we were to approach the stall from a steady 1-G deceleration in level flight, critical angle of attack and our pitch attitude would be roughly the same. We would “see” the stalling angle-of-attack by referencing the position of the wings or aircraft’s nose with respect to the horizon. But anyone who has practiced power-on stalls extensively knows that higher pitch attitudes are reached when the stall occurs, even though the wing always stalls at the same critical angle-of-attack. And if you have ever encountered a stall in descending flight or perhaps while recovering from a dive following a spin recovery, you know that a stall can occur at “negative” pitch angles, when the nose is definitely below the horizon. Consider again the DC-8 mishap. Because the stick shaker was inoperative, the pilot actually stalled the wings of the DC-8. Airspeed continued to decay, and, because lift decreased at the stall, the aircraft began descending. In the diagram in Figure 1, the aircraft on the left is at some angle-of-attack either below or at critical angle-of-attack. Since the flight path is level (as indicated by the relative airflow parallel to the horizon), the angle-of-attack and pitch angle are approximately the same (any typically small difference between the two would lie in the aircraft reference used to define pitch angle, which may not be the chord-line of the wing as it is for angle-of-attack, and in any built in angle-of-incidence of the wing as it is attached to the aircraft). The same aircraft depicted on the right side of the diagram has now entered a descent. Note that the relative wind is coming up at the aircraft from below. The pitch attitude of the aircraft has not changed, but the angle-of-attack has increased in the descent due to the upward flow of the relative wind. In the pilot’s desire to hold the DC-8 at constant pitch attitude he unwittingly allowed the angle-of-attack to increase further into the stall as the aircraft began a descent. This would not be readily apparent by noting the pitch attitude of the aircraft (and references were limited anyway), but there were other cues that the crew was not successfully recovering from the stall. They included:
- Continued aerodynamic buffet
- Un-commanded extreme pitch down moments (stall breaks) accompanied by un-commanded roll-off into steep banks
- Engine compressor surges
- Instrument indications of low airspeed and high sink rates
Figure 1: Pitch Attitude versus Angle-of-Attack in a Descent
The most prudent thing to do here would be to push forward on the yoke to reduce the angle-of-attack. Perhaps the pilot failed to do this, because it was not necessary to do so in his previous training, since the stick shaker kept the aircraft below critical angle-of-attack. The crew had plenty of altitude at the start of the stalled condition. As the DC-8 continued the descent, the un-commanded pitch down moments would have required pushing forward on the yoke to break the stall, not pulling back.
Regardless of the proximity of the ground, it is necessary to push forward on the yoke or stick enough to break the stall, while adding power to minimize altitude lost. Certainly, if the yoke is pushed forward excessively, the stall recovery will be successful, but excessive altitude loss will be inevitable in the ensuing dive recovery. However, if the yoke is not moved forward sufficiently, then the angle-of-attack may not be reduced sufficiently to break the stall. A pilot should not rely on pitch attitude to tell that the stall is broken. Other cues that can assist in determining that the aircraft is no longer stalled might include:
- Termination of stall warning horn or other aural warnings
- Reduction or termination of aircraft buffet
- Reestablishment of aircraft lateral and directional control
- Proper aircraft response to control inputs
The crew of DC-8 had several things going against them, including an inoperative stick shaker, lack of consistent guidance on proper stall recovery techniques, inadequate simulator training, and lack of adequate outside visual references. Still, had prompt and correct control inputs been applied to break the stall, the accident could have been avoided.
At APS, our instructors teach the following stall recovery technique:
- PRESSURE – Forward on the yoke or stick; enough to break the stall
- POWER – Full, to minimize altitude loss
- RUDDER – Enough to arrest any yaw and roll present while stalled
- LEVEL – Ailerons with coordinated rudder to level the wings after the stall has been broken
- CLIMB – Away from the ground
Notice that reducing angle-of-attack is absolutely the first and most important step in the recovery process. Minimizing altitude will at times be critical, and should be the emphasis of training, but if an actual stall occurs, altitude loss will be inevitable. You must react to all available cues that a stall exists, regardless of the aircraft’s attitude, airspeed, or proximity to the ground and you must be prepared to lower the angle-of-attack with sufficient forward movement of the yoke or stick. Proper training should involve stalls from a variety of pitch and roll attitudes and airspeeds. Your options are limited in most aircraft due to narrow structural and airspeed tolerances. A thorough Emergency Maneuver Training program in a structurally capable aircraft can help. APS offers such a program in the Extra 300L.
So what is your best defense in aircraft unusual attitude or upset conditions?
- First, attempt to avoid conditions that can induce unusual attitudes in the first place! Steer clear of thunderstorms and wake turbulence! Avoid IMC or flight into low visibility conditions if not properly certificated and trained. Avoid distractions.
- Second, get the proper training. According to an article in AW&ST (May 8, 1995 issue): “Training should include flights in aerobatic aircraft to practice recovery techniques because no simulator can model the disorientation of actually being upside down… recurrent training every two years, with time in an actual aircraft, would be a good start.” Regardless of the aircraft that you fly, proper training will enable you to learn to react decisively in a high-pressure environment, and to learn proper recovery techniques to avoid a “panic” response that could worsen the situation.
- Contact a APS – Emergency Maneuver Training representative. Certainly, we would like to take this opportunity to recommend our customized UPRT programs at APS. Please give us a call a 1-866-FLY-HARD and ask to speak with a flight training specialist or submit the form below for more information today!
Get this training somewhere. The life you save may be more than just your own.