Flight control failure
Rare, but worth your consideration
The accident was caused by the catastrophic loss of all hydraulic power, which made it impossible to move any of the flight controls. There are no backup cables with which to manually control a widebody jetliner. Not even an Olympic weightlifter could operate such massive control surfaces. The stabilator on a Lockheed L–1011, for example, weighs 7,000 pounds.
Captain Al Haynes and his crew discovered that the only way to control their crippled DC–10 was to vary the thrust of the two underwing engines. (The uncontained failure of the center engine in the tail caused the loss of fluid from all three hydraulic systems.)
Increasing and decreasing the thrust of both engines in unison caused the jetliner to pitch nose up and nose down, respectively. Applying power asymmetrically controlled the heading. When more thrust was generated by the right engine, the aircraft turned left, and vice versa. That the aircraft could be steered to a runway in this manner was incredible. That 184 people survived was a miracle.
This accident attracted the attention of Frank Burcham, chief of the Propulsion and Performance Branch at NASA’s Dryden Flight Research Facility. He initiated a program to investigate the handling qualities and effectiveness of controlling aircraft using only engine power. His goal was to develop techniques and technology to help those who might find themselves in similar peril.
Burcham invited me to participate in the research by flying a visual simulator programmed to investigate the engine-only handling qualities of several aircraft. He advised me that the primary flight controls were inoperative and that I was to attempt landing different aircraft using throttles only. This proved extremely challenging and highlighted how difficult it had to have been for Haynes to control his aircraft. Under the circumstances, he did a masterful job. I prefer not to discuss my performance in the simulator except to say that I am grateful that the results were simulated.
Burcham then offered me an opportunity to use a propulsion-enhanced flight-control system. In this mode, control-wheel inputs were converted into throttle commands. For example, pulling the wheel aft increased the power of both engines, thus causing the nose to rise, and vice versa. Turning the wheel right or left operated the engines differentially, which induced a roll in the desired direction.
It took considerable practice to adjust to the sluggish aircraft response and the tendency to overcontrol. You had to make small changes and wait for the results but operating the engines in this mode did make it a bit less difficult to land.
Flight-control failure on a jetliner is rare. It also is unlikely on general aviation aircraft. This is because the primary flight controls of a lightplane are independent of one another and not dependent on hydraulic power. Each flight control is operated by its own cables or push rods. The only way for a lightplane pilot to lose the use of all primary flight controls is to take off with the controls locked. It is not so impossible, however, for one control to fail.
Loss of aileron or rudder control usually is not catastrophic because one can be used in place of the other, but do not attempt to land on a narrow runway or with a crosswind with such a handicap. The most challenging problem is the loss of elevator control, and the most likely cause of that would be the failure of a cable. Such a failure, however, does not result in a total loss of pitch control.
An elevator control system consists of two cables. One raises the elevator when back-pressure is applied to the control wheel; the other lowers the elevator when forward pressure is applied. Assume, for example, that the down-elevator cable fails. The wheel can be moved forward but has no effect. At such a time, nose-down pitch control can be restored by applying substantial nose-down trim, enough to induce a significant nose-down attitude. This essentially restores total pitch control. To raise the nose, apply back pressure to the control wheel as you normally would. To lower the nose, simply release some of the back pressure necessitated by the nose-down trim. Landing in this manner is not difficult. On the other hand, if the up-elevator cable fails, pitch control can be restored by applying substantial nose-up trim and varying forward pressure on the wheel to control pitch attitude.
Such an emergency is thankfully rare and explains why many might tend to discount the possibility.
Captain Haynes might not have given it much thought either.
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