Perhaps the classic example of a failed flight control system involves United Airlines Flight 232, a scheduled domestic passenger operation of a DC-10. On July 19, 1989, while in cruise at FL370, the jet’s center, tail-mounted engine’s fan disk failed. The pieces penetrated the engine’s containment shroud and severed the airplane’s hydraulic lines. Those lines quickly leaked out their fluid, eliminating any ability for the flight crew to reposition the airplane’s primary control surfaces.
Using the only controls remaining—the left and right engines’ throttles—the crew managed to descend and approach the Sioux Gateway Airport/Col. Bud Day Field in Sioux City, Iowa, where the DC-10 crash-landed, killing 111 of the passengers and crew aboard. Survivors totaled 185, including the flight crew. Many lessons were learned from this accident, including alternatives to aircraft control when the primary systems have failed. To understand them, though, we first have to understand the systems themselves.
Cables, Pulleys And Pushrods
The typical personal airplane uses a system of cables and pulleys, often connected to pushrods, to transmit pilot input from the cockpit to the primary control surfaces. For reference, the graphic at right depicts a simplified version of a generic pitch control system. Systems used for yaw and roll control are similar. Basically, when a pilot pulls back on the pitch control, deflects the ailerons or steps on a rudder pedal, he or she is moving the control surface by activating its linkage.
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In addition to its relative light weight and simplicity, the typical system is robust, requiring little in the way of maintenance except lubrication, careful inspection and occasional checks to verify cable tension. But they can and do fail. Cables become worn, pulleys jam, rod ends can break, linkages established with nuts and bolts can come apart and loose items like screwdrivers or shop rags left behind can prevent full movement. Many of these kinds of failures can be traced back to poor maintenance techniques, yet another reason for the pilot/owner to closely monitor and inspect work performed.
That’s why we perform preflight inspections to verify the security and freedom of movement of each control surface. We then do an in-cockpit check before taking off to ensure the control system not only functions smoothly and freely throughout its range of movement, but that each surface deflects in the correct direction when we want it to. As we’ll discover, any question or concern about a control system’s function discovered during these checks should be investigated on the ground, not in the air.
Failure Modes
Perhaps the most common failure of a primary flight control surface involves a jammed hinge mechanism. (Although actual hinges sometimes are used—a piano hinge is common on many Cessna ailerons—we’re using the term generically here.) Every primary control surface and many secondary ones pivot in some fashion around attaching bolts, via torque tubes or in some other way. Of course, any time a mechanical object is designed to move, there’s a risk of it jamming somehow.
As mentioned above, another common failure mode can involve separation of a control cable or one that jumps off a pulley. In the former, it’s likely the surface will move of its own accord; in the latter, it may not move at all, along with the corresponding cockpit control. Partial failures also can occur, an example of which might be a binding control cable allowing movement but only with higher-than-normal force exerted from the cockpit.
Although they don’t involve the primary controls—elevator/stabilator, ailerons and rudder—secondary control failures involving trim or wing flap systems, among others, also may present challenges. The sidebar on the opposite page discusses them in greater detail.
A final type of control system failure involves external physical damage. This can be caused by hangar rash or some other ground-handling issue, in which case it should be discovered during a preflight inspection and repaired before flight. In-flight collisions—either with wildlife or another aircraft—also can cause failures. We’d guess the former can be just as unmanageable as the latter.
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In researching this article, we found a bunch of manufacturer recommendations on dealing with a jammed pitch control or runaway electric trim. Not so much with the other failures. That tells us there’s little consensus on how to deal with them.
Pitch Control Failures
Regardless of how the pitch control fails, you’re left without an effective way to decide the airplane’s attitude. Your task is to land the airplane using what’s left to control pitch. That’s usually power.
In conventional airplanes, presuming the elevator/stabilator remains fixed, adding power tends to raise the nose; reducing it lowers the nose. Notable exceptions to this rule can include airplanes with canard configurations or engines mounted high on pylons, like the Lake series of amphibians. If you fly what might be considered a non-conventional airplane and don’t know what happens when the pitch control remains fixed and power is added or reduced, now’s a good time to grab an instructor and go find out.
Wing flaps also can be used to manage pitch since the typical light plane wants to trend nose-up when wing flaps are deployed. In extreme conditions, feel free to ignore the airspeed indicator’s white arc but deploy only enough flap necessary to manage the situation. Some combination of power and flap probably will stabilize the airplane’s attitude.
Pitch trim, a secondary control, also can be used to control the airplane when the primary fails. With a conventional horizontal stabilizer, move the trim control opposite to the direction you want the nose to pitch. When flying an airplane with a stabilator, which includes Piper’s PA-28, -32 and -34 series, move the trim wheel in the same direction you want to pitch the nose.
Roll Control Failures
We see two basic failure modes of the roll control system in the average personal airplane. In one, the ailerons are jammed while deflected an equal amount in the normal, opposite directions. In the other, one aileron is jammed in a deflected position while the other responds normally. Regardless, handling failure of the roll control system usually involves the rudder. In extreme situations, you’re justified in holding a fully deflected rudder to counter resulting rolling tendencies.
Reducing power and thereby airspeed will minimize the adverse impact of the jammed control surface, but also may prevent the other aileron from exerting enough authority to keep the wings level. In such an event, you’ll need to find a sweet spot of power, airspeed and pitch that enables you to retain control and get the airplane on a runway. If you’re flying a conventional twin, differential power can make all the difference.
Yaw Control Failures
If we were to choose which control system failure we’d want to have, it would be the yaw control, or rudder. Such a failure is the most benign we can envision and usually means the least amount of effort required to get the airplane safely back on the ground. It can create its own set of problems, however.
In no event, for example, would we be tackling a crosswind landing. Similarly, the failure mode may also involve an airplane’s steering system, making it impossible to taxi off the runway after landing or cocking the nosewheel and ensuring an off-runway excursion upon landing.
If we’re flying a conventional twin, one with engines on the wings, differential power again is available to help control yawing about the vertical axis.
Finally…
Don’t forget managing power and airspeed can be a critical tool when dealing with a primary control system failure. Slowing down, for example, minimizes the effect of a deflected control (it also minimizes the other controls’ effectiveness). Slowing down also gives you more time to deal with the situation. We’d definitely want to get the airplane below its maneuvering speed, VA, where full control deflection won’t damage the airframe, if it’s not already that slow.
But don’t reflexively pull off all the power and hope for the best. Setting power should be part of an overall controllability check once you’ve recovered enough to perform one. Such a check only should be performed with adequate altitude and away from congested areas or traffic patterns.
Control system failures are rare, thankfully. To help prevent them, fly a well-maintained airplane. If they happen, remember you have several tools available. Use them all.
