From pushrods to bellcranks
Hang around the runup pad one sunny Saturday afternoon, and you'll discover that some pilots have a casual disregard for their airplane's flight controls. When they get to the "controls...free and correct" portion of the check list, they grab the yoke, twist and turn a bit, and then move on to the next item. Often, they never even look outside to see what the surfaces are doing. The vast majority of the time, the controls do what they are designed to do, and no harm comes from a fast-and-furious preflight.
Not always, though. Such incautious preflight of the flight controls was the undoing of a Piper Navajo Chieftain and its crew in 1983. The airplane, just out of the Piper factory, was undergoing its first production flight test when it crashed shortly after takeoff, killing its crew of two. Postcrash investigation discovered that the left aileron control cables had been connected backward, causing symmetrical rather than differential action of the left and right control surfaces. Piper took action to see that the controls could not be so easily misrigged by clearly marking the interchangeable parts, but pilots should take it upon themselves to make that one last continuity check before taking the runway.
Fortunately, such miscues are comparatively rare, as are outright failures of the control systems. A review of recent service difficulty reports showed that only two out of 90 indicated failure of a primary control system. And one of those, a broken control-column shaft on a Cessna 152, was discovered on the ground before flight. The other was an elevator-control failure in a sailplane; the pilot landed safely. Far more often, the reports disclose failures of trim systems, with the elevator or stabilator trim vastly overrepresented. Among the maladies: broken or frayed trim cables, actuators, and tabs. Usually, such a failure results in the airplane having to be landed with the trim set for something other than approach airspeed, which can be a nonevent or a thrill ride, depending upon the weight of the particular airplane's elevator and the point at which the pilot discovers the trim failure.
A basic lack of out-and-out failures highlights the light airplane control system's principal trait: Simplicity. No fly-by-wire or hydraulics, thank you, just a collection of cables, torque tubes, and pushrods to transmit movement at the control wheel to action at the wings and tail. And if boosting of control authority takes place, it is done aerodynamically. A Frise aileron is a good example. The aileron is hinged so that as its aft edge is deflected upward, its leading edge (ahead of the hinge line) protrudes into the slipstream. This helps reduce the control pressure needed to maintain this aileron deflection.
Two basic control methods are used in general aviation airplanes, and most models use elements of each. Cable control of flying surfaces is by far the most common, no surprise given that cables are generally easier to route around cabins and airframe structures than are pushrods. A cable is just what it sounds like, a collection of steel or stainless-steel strands with a total diameter ranging from one-sixteenth inch to a fourth inch in most airplanes. How the individual strands are wrapped influence the strength and flexibility of the cable; the most common are 7 x 7 and 7 x 19, meaning seven strands of seven-wire elements wound to produce a cable and seven strands of 19-wire elements, respectively. The 7 x 7 cable is generally stronger because each of the wire elements is larger, but 7 x 19 is more common in control systems because it is more flexible.
Another way to move a control surface is to push and pull on it with a rod or tube. Mooneys are the most common users of pushrod control. If the rod is used to move a surface by movement along its long axis, it is called a push/pull tube, but a tube twisted (by a handle or a motor) to actuate a control is called a torque tube. Flaps are frequently operated by torque tubes.
In most cable-operated systems, the vehicle/operator interface (okay, we'll call it a yoke) connects to the rest of the system through a sliding bar. Attached to each bar are linkages that typically run down behind the instrument panel where they join the elevator cables; they mate with the cables at a torque tube. (Put on some old clothes and crawl under the instrument panel some day, and you'll have a better idea of your particular model's system — and you will also have a fresh appreciation for avionics installers.)
There are two sets of elevator cables, because a cable is strong only in tension, and you need to make the control surface move in two directions. Heavier airplanes often have several sets of elevator cables, but most light aircraft have one each for up-elevator and down-elevator. (Meaning, of course, that a failure of one will render the system useless, and the control surface will seek its trimmed airspeed.)
These cables travel back to the tail held in place by a collection of pulleys under the cabin floor. Eventually, they meet with a bellcrank (which is simply a metal arm-like fixture that is connected at one end to the elevator and a cable at the other).
The ailerons are controlled similarly. Pilot and copilot yokes are linked by a small cable or chain — when it stretches, you often see sloppy controls or misalignment of the yokes — and thence to cables running into the wings. Normally, these cables are connected to bellcranks in the wings just ahead of the ailerons, which in turn act on short pushrods to the surfaces. Rudder and pedals are connected in much the same way, through a series of torque tubes, cables, and bellcranks.
Mooneys, ever the iconoclastic marque, use pushrods for elevator and aileron control. According to Dick Charon, service manager at Lake Aero Styling, a prominent West Coast Mooney shop, the M20 control system is durable. "Mostly we find that when the airplane has been through an annual or two at a non-Mooney shop, the elevator trim jackscrew hasn't been lubricated well enough. And sometimes we see evidence of corrosion at pushrod pivots, but otherwise, they [the M20s] are pretty rugged," says Charon.
Cable systems suffer different kinds of problems, but many originate with poor inspection and lubrication practices. John Frank, Executive Director of the Cessna Pilots Association, says that a number of different control system maladies are found through the firm's Santa Maria, California, tech center. "Corrosion is a problem, and we often find cable tensions aren't up to specification." Cable tension is especially critical in high-flying airplanes because the aluminum structure tends to shrink more so than the steel cables in the severe cold of the flight levels. Also, in many designs, any control flutter margin is based upon the proper balance of the controls and factory-spec cable tensions; loose or weak cables can cut substantially into this margin. Pulleys whose pivots have not been properly lubricated at the annual can seize, creating a flat spot in the pulley and eventually cutting through the cable.
Frank also notes that the CPA shop sees a number of botched rigging jobs, notably quick fixes that ultimately leave the airplane further out of rig and as slow as a tax return. Often, he says, the owner will tell his mechanic the airplane is flying, say, left-wing low, and the mechanic's response is to bend a fixed tab on the aileron to compensate. "But it's really the rudder that's out of rig," Frank says, adding that rigging itself is not particularly troublesome, but the flight testing to determine a course of action remains something of a black art. The CPA recommends only shops familiar with the rigging requirements of your particular model be used; to have anyone other than an expert attempt to rig the airplane is to invite failure.
So what should you look for during the preflight and your regular in-depth inspections? First, examine the general condition of the flight controls. Are there any dents or creases? How about corrosion? (Some foam- core Cessna elevators and trim tabs are notorious for corroding from the inside out.) The controls should be snug in their fasteners, but not binding. Each pivot point should be well lubricated — especially piano- hinged surfaces, which are subject to dissimilar-metals corrosion and benefit from an oil film. Pay particular attention to both vertical and lateral movement in stabilator-equipped aircraft because just one pivot point holds the horizontal element in place.
When you move the controls from the outside, you should not feel or hear any cable slap. (On some Cessnas, rudder-cable slap is almost inevitable, but it still shouldn't sound as though someone's cracking a whip inside the tailcone.) The controls should move smoothly to their stops.
Inside, look for symmetrical movement of the control wheels — although don't expect the ailerons to each move the same amount, as most production models have some differential action. Spongy controls or those that hit the stops at the control wheel rather than at the surface are cause for a closer look-see. Rudder pedals that bind or can be deflected simultaneously could point to broken torque tubes, mounts, or weakened, frayed cables.
Consult your service manual for the specified control displacements; you can make these measurements yourself with the help of a tape measure and a protractor. It's certainly worth an hour's effort to, literally and figuratively, get a handle on your airplane's flight controls.
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