Assessing the health of your airplane's control system
That day in February 1974 wasn't supposed to be quite so exciting for Phil Oestricher. As a test pilot for General Dynamics, Oestricher was slated to make a high-speed taxi run with the experimental YF-16, the airplane that would become the successful F-16 Falcon fighter jet. But despite countless hours of theoretical testing — the F-16 was supposedly the most simulated of all fly-by-wire airplanes to that time — this high-speed test quite unintentionally turned into the airplane's first flight. And not without considerable drama. Videotape of the event shows the airplane pitching and rolling violently down the runway, scraping its all-moving elevators on the asphalt until Oestricher decided that an early flight was better than a flaming ball at the far end of the runway. Afterburner engaged, Oestricher dragged the experimental fighter into the sky.
Oestricher's was a flight marked by a control system that let him down. Eventually, General Dynamics turned down the control gain and developed the F-16 into one of the world's premier fighters. Civilian pilots — even those who have built their own airplanes and have undertaken to prove them out — rarely experience such heart-pounding control-system malfunctions.
For the average owner, however, control system irregularities show up slowly as the result of aging or, perhaps just as often, as the side effect of improper maintenance. Airframe designers strive to make the control system rugged and trouble-free within the constraints of cost, complexity, and weight. As a result of their good work, most airplanes never have a serious control-system problem. But as the fleet ages, it's becoming obvious that time is indeed taking its toll, and the word from the maintenance shops is that extra vigilance is fast becoming necessary.
Common control system schemes use a variety of techniques to connect the yoke (or stick) and rudder pedals to the surfaces outside. Many homebuilts and a few production airplanes use a combination of pushrods and torque tubes. These are usually made up of hollow aluminum or steel rods that translate control-column motions along their length (a push rod) or through a twisting motion (a torque tube). Control systems made up predominantly of rigid members and typically quite solid feeling in the air but are harder to package into the airplane.
Far more prevalent are steel cables that translate commands from the cockpit to movement at the control surfaces. These cables can be either galvanized steel or stainless, and are, in fact, bundles of smaller strands. For example, a common flexible control cable is a 7x7, which is seven strands made up of seven wires each; 7x19 is another common configuration. These cables come in various diameters depending upon the required strength; a one-quarter-inch-thick 7x19 galvanized cable can handle a massive 7,000 pounds of tensile load. (A similarly sized stainless cable is some 10 percent weaker.)
Control cables can last a long time, assuming that they're well cared for. Protection from corrosion is high on the list for long cable life, as is assuring that they're properly routed. It's possible that during manufacture or major maintenance, cables can be misrouted so that they're not riding in their pulleys correctly, are incorrectly tensioned, or are contacting some part of the airframe. (See " Never Again: Trim Runaway!" page 142.) This last anomaly is perhaps the most common cause for premature cable wear. Be particularly cautious around autopilot installations because of the increased complexity.
Sometimes cable mix-ups happen as the result of maintenance, but mistakes can be made even on the production line. An entry in the FAA's Malfunction and Defect database elaborates. "Cessna 172R Skyhawk — While an inspection was being conducted, the autopilot servo control cable was found wrapped around the right aileron control cable. According to the manufacturer's manual, the cables should not be crossed. The control cables were inspected for damage and rerouted before the aircraft was returned to service. The submitter stated 'the aircraft was delivered by the manufacturer with this condition.' Part total time: 489 hours."
While this Cessna might have exhibited stiff ailerons, it might not have; the only tipoff could have been the alert technician who found the discrepancy. Nonetheless, it's important that you take the time to develop a feel for your control system, to understand what's normal and what's not. Amazingly, serious system flaws can be passed over from annual to annual. To wit: "Cessna Skylane — The pilot reported that the elevator trim system was inoperative during flight. The technician performed an inspection of the trim system, which revealed that the pivot pin on the last link of the chain had separated from one of the side links. Inspecting the elevator trim system chain assembly is difficult because it is located inside the center console; however, the submitter suggested expending extra effort during scheduled inspections. Part total time: 4,700 hours."
Begin your inspection by listening for strange noises and feeling for play or restrictions in the system. Outside of the effects of certain control-system tricks — like the elevator downspring that forces the stick forward or the aileron/rudder interconnect that ties the two axes together — the controls should be smooth and relatively free of friction. "Piper Saratoga — During a scheduled inspection, the aileron control system was binding at one point in its travel. Further investigation disclosed that the right aileron/rudder interconnect clamp was catching on a broken bracket. The submitter speculated that the bracket was broken by 'stepping on the flap handle or excessive force being applied during retraction and/or extension of the flaps.' Part total time: 1,946 hours."
You should not hear clanking or grinding noises, nor should you be able to discern cable slap — that's the sound of the cable touching the aircraft skin or structural member as the result of very low tension or a broken pulley. Listen also for clunking noises from pushrod- and torque-tube-actuated systems. Most of these setups use Heim joints at the ends of the pushrods, and the spherical bearing inside the joint can become loose over time.
Excessive play in any control surface should be cause for immediate investigation. How much is too much? Check with your A&P or consult your maintenance manual; different controls on different airplanes have different limits. "Cessna 310R — During a 100-hour inspection, excessive free play was found at the elevator control surface. Further investigation revealed that the elevator bellcrank pivot bolt holes were elongated where they pass through the bellcrank support brackets. The submitter believed this defect was caused by misrigging of the elevator control system and excessive cable tension. Part total time: 5,640 hours." Be looking for excessive play in hinges, too; the hinges that receive the most stress are those adjacent to the actuator, such as the center aileron hinge on the 100-series Cessnas.
On most conventional aircraft where the elevator halves are spliced together within the tail cone, it's a good idea to perform a "push-pull" test during preflight. Gently grasp the trailing edge of each elevator and try to push them in different directions. This test will tell you if there's something amiss in the torque tube or attaching hardware. Again, tag along on your annual inspection to determine the appropriate amount of slop. Stabilators can be tested as well, although you're looking for play in the main bearing area; try to move the tip fore and aft, as well as up and down. Listen carefully for knocking sounds.
Although the elevator and aileron systems get their share of attention, the comments from the shops reflect the fact that the rudder and nosewheel steering are often ignored. For example: "Piper Archer — The pilot reported that the rudder control "felt mushy" during a crosswind landing. An inspection disclosed that the left rudder bar and pedal assembly was severely cracked. The crack had progressed almost to the point of separation. A hard landing may have caused this defect. After a hard landing, the rudder control system should be closely inspected. Part total time: 3,445 hours." And this: "Cessna 150 — During a scheduled inspection, the rudder control cables were found excessively loose and were resting on the lower fuselage skin. An investigation revealed that the engine mount had separated from the lower strut collar. This allowed the strut to move forward causing excessive slack in the rudder cables. Part total time: 5,406 hours."
Nosewheel-steering systems vary, of course, with some — like the Piper Cherokee's — employing a direct connection between the rudder pedal assembly and the nosewheel fork. As suggested by the service difficulty report, care should be taken to center the nosewheel in a crosswind landing and, perhaps, inspection procedures need to be tightened, particularly for high-time aircraft.
If you do nothing else on your control-system preflight, check the security of any movable trim tabs. Why? "Cessna U206 Series — A parachute jump flight was being conducted, the jumpers had departed, and during the descent elevator flutter was experienced. The airspeed was 100 mph and the elevator flutter was marked by fast and heavy movements which were 2 to 3 seconds in duration. When the airspeed was slowed to 80 mph, elevator control returned to normal. After an inspection, the elevator trim actuator was discovered to be loose from the broken stabilizer bracket. This allowed the trim tab to have approximately 2 inches of free movement. Aircraft total time: 4,983 hours."
Control flutter can be a killer, and it's most likely to start with a loose or broken trim tab. (Poorly balanced control surfaces, from improper maintenance or loose balance weights, are also prime contributors to flutter.) Inspect these components carefully. There should be very little play in the system, and the play should not change significantly through the elevator's or aileron's range of motion. (On stabilator-equipped airplanes expect to see the trim tab move relative to the main airfoil; that's an intentional anti-servo function provided by the tab.) Gently squeeze fabricated trim tabs at the trailing edge to tell whether there's unseen corrosion (a crackling sound) or structural failure (a popping sound).
Ultimately, the control system of the typical GA aircraft is about as trouble-free as any subsystem, but it won't tolerate decades of benign neglect. Take the extra time to check it thoroughly before your next flight. You may be glad you did.
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