Force Times Distance: Articulating Control Surfaces

Until that defining moment which comes to some of us at some point during our training (somewhere between the day we start flying and when we inevitably must stop), we fail to grasp the importance of the rods and cables that carry our control inputs from inside the cockpit out to our wings. In my case, appreciation and awareness came rather suddenly.

During our pre-takeoff checks where we verify that our cockpit controls can move unimpeded and correctly, many of us are taught the “thumbs up” method of verifying correct aileron movement. We grasp the control yoke with both hands, thumbs pointing upward, and verify that our thumbs point in the same upward direction as the side on which the ailerons deflect upward. For example, when we turn the yoke or move the stick to the right, the aileron goes up on the right. Pretty basic stuff, right? Well, maybe not — especially when we’re rushed, and the eyes see what the brain expects them to see (unless perhaps we don’t even look) like the test pilots aboard the Piper Chieftain during one production flight test, about 20 years ago. The NTSB found the left aileron cable connections had been reversed, and two people died.

Primary control surface failures are rare (and we’ve pretty much covered the subject in earlier discussions), but control system degradation is not. That’s a good enough reason, for me at least, to bring the subject up again. Usually, it’s the trim system (elevators and stabilators, especially) that experiences difficulty: cables fray, get a bit too slack, or break. Usually, it’s no big deal. But since you’re better prepared to deal with something when you know how it works, it’s time to see how we wiggle our toes in an airplane.

Control surfaces articulate (as in connection and movement, not speech) either by a series of interconnected pulleys and cables, or else through interconnected arrays of pivoting tubes and bellcranks. Many airplanes use a combination of the two systems. A cable on pulleys is easy enough to visualize, and perhaps because it is intuitively simpler to route cables around and inside of an airplane than it is to do that with a bunch of interconnected rods, this is the system that is the most common.

What is a cable, exactly? A cable (as far as aircraft are concerned) is a spirally wound aggregation of thinner steel strands that is anywhere between one-sixteenth and one quarter inch in total diameter. Some comprise seven strands of seven-wire elements, and some are made up of seven strands of 19-wire elements. The former is stronger; the latter is more flexible. Pushrods, torque tubes, bellcranks, and the like are a bit less intuitive, but equally easy to understand, once given the proverbial thousand-word picture.

In fact, here are two drawings showing how such things are used to relay control inputs; in the case of an aircraft like the Robinson R-22 helicopter, where “slack” cables just wouldn’t do. (In fact, due to higher vibration levels and the number of consecutive tubes, the “play” in rod ends that would be acceptable on fixed-wing aircraft, isn’t, not where helicopters are concerned.)

Figures 1 and 2 (end of article) show how pitch change inputs are transmitted to the main and tail rotor control mechanisms, respectively. Both drawings show quite a few push-pull tubes for transmitting these inputs to the main rotor and tail rotor. Also shown is another type of junction in flight control systems, the torque tube. Mounted in a saddle, in this case perpendicular to the aircraft centerline, and attached at each end by a bearing, the shaft partially rotates within it, converting a rotary motion to a linear one (as in the use of the collective). A third type, namely bellcranks, often used with push-pull tubes, are also utilized to change direction or travel, as well as to provide a mechanical advantage.

In most systems using cables, taking an airplane elevator as an example, the stick or yoke controls things by means of a sliding bar, to which are attached linkages that run down behind the instrument panel. Those then join up with the elevator cables at something called a torque tube, which is just a pipe that exerts its influence by rotating. (Flaps can be controlled by the use of torque tubes.) Because cables work only when they’re in tension, you need a pair of them for each control surface (and in heavier airplanes, matched sets). Where there are only two cables, if one breaks, you lose both (unlike in a twin-engine airplane, where you’d probably still have a good engine), and the airplane would fly at whatever speed at which it had been trimmed. Continuing our dissection here, these elevator cables run back under the airplane floor and join up with a bellcrank, which is a hinged affair connected at one end to the cable, and in this case, at its other end, to the elevator.

Ailerons work in a similar way. In an airplane with side-by-side controls, usually linked by a chain or cable like the pulleys on an old-fashioned clothesline, and then on to cables out into the wings. Again, these attach to bellcranks in front of the ailerons, which then push on rods along their length connected to the ailerons. Rudder controls work by means of similar groupings of torque tubes, cables, bellcranks, or pushrods. (In the case of Mooneys, known for their strength and durability, pushrods predominate.)

Cables usually have problems such as insufficient lubrication and tension, as well as corrosion. In airplanes that fly high, cables can cause problems as the airframe gets cold-soaked and shrinks at a greater rate than does steel cabling. Since cable tensions are compromised, so are the safety margins against flutter. Pulleys that aren’t adequately lubricated can seize, become worn due to cable friction, or cut through the cable altogether. Problems related to rigging can also result in uncoordinated flight due to improperly tensioned or mismatched cables. The pushrod system can also degrade through neglect however, such as with corrosion at pushrod pivots, or insufficient lubrication on elevator trim jackscrews, but overall, they’re the tougher of the two.

What does this all translate to for a preflight? First, there are the normal things you’d look out for anyway, such as dents or creases in any surface (control or not), fretting rivets, or corrosion. As for the controls themselves, they should feel firm, but with no signs of snags or binding. All movement joints should be adequately lubricated. When moving the controls by wiggling control surfaces during the walk-around, you should not hear any “cable slap” (with the exception of some small smacking sound from rudder cables in aircraft having this type). Controls should move smoothly, all the way to their stops. Obviously, if the airplane has just come out of an annual or 100-hour, that “controls free and correct” stuff isn’t something to do while preoccupied. Be wary of any slop or excessive wiggle or misalignment between the yokes in a side-by-side arrangement. Control wheel movement should be symmetrical (although the deflections of the ailerons themselves might not be). If the control wheel hits the stops before the control surface does, this should be looked at by your A&P. Be sure there is no binding in the rudder pedals, and if they can be simultaneously deflected, that’s not good. Service manuals specify the amounts of control displacements, and these can be confirmed with a simple tape measure and protractor.

By the way, I didn’t experience anything that made my hair turn white overnight. My epiphany came a dozen years ago, when I happened to peer inside the fuselage and wings of a 172 during an annual inspection. It was then that I saw how my life depended on the integrity of cables and pulleys (not to mention the shock of seeing just how thin that aluminum fuselage and wing skins really were). Once you see just how many bends and corners have to be traversed before your control inputs make it out to the wingtips and tail feathers (as well as any flaps), you’ll become a big fan of continuity!

Figure 1:

Figure 1

Figure 2:

Figure 1