Controls Rigging: A Forgotten Chore

It was an unnecessary tail-chase, expense and inconvenience. A friend fetched his airplane from the avionics shop after a Garmin autopilot installation and on the way home discovered sizable excursions in both pitch and roll modes with the system engaged. He brought it back and the shop tried all the usual stuff—eventually swapping the flight computer and exchanging servos. After months of downtime for the initial install, this troubleshooting effort meant another three weeks on the hangar floor. The shop couldn’t fix it. Ultimately, he brought it to another shop and when Garmin got involved, it was discovered that it wasn’t really an autopilot problem at all, but instead the aircraft’s control cable tensions were well out of tolerance, and there was a handful of associated hardware that needed to be replaced. When he questioned the installing shop about not making sure the primary control cables were up to spec, he was told they didn’t have the maintenance manual for the aircraft and the procedures for performing the rigging. He finally ended up with a good-working system, but also with a $6000 invoice. In my view, the installing shop really wasn’t qualified to install the autopilot because the first step of any autopilot install should be to make sure the primary control cables are rigged properly. The autopilot install manual likely has instructions for resetting tensions so the autopilot flies true.

But autopilot installs aside, checking the aircraft’s rigging is often a forgotten task at some shops during routine inspections, which might leave an owner living with a sloppy flying (and dangerous) machine. Herewith is some basic advice to determine if it needs to see a qualified shop make straighten up and fly right.

When It’s Not Right

How do you know if an aircraft is out of rig? For many pilots, they don’t—until they fly a similar model that is rigged correctly. We all get used to the way our machines handle, but perhaps the first symptom might be loss of cruise speed. But there’s also a snowball effect where rigging problems lead to other ones—including premature failure of autopilot and trim servo motors. In many aircraft, and one that comes to mind is the Cessna Centurion, the trim servo works hard as it is. I remember changing an S-TEC pitch trim servo in one of them multiple times over a 10-year period, and the aircraft was maintained by a good shop that kept up with its cable tensions. Plus, like the example of the trouble that followed a Garmin digital autopilot installation, slop in the cables won’t only alter the performance of an otherwise excellent autopilot, but it puts strain on servo motors and clutches.

If the aircraft has an autopilot with automatic electric pitch trim, observe how frequently the trim motor runs when the aircraft is flying straight and level in smooth air. If the motor seems to constantly run in either direction, it could mean that the autopilot computer (via the servo) is trying to compensate. Some systems without auto trim might not hold altitude well at all, chasing the altitude a few hundred feet. Similarly, roll cables with excessive slop might cause the autopilot to blow through the inbound course on a coupled approach, or wander off heading. Since digital autopilots generally fly with more precision, you might not have noticed (or gotten used to) sloppy performance when you flew with the old autopilot—especially one that uses a spinning attitude gyro for roll and pitch reference.

Worth mentioning is that some autopilot servos have secondary bridle cables that connect the servo to the main control cables, and if the tensions on them aren’t rigged properly with a calibrated tensiometer (usually on the bench), the autopilot will also fly with slop. Yes, high-quality autopilot installations (and upkeep) require skill, so bring it to someone who knows what they’re doing. Let’s look at some basic checks you can do yourself.

More Than Cables

First, aircraft rigging is more than properly tensioned control cables. There’s control surfaces, flaps, and landing gear doors on retracs. Invest in a maintenance manual for your particular model—you’ll want to reference it. Put a sharp eye on the control surfaces, starting with ailerons. Begin at one aileron-flap juncture and hold their trailing edges together. Observe if the outer trailing edge of that aileron lines up with the trailing edge of the wingtip. If it does, is the control yoke or stick level? If you see the opposite aileron is level with the flap and wingtip, you may have picked up some valuable clues. A sagging aileron or flap may be the cause of tremendous trim drag. However, unless you have found something wonky, check the airframe manual before leaping to possibly bogus conclusions. For most designs, though, the trailing edges of the flap, wingtip and aileron should fair together to form a straight line or smooth curve. Check the flaps to make sure they retract at the same time and are even.

How Does It Fly? … Really

You’ve flown this airplane for thousands of hours, but have you really put a critical eye on how it flies? Go find some smooth air, and first top off the fuel tanks and empty the cabin and baggage area of unnecessary stuff. But think about how you usually operate. The idea is that if you nearly always have passengers, then you will want to proceed to check your rigging based on a balanced lateral load. That could mean doing the inflight checking with a passenger onboard, but if you fly solo almost all the time, check the rigging based on the unbalanced load of just you in the seat. Bring a roll of masking tape along to mark some reference points. In the air, accelerate to the normal cruising speed in level flight. I would trim it appropriately to maintain altitude, but don’t compensate with any rudder or aileron trim if equipped. If a wing is low, take your feet off the rudder pedals and roll the wings level using aileron referencing a properly working attitude instrument. With the heavy wing leveled and the skid ball centered (without you making roll inputs), note the rudder trim indicator’s position and mark the exact position using masking tape. Note whether the heavy wing is still heavy. Chances are it’s not, or at least not as bad as it was. If it still takes some aileron input to hold one wing up, adjust the aileron trim to compensate.

Do the same thing by noting the aileron trim indicator’s setting. An old fighter jock once schooled me that this is the correct way to trim an aircraft for level flight—and he called it the tail-first method. Trim the elevator to hold proper pitch, and then the rudder to get the ball centered. Last, do the ailerons to get the wings level. This is easier in aircraft with cockpit-adjustable trim tabs or no movable tabs. If the tabs are put into a neutral position and you still find that constant control pressure is required to keep the ball centered and the wings level, the airplane’s rigging really requires a closer look. It can also explain the aircraft’s inability to fly straight and level without having one or more trim tabs deflected and it likely means the plane is flying slower thanks to trim drag.

Do some ground checks, first

There are some basic things to look at before flight checking it (and heading to the shop) because it might solve some issues that could send the troubleshooting in a different direction. First, simply inspect the trim tab to make sure it isn’t damaged/bent. Are your instruments tricking you? Level the aircraft in the lateral axis. Taxi to a level area of the ramp where you can run the engine. One way to check for level ground is putting a level on a seat rail, but also make sure the struts are equally level, and that one is not stuck extended or retracted. For really light planes, you might need to have someone sit shotgun. If the airplane has a mechanical turn coordinator, you might have to adjust the instrument so the inclinometer is centered. You’ll be surprised at the number of techs who swap these out and don’t make sure they’re level in the panel. It’s easy enough to do by tweaking the mounting screws, but EFIS screens surely eliminate the step. You might solve a problem right here by leveling the mechanical turn coordinator instrument.

Next, get out of the airplane for some checks. Use a straightedge of the right length and check each trim tab for trueness. The positional indicators may say they’re neutral, but maybe they aren’t, so verify they are. If there is disagreement between what the indicators show in the cockpit and the actual position of the tabs, make a note of it and mark the actual neutral position on the trim housing in the cockpit. From the outside of the airplane, make sure that the cowl flaps retract uniformly and completely. Also check for looseness. Note that some cowl flap systems are designed so that the flap trails slightly in flight. But mis-rigged cowl flaps (and landing gear doors) are pretty common.

How Much for That?

If you’re buying an airplane, look through the logs to see if there are maintenance entries for control rigging. If the aircraft has been painted, you should certainly expect to see that the flight controls were balanced, but that isn’t the same as a rigging. Be especially mindful with aircraft that have been damaged and repaired.

It’s tough to say how much a full-up rigging job can cost because it really depends on the complexity of the aircraft and the experience level of the techs doing the work. It’s only one of many reasons to bring it someone familiar with the type. I’ve seen rigging jobs on Cessna twins take a full day or more, while more simple airframes can be done in a couple of hours. Regardless, ask the shop if it has the correct maintenance manuals and know-how for the task. Last, if an autopilot installation is the goal, you need to ask the installing shop if it has the ability to rig the cables. You might pay additional labor for them to do so, but it’s the only way to a flawlessly performing autopilot.