Aircraft Maintenance: Addressing control system issues

In the previous installment, we took the aircraft for a test flight to evaluate the control system from the pilot’s perspective. This time, we will look at the results of that flight and see what we should look into further when we’re back in the maintenance shop.

Rigging and control system issues are two maintenance categories that can be the source of issues discovered during a test flight.

Rigging issues

The term “rigging” has been around a lot longer than airplanes. It’s one of the many carryovers from the sailing world that aided aviation during its infancy and that still hold the two close together. In essence, rigging is the alignment and adjustment of both fixed and movable parts of the aircraft to bring the aircraft as close to the design specs as possible.

The hand-built nature of general aviation aircraft means that there are a lot of minor variations in the assembly of each aircraft when compared to the “ideal” design spec. Without the assistance of computerized, robotic assembly, it’s simply not possible to have every rivet in exactly the same spot and every part exactly the same within thousandths of an inch. The result is that no two wings, control surfaces, or even fuselages are exactly alike. This is one of the reasons that stall strips are added to the front of the wings of some aircraft during the test-flight phase before leaving the factory. The aircraft is test flown, and the positioning of the strips is adjusted until both wings stall at the same time. Hard landings, incidents, accidents, and major repairs all can affect the alignment of the aircraft fixed surfaces.

Next, there is the rigging of all the movable components of the aircraft. This includes the cockpit controls, control surfaces, gear doors, etc.—basically, anything that moves and would have an effect on the performance of the aircraft.

Start with the alignment checks for the fixed parts of the aircraft, including the position of the wings relative to the fuselage, the empennage, and even the propeller. On most aircraft, you can measure forward and toward the aircraft centerline from a specific point on the tip of each wing and see where the two points intersect. This intersection should be directly in line with the centerline of the fuselage. If it is not, you may have an issue. The angle of incidence of the wing also is a critical setting, but is more difficult to check and very difficult to adjust on most aircraft.

You should compare the alignment of the engine/propeller with the design specs. This alignment can vary because of worn mounts and can have a dramatic effect on performance. You also can do a simple “propeller tracking” check by putting a block of wood just below where the propeller blade passes and marking the exact location with a pencil. Rotate the propeller and ensure that the other blades cross at exactly the same location. If they don’t, you should ground the aircraft until the propeller is properly evaluated.

The maintenance manual for the aircraft should contain specific guidelines for checking the alignment of the fixed surfaces and rigging of the movable parts of the aircraft. In many cases, proper rigging requires special tools and jigs, but they often are available for rent from shops that specialize in specific aircraft types.

Control system issues

While rigging can bring the aircraft into alignment and adjust the control surfaces to their ideal positioning, it cannot make up for problems in the control system. The two most common issues in control systems that can cause poor handling are excessive play and excessive friction.

Control surface “play” is the ability of any control or control surface to move without affecting the other parts of the system. In a perfect system, the slightest movement of a control would move the control surface and vice-versa. Excessive play (or slop) is where you can move a control or control surface significantly without affecting the rest of the system. The manufacturer sets tolerances for most items, and they can be easily measured with the right tools. Excessive play in a control system essentially allows the control surface to move on its own in an unpredictable manner without input from the pilot. Loose cables and worn components are the most common causes of these issues.

Excessive friction occurs when the control systems cannot move freely as designed, resulting in controls that are difficult to move and do not return easily to their initial position. Worn cables, bad pulleys, poor lubrication, and improperly adjusted cable tensions often are the cause of excessive friction.

When an aircraft has excessive control system friction, it will not return to stable flight as quickly as it should after a disturbance. In addition, excessive control system friction can wreak havoc with autopilots because the autopilot servos are designed to deliver specific forces to the control system in order to deflect the control surface by exactly the amount commanded by the autopilot computer. This is especially true for elevator trim systems: If the trim control doesn’t move freely when manipulated by hand, don’t expect your autopilot to have more muscle than you do.

Fortunately, most rigging and control system issues can be easily adjusted or repaired in order to get the aircraft back in line with the manufacturer’s design. Patience and having the proper tools available are the keys to success in this project. However, the results are well worth the effort because few things are as satisfying as a great-handling aircraft. Happy flying!

Interested in aircraft maintenance? View the archives of Jeff Simon’s Aircraft Maintenance series.