Aircraft Maintenance: Evaluating control issues

In the previous installment of this series, we talked about how test pilots formally evaluate aircraft handling, and how you can use the same techniques to help figure out which make and model of airplane best suits your tastes in handling. These same techniques also can be used to evaluate your own aircraft and help you identify rigging and other control system issues that are more easily identified in the air than on the ground.

The goal of aircraft control design is to require a single input from the pilot for the desired action. The ideal aircraft would be incredibly simple to fly. It would be smooth, yet stable, and would naturally return to level flight in all flying conditions. The controls would be harmonious, with pitch, bank, and yaw control requiring the same amount of force. The pilot would never have to compensate for changes in power, airspeed, flap or gear extension, etc.

Alas, no aircraft is perfect and general aviation aircraft are far from it. Our aircraft were designed with severe budgetary and time constraints in order to be marketable, and most were designed before the advent of computers and advanced fluid dynamics modeling. You make a “real” model and put it into a wind tunnel. Then you let the test pilots do the rest.

In the real world of aircraft ownership, we’re not trying to design an aircraft; we want to diagnose one. The first step is to ready the aircraft by fueling and loading it in a balanced state. Set all cockpit-adjustable aileron and rudder trim to neutral. Choose a day with exceptionally calm and smooth air, and go flying.

Your first step should be to set the power for normal cruise and trim the aircraft for straight-and-level flight. Note whether the yoke/stick is perfectly level/centered when holding the aircraft in level flight. Observe the position of each aileron while continuing to hold the aircraft straight and level. Check the position of the trim indicator and, if you can, take a look at the alignment of the elevator trim tab to see how aligned it is with the elevator. Put your feet flat on the floor and check where the skid/slip ball is on the turn coordinator. You may want to have your passenger take pictures for evaluation later. (You loaded the airplane balanced with a friend up front, right?)

Next, let’s evaluate the play and friction in the control system. Move the yoke/stick around slightly to check the amount of play before movement of the control causes movement of the respective control surface. Note the force required to make control inputs and how smooth they feel. Check for any stickiness or “flat spots” in the control travel. Be sure to make notes; you’ll need them later. Move the elevator trim, evaluating the force required to move it in each direction. Reduce power and slowly trim the aircraft to full up trim while carefully monitoring the aircraft, but not putting any force on the yoke/stick (be prepared to intervene in the event of a stall). A properly rigged aircraft should not stall on its own with full up trim applied. Only a pilot pulling on the yoke should be able to induce a stall.

Put the aircraft back into straight-and-level flight, then take your hands off the controls briefly and see what happens. Does the aircraft roll off to one side? Does the skid/slip ball roll to one side? Does it pitch up or down with minor movement of the occupants in the cabin? Lower the flaps and check to see if they cause the aircraft to roll in one direction or the other. If properly rigged, flaps should have no effect on roll.

Take copious notes, pictures, and even video if possible. When you’re safely back on the ground you can go over the results and evaluate what might need attention, which is exactly what we will be covering next time. Until then, happy flying!

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

Jeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series: The Educated Owner. Jeff is also the creator of SocialFlight, the free mobile app and website that maps over 20,000 aviation events, airport restaurants, and educational aviation videos. Free apps are available for iPhone, iPad, and Android users, and on the Web at www.SocialFlight.com.