Pilot-induced oscillations

Although they helped to develop proficiency in normal and emergency procedures, we still had to fly the real airplane to experience its handling qualities. This was more fun, too. I recall one evening during my transition from jugs to jets when there were six Boeings in the pattern shooting touch-and-goes. Today a pilot can earn a type rating in a simulator without ever having been in the actual airplane.

It was during my Boeing 727 training in 1968 that I learned about pilot-induced oscillation, an interaction between man and machine that can have tragic consequences. It occurs when a pilot overcontrols while correcting an attitude excursion and inadvertently worsens the problem.

At 24,000 feet, my instructor turned off the trijet’s yaw dampers to provide me with the experience needed to recognize and recover from a Dutch roll. This behavior of many swept-wing airplanes is an unstable combination of rolling and yawing that can worsen on its own if not corrected. My challenge was to arrest the Dutch roll before it got worse.

During a roll to the right, I applied left aileron, but this input was too late and took effect after the airplane had already begun to roll left on its own. Instead of arresting the right roll, I inadvertently exacerbated rolling in the opposite direction. I then applied opposite aileron, this time with vigor. This was again excessive and too late. I began to lose control, and the instructor took over.

“Look, Barry,” he counseled. “Everybody has trouble with this at first. When the aircraft is rolling in one direction, just apply a jab of opposite aileron and quickly neutralize the controls. Continue doing this one jab at a time and in opposite directions until the airplane is under control.”

Pilot-induced oscillation (PIO), I learned, is a lag in pilot response to an undesirable motion of an airplane. This can induce oscillation or exacerbate the condition.

An example of PIO can occur when a pilot attempts his first takeoff in a taildragger. During the takeoff roll, such a pilot is usually unprepared for the instability of the aircraft. When the nose goes left, for example, he is late in adding corrective rudder to the right. The amount of rudder input usually is excessive. Seemingly with a will of its own, the taildragger then yaws right with increased determination. The yawing increases in amplitude and in opposite directions until the instructor takes over or the airplane comes to an ignominious rest in the weeds. Experience eventually improves a pilot’s timing and provides a better feel for the controls.

Consider a situation during which a pilot inadvertently lands nose first in a tricycle-gear airplane. The nose bounces skyward, and the pilot reacts with excessive forward pressure on the control wheel while at the same time the nose might have begun to lower on its own. As a result, the nose comes down more rapidly than the pilot expects. Recognizing that he is about to again hit nosewheel first, he pulls back on the wheel but is again too late. The resultant combination of the nosewheel tire bouncing off the ground and the applied back-pressure increases the amplitude of the wheelbarrowing cycle. Such oscillation continues and worsens until either something breaks or the airplane stalls. In the meantime, an inexperienced pilot might be advised to handle this situation by applying full power when the nose rises and establishing a safe go-around attitude. Experience with an airplane goes a long way to preventing PIO, which is most dangerous near the ground.

I recall when Merrill Wien checked me out in his Grumman Widgeon, an amphibian with a reputation for porpoising during water operations. Was I concerned? No. Not with my experience. But I failed to consider that I had no experience in that type of airplane.

True to its reputation, the Widgeon demonstrated its propensity for porpoising, and I demonstrated that I was my own worst enemy. Without Merrill in the right seat to save me from serious PIO, I probably would have deep-sixed his beautiful machine during my first few water takeoffs and landings. Keeping the airplane on an even keel required short, timely jabs on the elevator to prevent the nose from porpoising. This is similar to how short, timely rudder jabs can be used by a new taildragger pilot to prevent losing directional control.

Merrill knew that I wrote a monthly column and suggested that PIO might be a good subject. Great idea, I thought, but it would be a case of “do as I say, not as I do.”

www.BarrySchiff.com