When I was hired by TWA in 1964, type-specific simulators were not as effective as they are today. Although they helped to develop proficiency in normal and abnormal procedures, we still had to fly the airplane to become familiar with its handling qualities and quirks. It was fun, too. I recall one evening during my transition from jugs to jets, shooting touch-and-goes at St. Joseph, Missouri, when there were six Boeing jetliners in the pattern. Today, of course, a pilot often earns 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.
At 24,000 feet, my instructor turned off the trijet's yaw dampers to provide me with the experience needed to recognize and tame Dutch rolling. This natural behavior of many sweptwing airplanes is an unstable combination of rolling and yawing right and left that can worsen if left uncorrected. My challenge was to use the ailerons to arrest the Dutch roll before it got out of hand.
During a roll to the right, I applied left aileron, but this input was too late and took effect after the airplane had begun its roll cycle in that direction. Instead of arresting the right roll, I inadvertently helped the airplane to go in the opposite direction, but with increased enthusiasm. I again applied opposite aileron, this time with greater vigor. But this was again excessive and too late. The Dutch roll got much worse, and the instructor had to take 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. Keep doing this — one jab at a time and in opposite directions — until the airplane is under control. PIO just makes it worse."
"Huh?"
Pilot-induced oscillation, I learned, is a lag in pilot and control-system response to an undesirable motion of an airplane. It can either induce an oscillation or exacerbate an existing one.
A classic example of a form of PIO often occurs when a pilot attempts his first takeoffs and landings in a taildragger. During the takeoff roll, such a pilot is usually unprepared for the natural instability of the aircraft. So when the nose goes left, he is late in adding corrective rudder to the right. The amount of rudder input usually is excessive as well. Seemingly with a will of its own, the taildragger then yaws right with increased determination. This PIO increases in amplitude in opposite directions until the instructor takes over or the airplane comes to an ignominious rest in the weeds.
Experience eventually allows a pilot to improve his timing and gain a better feel for the effect of 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 too late by pushing forward on the control wheel at the same time that the nose might have begun to lower on its own. As a result, the nose comes down considerably faster than the pilot had expected. Recognizing that he is about to again hit nosewheel-first, he pulls back on the wheel with a mighty tug 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 subsequent nose-high cycle. Such an oscillation continues and worsens until either the nose-gear strut breaks or the airplane stalls. Experience with an airplane is the best PIO preventative. 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. PIO is most dangerous near the ground.
Several months ago, Merrill Wien checked me out in his pristine Grumman Widgeon, an amphibious flying boat with a reputation for porpoising during water operations. But was I concerned? Nah. Not with my experience. But I failed to consider that I had had no experience in this type of airplane.
True to its reputation, the Widgeon demonstrated its propensity for porpoising, and I demonstrated that I was once again my own worst enemy. Without Wien in the right seat to save me from PIO, I probably would have deep-sixed his beautiful machine during my first few takeoffs and landings. Keeping the airplane on an even keel requires quick, short jabs on the elevator to prevent the nose from bobbing in the same way that short, timely rudder jabs are required by a new taildragger pilot to prevent losing directional control.
The next day I was flying John Deakin's Beech Bonanza and mentioned to him during the flight that my experience in Wien's Widgeon would be a good basis for a column.
I was soon lined up on final approach to Tacoma International Airport in Seattle. Deakin's STOL-converted airplane has spoilers instead of ailerons, and these become very sensitive when the full-span flaps are extended.
Noting that I was overcontrolling and having some difficulty holding the wings level, Deakin asked with raised eyebrows, "So you're going to write about how to avoid PIO, eh?"
Clearly, I thought, it will be a case of "Do as I say and not as I do."
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