Porpoising PIOs During Launch Pilot-Induced Oscillations (PIOs) Factors Influencing PIOs Porpoising is a general term that refers to pitch oscillations  that can occur in gliders and aircraft in general. In most cases,  pilots induce these oscillations through overcontrolling the  glider as they attempt to stop the oscillations from occurring  in the first place. The instability of a glider’s attitude that arises when the  pilot fails to recognize the lag time inherent in controlling  the glider is known as a pilot-induced oscillation (PIO).  Typically, PIOs occur when the glider fails to respond  instantly to control input and the pilot quickly increases  the pressure on the controls. By the time the pilot judges  that the glider is responding satisfactorily, the extra control  pressures have resulted in such a vigorous response that the  glider overshoots the desired flight attitude. In an attempt to  correct the situation, the pilot moves the controls rapidly in  the opposite direction, overcompensating for the mistake.  The undesired glider motion slows, stops for an instant,  and then reverses. The alarmed pilot maintains significant  control pressures to try to increase the rate of response.  The glider, now in rapid motion in the desired direction in  response to heavy-handed control inputs, again shoots past  the desired attitude as the now thoroughly alarmed pilot jerks  the flight controls in the opposite direction. Unless the pilot  understands that these oscillations are the direct result of  overcontrolling the glider, it is unlikely that the oscillations  will cease. More likely, they increase in intensity until there  is a complete loss of control. Although PIOs can occur at anytime, these situations arise  most commonly during primary training. Pitch instability is  another result of center of gravity aft beyond limits. If a pilot  encounters PIO, they should ensure the CG is within limits.  Pitch instability tends to disappear as pilot experience grows  because pilots gain familiarity with the lag time inherent in  the flight controls. These types of oscillations may also occur  when a pilot is making flights in unfamiliar types of gliders.  For this reason, particular care must be taken when the pilot  is preparing to fly a single-seat glider in which the pilot has  no prior experience. When checking out a new make of  single seat glider, the lag time of the flight controls must be  learned without the obvious benefit of having an experienced  glider flight instructor aboard during flight to offer advice or,  if necessary, to intervene. While most PIO discussions are  devoted to pitch oscillations, consideration should be given  to roll-and-yaw induced oscillations. The first step toward interrupting the PIO cycle is to recognize  the lag time inherent in the glider’s response. Any change in  glider flight attitude takes an appreciable amount of time to  accomplish as the flight controls take effect, and the mass of  There are several techniques that reduce the likelihood and  severity of PIOs during aerotow launch. A pilot should  not try to lift off until confident that flying speed and good  aerodynamic control has been achieved. Also, just after the  moment of lift-off, allow the glider to rise several feet above  the runway before stabilizing the altitude of the glider. Two  to three feet is high enough that minor excursions in pitch  attitude, if corrected promptly, do not result in glider contact  with the runway surface, but not high enough to lose sight  of the towplane below the nose of the glider. Caution should  be exercised if attempting to stabilize the glider just a few  inches above the ground because it provides little margin for  error if a PIO occurs. To minimize the influence of the towplane’s wake, use a  towline of adequate length—200 feet is the minimum length  for normal towing operations. A longer towline provides  more isolation from towplane wake during aerotow launch.  Short towlines, on the other hand, keep the glider closer to the  towplane and its turbulent wake, complicating the problem  of controlling the glider. The characteristics of the towhook/towline combination on  pitch of the glider being flown may cause uncommanded pitch  excursions if the pilot does not compensate for those effects.  This can contribute to PIO during aerotow launch. In addition,  the propwash and wing vortices of the towplane, through  which the glider must pass if there is little or no crosswind,  affect the flight attitude and control response of the glider,  especially at low speeds during the start of the takeoff roll.  PIOs are most likely to occur during launch because the  glider’s lag time changes rapidly as the glider accelerates.  During the first moments of the takeoff roll, aerodynamic  control is poor, the control feel of the glider is very sluggish,  and lag time is great. Flight controls require a wide range of  movement or input to have an effect on the glider’s flightpath.  As the glider gains speed, aerodynamic response improves,  control feel becomes crisper, and lag time decreases. When  the glider has acquired safe flying speed, lag time is short,  the controls feel normal, and PIOs become much less likely. the glider responds to the pilot’s control inputs. The second  step is to modify control inputs to avoid overcontrolling the  glider. The correct technique is to pressure the controls until  the glider begins to respond in the desired direction, and then  ease off the pressure. As the glider nears the desired attitude,  center the appropriate flight control so that overshooting  does not occur. 8-2