the top of the canopy if the seat belt and shoulder harness are loosely fastened. 

During these excursions, the weight of the pilot’s hand and arm on the control stick may cause the control stick to move a significant distance forward or aft. During positive G-loading, the increased apparent weight of the pilot’s arm tends to move the control stick aft, further increasing the angle of attack of the wing and G-load factor. During negative G-loading, the reduced apparent weight of the pilot’s arm tends to result in forward stick motion, reducing the angle of attack and reducing the G-load factor still further. In short, this rapid cycle of induced flight control input affects load factor and increases the intensity of vertical gusts on the glider’s airframe and the pilot. One protection against this is to reduce speed when cruising through turbulent air. Another protection is to brace both arms and use both hands on the control stick when cruising through turbulent air at high speed. It is worth noting that some glider designs incorporate a parallelogram control stick linkage to reduce the tendency toward PIO during high-speed cruise. 

## Pilot-Induced Pitch Oscillations During Landing 

Instances of PIO may occur during the landing approach in turbulent air for the same reasons previously stated. Landing the glider involves interacting with ground effect during the flare and keeping precise control of the glider even as airspeed decays and control authority declines. A pilot can cause a PIO by overcontrolling the elevator during the flare, causing the glider to balloon well above the landing surface even as airspeed is decreasing. If the pilot reacts by pushing the stick well forward, the glider will quickly dive for the ground with a fairly rapid rate of descent. If the pilot pulls the control stick back to arrest this descent while still in possession of considerable airspeed, the glider balloons again and the PIO cycle continues. If airspeed is low when the pilot pulls back on the stick to avoid a hard landing, there will probably be insufficient lift available to arrest the descent. A hard or a nose-first landing may result. 

To reduce ballooning during the flare, stabilize the glider at an altitude of 3 or 4 feet, and then begin the flare anew. Do not try to force the nose of the glider down on to the runway. If airspeed during the ballooning is low and the ballooning takes the glider higher than a normal flare altitude, it may be necessary to reduce the extension of the spoilers/dive brakes in order to moderate the descent rate of the glider. Care must be taken to avoid abrupt changes. Partial retraction of the spoilers/dive brakes allows the wing to provide a bit more lift despite decaying airspeed. 

Another source of PIOs during the approach to landing is overly abrupt adjustment of the spoilers/dive brakes setting. 

The spoilers/dive brakes on most modern gliders provide a very large amount of drag when fully deployed, and they reduce the lift of the wing considerably. Excessive use of the spoilers/dive brakes during the approach to land can easily lead to oscillations in pitch attitude and airspeed changes. The easiest way to guard against these oscillations is to make smooth adjustments in the spoilers/dive brakes setting whenever spoilers/dive brakes adjustment is necessary. This becomes particularly important during the landing flare just prior to touchdown. A sudden increase in spoilers/dive brakes extension results in a high sink rate and possible hard contact with the runway. This can lead to a rebound into the air, setting the stage for a series of PIOs. As before, the cure is to stabilize the glider, then resume the flare. If the spoilers/ dive brakes are retracted abruptly during the flare, the glider will probably balloon into the air because of the increased lift provided by the wings. Remember, spoiler/dive brakes provide drag and reduce lift. This extra lift and pilot reaction may result in overcontrolling and PIOs. The use of spoiler/ dive brakes should be determined by flight conditions. If a wind gust has caused the glider to balloon, then spoiler/dive brake use is an option for the pilot to reestablish the flare attitude. If the spoilers/dive brakes must be adjusted, do so with a smooth, gentle motion. 

## Glider-Induced Oscillations 

## Pitch Influence of the Glider Towhook Position 

The location of the glider’s aerotow towhook influences pitch attitude control of the glider during aerotow operations. During these operations, the towline is under considerable tension. If the towline is connected to a glider towhook located more or less directly on the longitudinal axis of the glider, the towline tension has little effect on the pitch attitude of the glider. 

On many gliders, the tow hook is located below the cockpit or just forward of the landing gear. Many European gliders have the towhook located on the belly of the glider, just forward of the main landing gear and below the longitudinal axis of the glider. The glider’s center of mass is above the location of the towhook in this position. In fact, virtually all of the glider’s mass is above the towhook. The mass of the glider has inertia and resists acceleration when the towline tension increases. In these tow hook configurations, an increase in tension on the towline causes an uncommanded pitch-up of the glider nose as shown in Figure 8-4 . Decrease in towline tension results in an uncommanded pitch-down. 

Rapid changes in towline tension, most likely to occur during aerotow in turbulent air, cause these effects in alternation. Naturally, on days when good lift is available, the aerotow is conducted in turbulent air. The potential for inducing pitch 

