the top of the canopy if the seat belt and shoulder harness are  loosely fastened.  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. 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. 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. 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. 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  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.  8-6 Glider-Induced Oscillations Pitch Influence of the Glider Towhook Position Pilot-Induced Pitch Oscillations During Landing