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the glider. The correct technique is to pressure the controls
until the glider begins to respond in the desired
direction, then ease off the pressure. As the glider
nears the desired attitude, center the appropriate flight
control so that overshooting does not occur.
PILOT-INDUCED PITCH
OSCILLATIONS DURING LAUNCH
Pilot-induced oscillations 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. 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 pilot-induced oscillations become much less
likely.
FACTORS INFLUENCING PIOS
The pitch effect of the towhook/towline combination
characteristic of the glider being flown, which may
cause uncommanded pitch excursions, contributes to
PIOs 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. To
8-2
minimize the influence of the towplane’s wake, use
a towline of adequate length—200 feet is the minimum
length for normal 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.
There are several techniques that reduce the likelihood
and severity of PIOs during aerotow launch. Do not try
to lift off until confident that flying speed and good
aerodynamic control have been achieved. Also, just
after the moment of liftoff, 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 the glider smacking back to
the ground, but not high enough for you to lose sight of
the towplane below the nose of the glider. Although
visually attractive to onlookers, the practice of trying
to stabilize the glider just a few inches above the
ground provides little margin for error if a PIO occurs.
IMPROPER ELEVATOR TRIM SETTING
The elevator trim control position also contributes to
PIOs in pitch attitude. The takeoff checklist includes a
check to confirm that the flight controls including elevator
trim are properly set for takeoff. If the trim is
properly set for takeoff, elevator pressure felt through
the control stick is normal and the likelihood of PIO is
reduced. If the elevator trim is set incorrectly, however,
elevator pressure felt through the control stick may contribute
to PIOs. If the trim is set excessively nose-down,
the pilot needs to hold back pressure on the control stick
to achieve and maintain the desired pitch attitude during
launch and climb-out. If the trim is set excessively
nose-up, the pilot needs to hold forward pressure. The
more pressure that is needed, the more likely it is that
the pilot will over-control the glider.
Although all gliders exhibit these tendencies if the trim
is improperly set, the effect is most pronounced on
those gliders with an aerodynamic elevator trim tab or
an anti-servo tab on the elevator. The effect usually is
less pronounced on those glider fitted with a simple
spring system elevator trim. Regardless of the type of
elevator trim installed in the glider, error prevention is
superior to error correction. Use a comprehensive pretakeoff
checklist and set the elevator trim in the appropriate
position prior to launch to help prevent PIOs
attributable to elevator trim miss-use. [Figure 8-1]
IMPROPER WING FLAPS SETTING
The likelihood of PIOs increases if the wing flaps
are not correctly set in the desired takeoff position.
For the majority of flap-equipped gliders, most
Glider Flight Manuals/Pilot Operating Handbook
(GFM/POH) recommend that flaps be set at zero
degrees for takeoff (check the GFM/POH for the
manufacturer recommendations). If the flaps are
incorrectly set to a positive flap setting, which
increases wing camber and wing lift, the glider
tends to rise off the runway prematurely, perhaps
even before the elevator control is sufficient to control
the pitch attitude. Attempting to prevent the
　
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