Caution: The pilot should not exceed the glider’s tow speed limitations when adding speed for takeoff in windy conditions.
Pilot Induced Roll Oscillations During Launch
As the tow pilot applies power, the glider moves forward, balanced laterally on its main wheel by the wing runner.
After the wing runner lets go and before the glider achieves significant speed, a wingtip may drop toward the ground.
In response, the pilot may apply considerable control displacement leading to a series oscillations and potential wingtip
ground contact. [Figure 7-7]
Pilot-induced roll oscillations during initial portion of
takeoff ground roll. Momentum of long, massive glider
wings tends to overshoot desired wings-level condition.
This tendency is aggravated if the pilot holds corrective
aileron pressure too long.
Figure 7-7. Pilot-induced roll oscillations during takeoff roll.
If a glider’s wingtip contacts the ground during takeoff roll, the drag of the wingtip on the ground induces a yaw in the
direction of the grounded wingtip. Mild drag and yaw result with a wingtip on smooth pavement, but strong drag and yaw
develop from a wingtip dragging through tall grass. If appropriate aileron pressure fails to raise the wingtip off the ground
quickly, the pilot should release the tow line before losing control of the glider.
Pilot Induced Yaw Oscillations During Launch
If the glider veers away from the tow plane while on the ground, rudder application in the appropriate direction corrects
the situation. As the glider continues to accelerate, the effect of the rudder increases, and the lag time decreases. The pilot
should anticipate the momentum of the glider about the vertical axis and reduce pressure on the rudder pedal when the nose
of the glider begins to yaw in the desired direction. [Figure 7-8] If the pilot holds rudder pressure too long, momentum of
the glider results in an overshoot of the desired yaw position. In extreme cases, and after a series of PIOs, the glider may
veer off the runway or force the tow plane off the runway.
Yaw momentum of the
mass of the glider’s wings
and fuselage contributes to
overshooting the desired
heading. This tendency is
aggravated if the pilot holds
corrective rudder
pressure too long.
Figure 7-8. Pilot-induced yaw oscillations during takeoff roll.
Common Errors
Common errors in aerotow takeoffs include:
• Improper glider configuration for takeoff.
• Improper initial positioning of flight controls.
• Improper alignment of the glider (unassisted takeoff).
• Improper use or interpretation of visual launch signals.
• Failure to maintain alignment behind towplane before towplane becomes airborne.
• Improper alignment with the towplane after becoming airborne.
• Climbing too high after lift-off and causing a towplane upset.
Aerotow Climb-Out
The towplane’s wake drifts down behind the towplane, and the glider can climb either above or below that wake. During
high-tow, the glider pilot maintains a position slightly above the wake of the towplane. During low-tow flight, the pilot
positions the glider just below the wake of the towplane. [ Figure 7-9] Pilots should use both positions when learning
coordinated towing procedures and aerotow dynamics. For gliders with retractable gear, the pilot normally leaves the
undercarriage down until after release.
High tow position
Wake
Low tow position
Wake
Figure 7-9. Aerotow climb-out positions.
The tow pilot strives to maintain a steady pitch attitude and a constant power setting for the desired climb airspeed. Any
excessive deviation from the low- or high-tow position by the glider causes the tow pilot to use abnormal control inputs,
which generate more drag and degrade climb performance during the tow. The glider pilot uses visual references on the
towplane to maintain a proper lateral and vertical position. The glider pilot may use different sight pictures, including
adjusting relative to the image of the towplane’s wings on the horizon or maintaining the towplane’s rudder centered over
a point on the fuselage of the towplane.
Low-tow offers the glider pilot a better view of the towplane and results in a more aerodynamically efficient tow, especially
during climb, as the towplane requires less upward elevator deflection due to the downward pull of the glider. However,
low tow increases the risk of towline fouling from a broken towline or release by the towplane during a climb. Low tow
works well for a level-flight tow during a cross-country flight.
The tow pilot normally makes climbing turns using shallow bank angles. During turns, the glider pilot observes the
towplane, matches the bank angle in a coordinated turn, and aims the nose of the glider at the outside wingtip of the
towplane. [Figure 7-10]
During a turn, the glider flies
the same path through the
air that the towplane flew.
Figure 7-10. Aerotow climbing turns.
If the glider pilot uses a steeper bank than the towplane, the glider's turn radius becomes less than that of the towplane.
[Figure 7-11] If this occurs, reduced tension on the tow line causes the line to bow and slack and allows the glider’s
airspeed to slow. As a result, the glider may begin to sink relative to the towplane. The glider pilot can correct by reducing
the glider’s bank angle, so the glider flies the same radius of turn as the towplane. A following section in this chapter
describes how using the spoilers or performing a slip can correct for slack line.
Glider turns toward inside of
towplane turn. Glider slows,
slack towline develops.
Figure 7-11. Glider bank is steeper than that of towplane, causing slack in tow line.
If the glider pilot uses a shallower bank than the tow plane, the glider’s turn radius exceeds than that of the tow plane.
[Figure 7-12] If this occurs, the increased tension on the tow line causes the glider to accelerate and climb. The glider pilot
can correct by increasing the glider's bank angle, so the glider flies the same radius of turn as the tow plane. Without timely
corrective action and if the glider climbs too high above the tow plane, the tow plane may lose rudder and elevator control.
If this occurs, the glider pilot should release the tow line and turn to avoid the tow plane.
Glider turns toward outside of
towplane turn. Towline tension
increases causing glider to
accelerate and climb.
Figure 7-12. Glider bank too shallow, causing turn outside towplane turn.
Towline/Tow Hook Characteristics and PIOs
A short tow line keeps the glider close to the towplane and its turbulent wake and complicates glider control. Using a tow
line of adequate length—200 feet minimum for normal towing operations—minimizes the influence of the towplane’s
wake and reduces the likelihood of PIOs.
The characteristics of the tow hook/tow line combination may cause changes in pitch attitude during the tow. On many
gliders, the tow hook resides below the pilot enclosure or just forward of the landing gear. An increase in tension on the tow
line causes an uncommanded pitch-up of the glider nose as shown in Figure 7-13. Decrease in tow line tension results in an
uncommanded pitch-down. Even if rapid changes in tow line tension during a turbulent aerotow of a bellyhook-equipped
glider leads to these pitch changes, the pilot should make control inputs that avoid overcontrol and PIOs.
The CG hook or bellyhook is located well below the longitudinal axis of the glider. The long moment arm from the CG hook to
the center of aircraft mass influences pitch attitude during aerotow. Increased towline tension increases tendancy to pitch up.
Towline force
Towline
Center of massLongitudinal axis
Figure 7-13. Effects of increased tow line tension on pitch altitude of bellyhook-equipped gliders during aerotow.
Common Errors
Common errors in aerotow climb-out include:
• Not maintaining proper vertical and lateral position during high- or low-tow.
• Inadvertent entry into towplane wake.
• Failure to maintain glider alignment during turns on aerotow.
Slack Line
Most cases of slack line are minor, require no corrective action on the part of the glider pilot, and resolve using a stabilized
flight path. If severe enough, slack line or reduced tension in the tow line might entangle the glider and result in damage
to the glider or towplane. Therefore, if the pilot loses sight of either the rope or the towplane, an immediate release should
be accomplished.The following situations may result in a slack line:
• Abrupt power reduction by the towplane
• Aerotow descents
• Glider turns inside the towplane turn radius [Figure 7-11]
• Updrafts and downdrafts
• Abrupt recovery from a wake corner
If the towplane precedes the glider into an updraft, the glider pilot first perceives the towplane climbing faster and higher
than the glider. Then, as the glider enters the updraft, it climbs more efficiently than the towplane. As a result, the glider
pilot pitches the glider over to regain the proper tow position. A resulting increase in airspeed creates the slack tow line.
[Figure 7-14] To avoid slack, the glider pilot should control the descent and closure rate to the towplane.
Glider, having risen too high above towplane,
dives down on towplane, inducing slack towline.
Figure 7-14. Diving on towplane.
The glider pilot should initiate slack line recovery procedures as soon as possible. The glider may try slipping back into
alignment with the towplane. If slipping fails to reduce the slack sufficiently, careful use of spoilers or dive brakes can
decelerate the glider to gently take up the slack. When the tow line tightens and the tow stabilizes, the glider pilot gradually
resumes the desired aerotow position. When slack in the tow line becomes excessive or beyond the pilot’s capability to
safely recover, the glider pilot should release from the aerotow.
Common errors regarding a slack line recovery include:
• Failure to take corrective action at the first indication of a slack line.
• Improper procedure to correct slack line causing excessive stress on the tow line, towplane, and the glider.
Boxing the Wake
The towplane generates two types of wake turbulence. Propwash generates a light chop while wingtip vortices induce a
strong rolling motion. Boxing the wake demonstrates a pilot’s ability to maneuver the glider around the towplane's wake
accurately and safely during aerotow. [Figure 7-15] A pilot can maneuver either clockwise or counterclockwise around the
wake. The example below uses a clockwise example.
Normal high tow position Upper right cornerUpper left corner
Low tow position Lower right cornerLower left corner
D A
C B
Figure 7-15. Boxing the wake.
Boxing the wake involves flying a rectangular pattern around the wake of the towplane. Prior to takeoff, the glider pilot
should advise the tow pilot of the intention to box the wake. Boxing the wake should commence outside the traffic pattern
area and no lower than 1,000 feet AGL.
Before starting the maneuver, the glider should move to the high tow position [Figure 7-15 A] and descend from the high
tow position through the wake to the center low-tow position [Figure 7-15 B] as a signal to the tow pilot that the maneuver
will begin. The pilot uses coordinated control inputs to move the glider over to the left side of the wake and holds that lower
corner of the rectangle [Figure 7-15 C] momentarily with sufficient rudder and aileron pressure.
The pilot applies sufficient control stick back pressure using the elevator to start a vertical ascent. During the ascent, the
pilot uses aileron and rudder pressure to maintain constant lateral distance from the wake. The pilot holds the wings near
level with the ailerons. When the glider reaches high left corner position [Figure 7-15 D], the pilot momentarily maintains
this position with sufficient rudder and aileron pressure.
As the maneuver continues, the pilot reduces the rudder pressure and uses coordinated flight control inputs to fly along the
top side of the rectangle. The glider proceeds to the top right corner [Figure 7-15 E] using aileron and rudder pressure, as
appropriate. The pilot maintains this position momentarily with aileron and rudder pressure.
The pilot applies sufficient control stick forward pressure using the elevator to start a vertical descent. During the descent,
the pilot uses aileron and rudder pressure to maintain constant lateral distance from the wake. The pilot holds the wings
near level with the ailerons. When the glider reaches the low right corner position [ Figure 7-15 F], the pilot momentarily
maintains this position with sufficient rudder and aileron pressure.
As the maneuver continues, the pilot reduces the rudder pressure and uses coordinated flight control inputs to fly along the
bottom side of the box until reaching the original center low tow position [Figure 7-15 B]. From center low tow position,
the pilot maneuvers the glider through the wake to the center high tow position [Figure 7-15 A], completing the maneuver.