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General
towing, tow system safety, requirements and operation are covered in the book's towing
chapter, this is only meant to explain what peculiar risks come with using
a turn-around pulley system. Like all risks, they can be managed, but the
first step is understanding. Also, this is
about towing for training, not for soaring.
See the diagrams at right for a basic description of turn-around
towing and straight-in towing.
It is hugely valuable to get a student airborne before
strapping him into a motor. Some instructors do so through towing and a
few favor the turn-around pulley
since it puts the student starts beside them instead of starting farther
away. Plus, it is more convenient as it requires one less person. However, there are
inherent
risks that should give pause to the practice or at
least give it more careful attention.
Much of my information has come
through discussions with several instructors that do towing and Stu
Caruk (www.TowMeUp.com). Although his goal
is getting free flyers high enough to soar or do maneuvers, much of that
wisdom can be applied here. I have no relationship to his company but
respect what he has learned over the years. I believe that knowledge can
help make towing for training safer if we're willing to apply it.
Turn-Around Risks
Lockout (see diagram) is the biggest towing risk
and is aggravated by the turnaround pulley. Mind you there are many other
serious risks that must be managed, too.
1. The riskiest part of tow is when the pilot
gets closer to the pivot point because small lateral displacements make
for larger angular changes. With a turn-around system the pilot is getting
farther away from the tow operator so it's harder to tell distance. So as
it gets more critical, the operator is less able to tell how far the pilot
is from the
pulley. With straight-in towing it's obvious when the pilot is getting
close because he's coming towards the tow operator.
2. Overflying the pivot point can become
catastrophic almost immediately. The release mechanism gets pulled down
between the pilots legs and away from its
normal position. If the panicked pilot cannot find the release line,
he's probably doomed.
3. The turn-around pulley adds friction. So if a
lockout begins, even if the tow operator lets off the throttle, the tow
line will be pulling harder through that friction.
4. In an emergency, even if the tow operator cuts
the line, it may foul at the pulley, negating the cut. Stu Caruk has
reported seeing this happen at least twice so it's not unheard of.
Mitigation
Acknowledging the extra risks of using a
turn-around pulley, there are ways to improve safety. One key element is
ending the tow before the towee gets halfway to the turnaround
pulley. Just "eyeballing" it by estimating distance from
glider size has proven inadequate. Here are some suggestions:
1. Have an obvious marker (see
diagram at right) on the line so that, once
it gets reeled in, the tow operator know it's time to ease off the
throttle. It should be placed about 25% of the way out the full length of
the towline so that seeing/hearing it means that the towee is now halfway
to the turn-around pulley and needs to be landed. The operator should be able
to hear it go onto the drum or have a dedicated second person who's
main job is monitoring the tow to include an "approaching
halfway" call.
2. Have a hook knife out and ready to use. The
tow operator will likely only have 3 seconds to recognize the need, grab
the knife and cut the line. If this is not easy and rehearsed, at least
mentally, it will not likely happen in time. Even if cut there is a
reasonable chance the line will spring forward and foul the pulley which
would be catastrophic.
3. Tow gently. If the towee starts getting
off-line (not pointed at the tow line), reduce tension. The student gets
rewarded by continued thrust as long as they keep steering towards the
line.
4. Have a person stationed or a remote method of
cutting the line at the turn-around pulley. If a lockout happens, the line
is cut from there.
5. Besides normal tow instructions, inform the
student about the risk of getting close to the pulley and have them
rehearse what to do (release) if they sense that's happening. Of course
new students are in sensory overload and can't be counted on but it may
help. Plus, just briefing it this way reinforces the risk to everybody,
including the tow operator.
6. Use a Tow
Assist Bridle. This device does two things: 1) it primarily keeps the
wing from falling back, and possibly going parachutal, by applying
speedbar if it starts and 2) applies differential speedbar if the wing
banks away from the tow line which tends to pull it back in line by
speeding up that side.
7. Never get the student (towee) higher than
about 20 feet. Reduce throttle at the first sign of turning offline.
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A
typical stationary winch tow system with turn-around pulley. The drum is frequently
attached to a converted motor scooter where the back wheel is made into
the drum (spool). Thus the term "scooter tow." In this depiction
it is attached to an electric motor and driven through a chain.
.
Straight
in towing, depicted here, means the pilot starts from 1000 feet or more downwind and is
towed towards the operator. The riskiest part of a tow is when the pilot
gets closest to the pivot point (drum in this case). With straight in
towing, the pilot is easier to see at the riskiest point. With turnaround
towing, the pilot is at his farthest, most difficult time to see.
Lockout
is, by far, the most common and most dangerous aspect of turn-around
towing although there are certainly other significant risks.
If
a pilot on tow begins to turn left, for example, the tow line starts
pulling the pilot to the right which makes his glider want to turn left
even more. The process can quickly spiral out of control, beyond the pilots
brake authority to
recover. This progression can be
fatal within just a few seconds if not immediately corrected.
Pilots
who wind up flying away from the
pivot point will likely be pulled into the ground since line can't come
off the drum easy enough. Cutting the line must be done before it
progresses this far.
An
airbag harnessed pilot being towed aloft. 
Line
mark at 25% point. When the mark reels onto the drum, the operator should
end the tow smoothly. This helps prevent the dangerous situation where the
towee gets too close to the turn-around pulley, a problem especially prevalent
on low tows. |
