All you ever wanted to know about Lockouts and some things you didn't but are going to hear anyway.
Recently, there has been a lot of discussion on lockouts and
what they are. To this end, I have defined a new science, lockoutology.
Lockoutology is the exhaustive study of the history of, and the
mechanisms behind lockouts to help people avoid Close Encounters
of the Fourth Kind, the encounters with lockouts that put you
in the hospital or worse.
It always helps to put a sugar coating on bitter medicine, so
I may joke around while discussing this subject, but it is a serious
subject. Most serious tow accidents end up in a lockout. This
is not to say that they are caused by lockouts. Most car accidents
in which a car hits a bridge abutment at high speed are fatal,
but the bridge abutment is not the cause of the accident. A bridge
abutment is always there. If you don't pay attention, and wander
from the center of the road, you encounter the abutment. No real
surprise, you knew it was there, and you have passed it hundreds
of times, but damn, there it is right in front of you. Good-bye!
Likewise, the lockout is there. All the time! A little less visible
than a bridge abutment, but you know it's there. You have passed
it hundreds of times. The first rule for avoiding lockouts is
to pay attention. The 100% rule is in effect anytime you tow.
Give 100% of your attention to the successful completion of the
tow, to the exclusion of all else. If your dog bit you this morning,
and you found one half of a cricket in your waffle at the local
pancake house, and you got a ticket for speeding on the way to
the tow site, and you lost 3 safety rings while setting up your
glider, and the only person that has any at launch wants two bucks
apiece for them, STILL, you have to put all this aside when you
start to put your glider under tow.
The towline is not your friend; you are adding
an extra 100 to 200 pounds of force to your glider in a direction
which will change from moment to moment, and it certainly makes your life more complicated under tow.
A hang glider under tow is an unstable system. It
requires constant pilot input of the correct sort to keep the
glider where it is supposed to be. This is not an impossible task,
anymore than driving down a 2 lane highway, 8 feet away from cars
going in the opposite direction with a relative speed to you of
140 mph is difficult, but in both cases, the price for inattention
Rule number one: during a tow, you must give 100% of your attention to flying that tow,
until you release. If you think towing is easy and you can do
it with 10% of your attention, then meet Mr. Lockout.
Pay attention to what?
Rule number two: Keep the glider in the correct tow position. This
is important both during ground based tow, and during aerotow.
If you are out of position, your options are to get back into
position or release from tow.
If you can't stay in position, then meet Mr. Lockout.
WHAT IS A LOCKOUT?
So now we get down to the nitty gritty. A lockout is the attitude
of a hang glider under tow, in which the tow rope is applying
a force which is not in line with the center of pressure of the
glider, and causes it to roll to the outside with more force than
the pilot can exert to correct the roll. The consequence of a
lockout is that the glider does a slipping turn into the ground
or in the case of an aerotow lockout, may actually roll inverted
while creating so much drag that the tow plane is put into trouble.
So what are the dynamics of a lockout? The glider and tow rope
combine to create what engineers call a positive feedback condition.
When the condition of a lockout occurs, suddenly things start
getting worse very fast. Why? Well, consider the worst case condition,
a hang glider which has the tow rope attached to the control bar
in the center, and that the rope is attached to some sort of moving
vehicle without any tension limiting devices. In this case, which,
incidentally, replicates the way we used to tow in the '70s, any
time the glider is not being flown right directly along the tow
rope, the glider tends to roll to the outside. Actually, there
is some force via the dihedral, sweep back and washout of the
glider which tends slightly, to keep the glider centered. Old
standard gliders from the early '70s had lots of dihedral, sweep
back and washout, and were relatively easy to keep centered. Still,
several pilots during that period were killed in lockout related
towing accidents. Modern gliders, even including some of those
in the late '70s had much less of all of the above. Modern competition
gliders with relative little washout, sweep back and dihedral
require a lot of pilot input to stay centered on tow even with
some new innovations which improve things a lot.
However, back to the worst case condition. The glider is rolled
or banked to the outside. Due to the physical attachment of the
rope to the control bar, the tension is out of line with the center
of pressure of the glider. The
pilot corrects. This means that the pilot moves his/her body to
the high side of the control bar. If the pilot is able to move
the weight of his/her body far enough to the side to counter the
rolling moment generated by the tow tension, then the bank is
corrected. If not, the bank either continues or gets worse. Once
the bank starts getting worse, the glider tends to slip to the
outside. This causes an increase in airspeed. Why? Because the
tow vehicle's speed is constant, and the glider's speed is a combined
vector sum of the vehicle speed and it's movement to the outside.
The increase in speed increases the lift of the wing and makes
the tow rope tension even higher, the higher the tow rope tension,
the more it banks to the outside. The more it banks to the outside,
the more it slips, the more it turns to the outside and the more
the speed increases. The vehicle, very great in weight and power
compared to the hang glider, is not retarded much by the increase
in drag of the hang glider, thus the hang glider never slows down.
This means that although the hang glider may well exceed the critical
angle of attack for stall, that it doesn't stall. (Because
stall is caused by the rapid increase in drag and subsequent loss
of airspeed as the critical angle of attack is exceeded.) Thus,
as long as the glider remains attached to the tow rope, it experiences
relatively unlimited and increasing acceleration, until something,
often the glider, breaks.
Notice that the pilot's strength is not an issue so much as the
limitation to the space a pilot has for movement to the side.
No matter how strong the pilot is, he isn't going to push his
way through the flying wires and downtube.
So why weren't early pilots plagued by lockouts? Well, these
pilots, well aware of the risks of getting too far off the center
of the tow, generally flew the glider in the correct position
during the tow. In addition, they usually flew over lakes or open
water, where the air is more stable and were less likely to encounter
a strong thermal. Further, standard gliders had a lot of built-in
stability which required a lot less pilot input to keep the glider
stable. As the years wore on, gliders evolved. To provide better
performance and handling, the glider's aspect ratio was increased,
the sweepback, dihedral and washout were decreased and gliders
became harder to fly under tow because of decreased stability.
The payout winch, developed in the '70s. greatly improved the
situation. The payout winch is a drum of rope which has drag applied
to it by a disc brake or hydraulic pump, and which is designed
to release rope at a constant tension. Since the payout winch
provided a relatively constant tow tension even under lockout
conditions, the positive feedback effect is greatly lessened.
This doesn't mean that lockouts are eliminated, just that if
a lockout occurs, the glider doesn't accelerate without limit
until something breaks.
In the late '70s, a pilot and physics professor from Texas, Donal
Hewett, came up with the idea of towing from the pilot's body
and the top of the control bar, using a V-bridle sort of arrangement.
The idea was that this would provide stability to gliders by providing
negative feedback in towing roll instability. The way it worked,
was that the tow rope was attached to the pilot's body, and if
the glider banked, the tow force would pull the pilot over to
cause the glider to turn back into the center. It was thought
at the time that this system would eliminate lockouts. In fact,
experience proved that the center of mass bridle, did make it
easier to avoid entering a lockout while under tow. Up to a point,
the rope, by pulling the pilot back to the center, assisted the
pilot in causing the glider to automatically roll back to center
if it became banked. Once, however, the glider was banked to the
outside and low and the rope pulled the pilot against the flying
wires and control bar as far as he/she could go, then Katy, Bar
the Door. The lockout would come on full force as soon as the
glider got banked enough that the pilot couldn't move any more
to the side.
This means that there is a difference between towing from the
basetube and towing from the center of mass. The difference is
that when towing from a fixed point on the basetube, the force
rolling the glider to the outside increases constantly as a function
of the angle the glider is off center from the towing vehicle.
The farther over to the side the glider is, the harder it is to
roll the glider straight.. While with a center of mass bridle,
the glider is very easy to turn into the center until the glider
is off center enough that the pilot can not move enough to compensate.
Once the pilot is against the side wires and downtube an almost
instant lockout occurs. The pilot can get pretty far out of shape
before he/she is in trouble, but once there, the situation degenerates
To quote Gilbert Griffith of Australia, "As
far as theory of lockout is concerned, I doubt whether there is
any cure and bugger-all time for theory if you're stuck in it.
All the ones I have seen are over within seconds."
What impact do pitch, pitch pressure, and pitch stability have
on lockouts? Well, first, the increasing speed of the glider in
a lockout causes the glider to increase it's pitch. The increasing
pitch causes the glider to turn away from the direction of tow
even faster. Pitch stability is the name for the tendency of a
glider to increase it's angle of attack and slow down when it
is going faster than trim and nose down and speed up when it is
going slower than trim. Obviously, a pitch stable glider, such
as a novice rated glider, once it is in a lockout would tend to
nose up and slip even more than a higher performance glider with
less pitch stability. On the other hand, a stable glider is less
likely to get into a lockout in the first place.
Recovering from a lockout requires pulling the nose down to reduce
the tendency of the glider to turn to the outside, which allows
the pilot to roll the glider in before the lockout condition worsens.
It is common knowledge that when towing with a static line tow,
the shorter the towline, the more rapid the onset of a lockout.
However, the natural tendency of pilots everywhere is to think
that whatever they are doing is at the limit for safety, so if
they usually tow with a 1000 ft. line, then less is dangerous.
I have heard critical comments about using a static towline only
200 ft. However, in aerotow, which is very definitely a static
line tow, just with a lighter, flying tow vehicle, it is commonplace
to use a 200 ft. towline, or even a 100 ft. towline. when more
maneuverability is needed. When aerotowing, avoiding lockouts
is paramount. Yet, pilots, to the chagrin of the tug pilot, quite
often oscillate all over the place. This is however without locking
out or even breaking the weaklink. What this shows is that it
is rather easy to recover from lockouts with light tow tensions
and in level flight, even with a short towline. On the other hand,
aerotow gives us the opportunity to observe that lockouts do regularly
occur with a center of mass bridle, and also that occasionally,
a glider will get into a really bad attitude before the weaklink
breaks. This means that it is extraordinarily important for the
tug pilot and the glider pilot to be very conservative until enough
altitude is attained to prevent the glider pilot from hitting
the ground if he has a lockout.
Another benefit of the payout winch besides constant rope tension
is more subtle. In the '80s, platform launch was invented. While
no one in their right mind would tow a glider with a 5 ft. long
static line tow rope, due to the rapidity of the onset of lockouts
with such a short rope, at some point, pilots found that a glider
could be flown with a very short rope from a payout winch. Why?
Because the payout of the rope, and the constant tension feature,
provided an effectively infinite towrope length. This is not intuitively
obvious to any but the greatest thinkers, but in Texas, some great
thinkers, the author not among them figured this out, either mathematically,
or empirically and from this bit of news, figured out platform
launch. It took a brave person to eat the first raw oyster and
another to do the first payout winch truck launch.
There has been a lot of discussion of lockouts and methods to
recover from them. As this article points out, with a center of
mass tow system, up to a point, the tow system tends to cause
the glider to recover from unsafe attitudes pretty much automatically,
to the extent that some gliders even require that you let them
recover without much help or they will oscillate all over the
sky. However, when you truly lock out, it is sudden and gets worse
fast. Also, some gliders, especially those with little yaw stability
and some resistance to rolling, won't auto-recover well. Further,
it is difficult to know when you are approaching the point of
no return when you have the glider in a difficult attitude. Thus
it makes good sense to deal with bad attitudes by releasing with
only mild provocation. Good pilots, flying from platform launch, who are aggressive,
quite often can fly the glider out of almost anything that they
encounter. Great pilots don't waste their energy. Rule number three: RELEASE EARLY.
.Pilots should prime themselves
to release instantly at the first sign of trouble.
An interesting aside, is that if the rope is going above the basetube,
the glider is much harder to lock out than if the rope is going
only underneath the basetube. Why this is the case, is because
if the rope is routed entirely below the control bar, and the
glider is at the same altitude as the tow vehicle, the rope will
exert a force on the control bar which noses up the glider. Insert
illustration here .Illustration should show that if the glider
is close to the same altitude as the tow vehicle that the control
bar is pulled up. Counterpoint showing the rope through the control
bar. If the rope is routed through the control bar, the
glider is much easier to nose down as the glider gets into a difficult
attitude. Thus, in aerotow, where the rope is always routed through
the control bar or in a V-bridle arrangement which has the line
attached to the pilot routed above the base tube, the pilot can
recover from an attitude which would surely result in a lockout
if the tow rope were routed entirely below the control bar. Therefore,
in aerotow, pilots are sometimes told not to release, but to let the
tug pilot fly them out of their problem. Recent accidents
indicate that this may not be a wise policy. The reason why is
most probably because that once the glider gets into a really
bad attitude from which a lockout is inevitable, the ensuing reaction
after the weak link breaks may require a lot of altitude to recover.
If the pilot is at a low altitude, he or she may hit the ground
before recovering. A glider pilot in great difficulty, may put
the tug pilot in jeopardy as well. So, unless the pilot is very
sure that he or she can recover from whatever is encountered,
then he or she should GET OFF THE ROPE. In aerotow, the rope is
relatively short, commonly 100 to 200 ft. thus, a lockout is quicker to occur.
Likewise, if the tow bridle is arranged in a skyting bridle arrangement,
since the force is partially to the glider, at the keel, this
also makes the glider much easier to nose down and thus harder
to lock out. But likewise, if you get into an attitude which will
cause a lockout, things will get bad really fast.
Maximizing tow altitude for a given length of tow road has been
the subject of debate. It is pretty well known that a static line
tow, with the tow tension limited to a value similar to that for
a payout winch tow will climb a glider higher in a given distance
than will the payout winch. Thus, it has been a practice of some
in the platform launch community to launch a pilot, drive really
fast and payout a lot of line at a relatively low tension, then
jack the tension up, and climb the glider rapidly while paying
out relatively little rope. This practice has some risk involved.
As mentioned before in this article, when the glider is not much
higher than the tow vehicle and the tow rope is below the basetube,
then the tow force tends to nose the glider up and make it vulnerable
to tip stall and then lock out. This is not a function of the
length of the rope, but entirely a function of the vertical angle
of the glider to the tow vehicle. Thus, a safe guide line is that
the tow tension should be limited until the glider can be flown
at a safe angle of attack, with the
tow rope not touching the base tube of the glider. If the tow rope
touches the basetube with the rope underneath the basetube, it
limits the pilot's ability to pull in and recover from a turn
to the side.
In conclusion, a lockout doesn't just magically appear from thin air. It usually happens because a pilot isn't following some basic principles.
Rule number one: Give 100% of your attention to the successful completion of the tow.
Rule number two: Stay in the proper position.
Rule number three: Release early, before you are locked out.
thing. Mentally rehearse the limit of how far off line or out
of position you will allow yourself to be. Visualize that, and
visualize releasing instantly when you pass that point. Then in
real life, when you pass the point, DO IT.