MEET MR. LOCKOUT

All you ever wanted to know about Lockouts and some things you didn't but are going to hear anyway.

Dave Broyles © 1996

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 is high.

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 very rapidly.

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.

One last 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.

 

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