Weak Links

In all aviation the pilot is usually the weakest link…

The purpose of a weak link is solely to prevent the tow force from increasing to a point that the glider can be stressed close to or beyond its structural limits. Lockouts can and do occur without increasing tow tension up until the point where the glider is radically diverging from the direction of tow. At this point tension rises dramatically and something will give – preferably the weak link. Given that a certified glider will take 6-10G positive a 1.5G weak link as opposed to a standard 1G weak link should not significantly increase the risk of structural failure. It will however significantly decrease the probability of an unwanted weak link break.

Consider a standard tow force of up to 0.8G. A 1G weak link gives a 25% strength margin. A 1.5G weak link gives 88% strength margin or 300+% more than for the 1G link. At weak link break a 1G link will give a total glider load of 2G, a 1.5G weak link a total load of 2.5G or 25% more. The stronger weak link therefore gives a 300+% increase in strength margin for a 25% increase in ultimate load.

I use a stronger weak link for the simple reason that low level weak link breaks are dangerous as the glider may not recover from the probable ensuing stall prior to hitting the ground. They are also dangerous in tailwind launch conditions although you should never take of tailwind in anything stronger than you are willing to land in. In my opinion these problems presents a far bigger risk than that of structural failure. In my experience weak link breaks cause numerous problems, whereas thanks to the quality of today’s gliders structural failure is almost unheard of. Yeah, I do know of on incident where a pilot locked out and was pulled through a previously straightened down tube leading to partial structural failure – the pilot was OK. In the event of a lock out I plan to release, and release early, because I know that by the time my weak link breaks (no matter how weak) the glider is likely to be diverging from the tow direction in a steep bank and dive. If my release fails the weak link will still break before the glider. This being said it is still far preferable to use low tow tension, headwind launch, and a standard weak link.

  1. The bridle set up is the pilots responsibility – after all they depend on it
  2. The weak link forms an integral part of the bridle set up. It should be made and tested by the pilot. If the knot comes undone then the pilot should learn to tie a better one. The weak link will only break at a consistent load if the string snaps, not if the knot comes undone.
  3. Higher climb rates and heavier glider/pilot combinations will inevitably need stronger weak links. One size weak link does does not fit all. A 1G weak link for a heavy pilot may well represent 1.5G for a light one. If they keep breaking make them stronger. Alternatively reduce tug climb rate to decrease tension or fly a lighter glider.
  4. Experience has shown that different batches of weak link line from the same manufacturer break at quite different loads. Test each batch yourself using exactly the set up you use in the glider. Use your personally tested stuff exclusively.
  5. The way the weak link is set up does influence its breaking load. The knot is the weakest point and decreases the strength of the line to ~50-70% of its un-knotted strength. Different types of knots weaken the line to different degrees. Because of this it is important to tie the same knot/s every time and position the knot/s consistently. If the knot is under tension a given number of strands of weak link will be weaker than if the knot is not under tension (i.e. excluded from the tension strands). It really doesn’t matter what type of knot you tie (provided it doesn’t come undone) or where you position it provided you are consistent and you test your system as in 4 above.
  6. Have a bridle system that is as foolproof as possible. Remember nothing is absolutely foolproof because fools are just too damn ingenious. A system that only requires a snap hook to be attached to the tow line is almost foolproof. The pilot can then personally check his/her system during their preflight.
  7. Make sure all your stuff is good stuff. Check you stuff works. Personally put your own stuff together. Check you have put all your stuff together right – remember glider, harness and tow bridle are all essential elements of your stuff.
  8. Accept that tug pilots come in all flavours – the good, the bad and the ugly. So do tow pilots. Do your part right and make their job easy. If you feel they could give a better tow give them the feedback they need to do this. Diplomacy may be required. Remember hooking onto tow is an article of faith on both ends of the line.
  9. None of this is rocket science. It is basic physics combined with elementary mechanics. It applies to all aerotowing and to towing in general. If things go wrong analyse why and fix the problem. Develop the most foolproof system possible. Do it the same way all the time, every time. Remember Murphy’s extended law which states: “If anything can go wrong it will go wrong. It will also be the worst possible thing that could go wrong and will inevitably occur at the worst possible moment”. Also remember Sod’s law which simply states: “Murphy was an optimist”
  10. Follow 1-9 then relax and enjoy stress reduced towing.

Tags: , , ,