I have a number of questions about tow ropes on my Pre solo written test which I give to my students.

After explaining the FAR's , I then ask them, "thinking as a glider pilot, would you rather have a tow rope which is on the stronger side, or on the weaker side?" They almost always answer "stronger", and I agree. But I caution, stronger within reason. In my way of thinking...I have the option to pull the release if things get too gnarly...but at least that release is on my terms, not the rope's.

My rough calculations show that the "normal" tension on a tow rope in smooth tow is very low...It is basically the drag on the glider, plus a tiny bit extra due to the climb vector...so maybe 60 lbs tension...

Almost all ropes I've seen at glider ports are in the 2000 lb breaking strength area....so rope breaks "should" be unlikely. But obviously they do break...so I attribute this to wear of damage....

I think the 80% ~ 200% is a pretty good "rule"...but again I want close to the 200%.

In 8000 flights, I have had 5 (if I remember correctly) tow rope "incidents".

Only two were classic rope breaks...and these happened on the ground roll..where the rope tension is quite high.

Another was a failure at the attachment of the rope to a weak link, not the rope actually breaking...(long story here)

another was the failure of the tow ring...but this was not an "approved" tow ring...it was somebody's idea of a "new and improved" ring...which of course failed...luckily at 400' and we were certainly expecting it. Funny thing about this one...it was in 2-33....at 400' I heard "tink"....which I figured was the tow ring breaking. We returned to the airport to find that the ring was in perfect condition. We looked at the tow hook on the Schweizer and it was still closed and in perfect condition. The guy in front was flying at the time...he got a bit of slack in the rope, when the slack come out, the rope was disconnected. What I think happened was the slack allowed the ring to twist around and back release the schweizer hook. They have a habit of bouncing back closed so it looked like it never released and the ring separated as if by magic.

Lastly, while doing a scenic ride, I noticed something first near the tow plane, which then seemed to be headed toward me...a black thing....turns out this was part of the tail spring, to which the tow rope was attached. It came straight and level back toward me for quite a while, but then finally arced and fell under the glider. So again, not a real rope break.

As for rope length..some operators will cut out a short section of worn rope at the end, and splice it back on the ring...so over time the rope gets shorter and shorter. I has a tow just last week with a student...and a really short rope. First of all, that Pawnee looked really "big"...but the student had a bit more trouble on tow than usual.

I've done long distance tows, where we attached two 200' ropes together to give 400' tow rope. This is luxury! The interaction between glider and tow plane is cut down dramatically!


Cookie








On Tuesday, November 12, 2013 11:22:14 AM UTC-5, son_of_flubber wrote:
With the dewpoint at 15d F. today, I'm ready to heat things up.



Regulations on aerotow ropes vary by region. What is the evidence to support the various approaches?



PTT (Premature Termination of Tow)(aka rope break) correlates with a number of fatal accidents. How often has a hard_to_break towrope led to a fatal accident?



Options to land after a PTT vary by airport and can be non-existent at certain altitudes. Why is an extra strong towrope not an option where the straight-ahead landing option is poor or non-existent?



In what year did the FAA set the FAR for towropes and what was the quality of towropes at that time? (obscure, but interesting question).



At the airport where I am towed, the same towrope satisfies the FAR for both the frequently towed heavier two-seater, and my less frequently towed lighter weight single place glider. Does this mean that the two-place is more likely to break the towrope?