http://www.birrendesign.com/rhgpa_criteria.html

These are Donnell Hewett's original twelve criteria for a good towing system. Let's look at them with respect to aerotowing and our current practice:

#3: C-M (Center of Mass)

DISTRIBUTION The towing force must be distributed between the components of the flying system proportionally to the masses of the respective components. (Pilot and glider move in relation to each other and the tow force needs to be evenly distributed. 2:1 bridle is quite effective at this – 2/3 force on the pilot, 1/3 force on the glider.)

Pro tow type bridles are the most widely used type of towing bridles in aerotow competition. For the most part competition pilots don't want to deal with all the extra strings that come with 2:1 and even 1:1 bridles. 1:1 bridles are used with the "Moyes type system.

#4: C-M ATTACHMENTS

The tow line/bridle must be attached as closely to the effective center-of-mass of each of the components and must not be allowed to touch any other part of the flying system. (Violations of this produce lockouts, adverse yaw and other loss of control problems.)

The pro tow puts all the forces from the tow on the pilot and the pilot has to do the balancing act. The pro tow is connected close to the pilot and doesn't touch any other part of the glider and is the system most unlikely to do so.

#7: INFALLIBLE WEAK LINK

The system must include a weak link which will infallibly and automatically release the glider from tow whenever the tow line tension exceeds the limit for safe operation. (There is always the possibility something unexpected can happen. Breaking point should be appropriate for the weight and experience of the pilot, not to exceed 1G – sum of all towed parts.)

The pro tow and the "Moyes" type system require that the bridle go through the "ring" at the end of the tow line if the weaklink breaks. They are therefore not infallible. Other weaklink release systems are also not infallible. Infallible is a pretty high barrier, and we might want to speak of more or less fallible.

#11: RELIABLE COMMUNICATIONS

The system must provide a means whereby the pilot can reliably communicate to the rest of the crew. (Anything longer that 500 feet of tow line requires a radio – minimum from pilot to driver – for pilot-in-command. Everyone must agree on what signals or commands that will be used.)

There is only minimal communication used in an aerotow operations. Two signs/signals used by the ground crew, one verbal command from the pilot, three signs used by the ultralight pilot in the air. No radios.

I look forward to any response from the HGFA or other interested persons. Again, I have a direct personal and minor financial interest in the issues raised by this discussion, but no financial interest when it comes to weaklink material.