On Thursday, May 14, 2020 at 12:28:22 PM UTC-7, Steve Koerner wrote:
An artificial horizon and electric guillotine is a complex solution. Is the tow rope at a sufficient angle in these situations to simply position a sharp knife above the rope such that it cuts itself if the angle is too high? Surely that has been thought of and rejected for good reasons?


I'd been thinking along the same line. I don't think it's quite that simple though. No matter how sharp the blade, it won't cut a line that is merely kissing against it. The blade needs to be hot enough to melt through the line or actively sawing. And, obviously it has to all happen very quickly.

How about this: Imagine two permanent magnets (strong magnets) that when joined together create a magnetic junction in the tow line just a bit aft of the tugs two hook. Similar to the knife blade idea, a horizontal bar is mounted off the back of the tow plane above the tow rope such that when the rope raises to a critical angle, the bar presses along the alignment between the two magnets. The relatively small orthogonal force exerted at the magnetic juncture will have a leverage effect to pry the magnets enough to break the magnetic circuit. To improve and optimize the leverage action on the magnets, each may have an attached bar of defined length that is rigidly affixed such the tow line attachment point is positioned at the distal end of these leverage bars. In fact, the tug side's lever bar may have a ring at the end to be attached directly to the tug's tow release. The tug will be instantly freed predicated on the existence of a strong enough pull on the line above the critical angle of bar contact.

Perhaps simpler and smaller than the magnetic method would be to instead incorporate a metal weak link analogous to a Tost weak link used on winch tows. In this case, the link is designed to bust specifically by leverage action across the weak link junction. There will be leverage bars on both sides of this angular weak link. The fulcrum of action could comprise two annular elements with the weak link element joining the two sides in the center of the annular elements. As with the magnetic case, a bar above the line imparts an orthogonal force that results in a powerful leverage action that busts the weak link when the combination of line angle and pull force exceeds the intended critical threshold.