HyperCut – radio operated tow disconnection device

design and engineering – Had Robinson
RC setup – Lee Boone
test pilot – Sam O’Donnell
updated September 18, 2021


The HyperCut instantly ends a tow by disconnecting the towed pilot at the drogue parachute.  The only hardware that remains connected to the pilot is the weak-link assembly.  When seconds count, the HyperCut can help prevent an accident.  The HyperCut manual gives additional information on its installation and use.

There was a recent death in our region caused by a tow operator (untrained and uncertified among other things) that could have been prevented had an observer been stationed behind the towed pilot with the HyperCut ready.

When a tow does not go as expected, the tow operator (TO) can shutdown the winch or, if things get particularly out of control, cut the towline.  However, these procedures can take a few seconds which is long enough for a lockout (LO) to dramatically increase in intensity.  A minor issue after a cut in the towline is repairing the Spectra.  It must be carefully spliced back together which can take (15) minutes.  Unfortunately, the splice weakens the line and, over dozens of tows, the splice may fails.

Principles of design and operation

The HyperCut (HC) has (2) important functions: 1.) it cuts an inexpensive replaceable link between the pilot's weak-link assembly and the drogue parachute and 2.) it cuts the line very quickly, as fast as the tow operator can actuate the switch on the radio (about 500 ms).  With the HC, the towed pilot is completely free of the towline and all associated hardware except the weak-link assembly.  It can be used for both pay-in and pay-out tows.

HyperCut details

  1. Foam container inside the drogue parachute which protects the HC electronics (battery, receiver, and on-off switch)
  2. Heavy duty nylon webbing that connects the drogue to the HC
  3. Heavy duty screw-link maillon
  4. Dyneema line that secures the HC to the maillon and drogue after the HC is activated
  5. the HyperCut – it consists of a pair of razorblades enclosed in a plastic tube (LinkKnife™), an RC servo to actuate the LinkKnife™, monitoring red LED, aluminum housing, and a foam protective tube (removed in the photo above for clarity).
  6. Entrance hole that goes through the center of the HC
  7. Replaceable line that goes through the HC and then to the weak-link assembly (not shown) that is attached to the tow bridle.  (The replaceable line is long because of repeated activations and testing of the HC.)  For strength, it can be tripled with just one strand going through the HC.

Protective foam tube that surrounds the HC during normal operation of the tow.  When the drogue falls to the ground it helps protect the mechanical parts of the HC.


Protective foam container inside the drogue.  The electronics have to be carefully positioned so that they do not exit the container when the towline is released or cut.


The HC consists of a model airplane RC servo which is operated by the "GEAR" switch on a commonly available RC radio (a Turnigy 9XR).  The advantage of using these RC radios is that they are inexpensive, widely available, and easy to program.  The radio is kept adjacent to the TO at all times.

Instantly cutting the towline close to the pilot's bridle eliminates the substantial drag of the towline and drogue on the pilot who may be recovering from severe pitch and roll movements that can accompany a lockout.  He also will appreciate not having thousands of feet of towline following him around while he figures out what to do next.  The weak-link is always present so that a sudden snag of the towline from some object on the ground should break it – but not before increasing the pitch of the glider.  It might not matter if there is altitude but what if the snag happens just before the pilot lands and he fails to check the resultant surge?  The HC helps eliminate this risk.  This sort of scenario has never happened at Southwest Airsports after thousands of tows but one never knows what may happen in the future, especially during cascading events.

The battery and receiver are in a foam container safely inside the drogue. This container was developed at Southwest Airsports years ago for a sat-com (SPOT) that is needed when doing recreational high-tows (+1 mile AGL).  We sometimes have a break in the towline when it was a mile or more out.  The drogue (which actually flies) would go downwind a great distance, often out of site.  Instead of losing the drogue, the SPOT would tell us exactly where it was.  The drogue jerks severely when it is released and also when it opens.  The foam container was designed so that the SPOT would not be ejected and would be protected in case the drogue hit the ground with force.

This same design works for the HC electronics which are now in the same container.  The loop in the black wire loom helps keep the control wires from disconnecting from the HyperCut due to the severe jerk on the drogue assembly when it is released or when the HyperCut is activated.  I considered mounting the receiver directly on the HC but the HC undergoes a significant shock load nearby (along with everything else) and it seemed best that it be located inside the foam container.

A hazard could occur if maillon #3 or the HD line #2 between the drogue and HC fail.  Parts could slam back into the pilot (this is why pilots must always have eye protection when towing).  Below is a photo of the test pilot after an HC activation in flight.  He is holding what is left of the tow equipment that was attached to his bridle.


Because tows should always be done moderately (<600’/min), a failure of this kind should not be dangerous to the pilot.  Over a decade, there have been failures of various parts of the drogue assembly but they did not result in an injury of any kind to the pilot.  Limiting tow pressure is critical in preventing all sorts of mishaps.

Before a tow begins, the monitoring light on the HC should be checked.  If the HC receiver battery is low in charge or dead, the red LED will be off or dim instead of blinking every second.

Even under full tow pressure the surgical razor blades of the LinkKnife" are not able to cut through the protective covering that surrounds the Dyneema fibers inside.  I was surprised finding this out.

Generally, pay-out towing should be limited to P3’s who have the ST rating.  No matter how it is done, newer pilots must simply be watched with the utmost attention i.e. launched with pay-in only.  Maybe it would be safe for P1/2’s if pay-out is done over water with the strictest adherence to emergency preparedness.  However, I would not be in favor of this because of the unlimited way things can go wrong in this sport.

The type of tow equipment used by the TO is a prime factor with regards to the overall safety of his operation.  Use of electric motors or hydraulics to do both pay-in and pay-out are the only types that will endure weak-links that are barely stronger than the tow force.  All other types of towing equipment can be rough and temperamental.  The hang glider pilots we have towed could not believe that we could get them off the ground with a 70 lb. weak-link.  We rarely break a weak-link even though they are about 30-40% of the pilot’s weight.  This is because electric & hydraulic winches can go in and out as needed.  I call it a “constant force winch” (CFW).  Winch speed is irrelevant. The tow force is what matters.

The HC must be activated at the right time.  Is the pilot too close to the ground?  Releasing tow pressure must be done first if there is any chance a sudden loss of tow might pendulum the pilot into the ground.  If the pilot is high enough to recover from a 100% loss of tow (and why ONLY moderate tow forces should ever be used) then activating the HC the moment a lockout (LO) begins will not cause more problems i.e. a collapse.  LO’s close to the ground require great skill for the TO to mitigate. I think that is the hardest thing to do for a TO.  Moreover, the TO should never apply full tow pressure until the pilot is away from the ground.  A sudden failure just after launch would not endanger the pilot.  This and other reasons should require that towing equipment be approved by the USHPA towing committee.

pilot being towed with the HperCut installed

It makes me also think about the value of a hook-knife?  It will take some force to cut the glider lines or the risers in an emergency.  The lines would have to be under load which they are not sometimes.  The movie "Master & Commander" has a tragic scene of some rigging being cut under load.  Hemp rope does not stretch much but I wonder if the actors and movie staff thought about it?  If you sever a Nylon rope under load you better consider where the cut is made or people can get decapitated.  I’ve seen nylon rope imprint itself in steel sheeting when cut under load.  “It don’t take much to do it.”