The Skyhook is United Parachute Technologies version of a Main-Assisted Reserve Deployment system (MARD), a safety feature on skydiving parachute systems. It builds on the concept underlying an ordinary reserve static line (RSL), which uses the force of the departing main parachute to open the reserve parachute compartment after the malfunctioning main parachute is cut-away, by further using the force of the departing main parachute to extract the reserve parachute out of the reserve compartment. This greatly decreases the time, and hence loss of altitude, required to fully open the reserve parachute.
The Skyhook system is engineered so that it should not interfere with reserve deployment activated by directly pulling the reserve rip cord in situations where no main parachute had been deployed. The key component in the system, from which the Skyhook derives its name, is a cantilevered hook that grasps the reserve bridle about midway between the reserve pilot chute and the bag containing the packed reserve chute. If the departing main parachute applies more pull force on the bridle than the reserve pilot chute, then the main parachute will remain hooked onto the reserve bridle, and so it will pull the reserve parachute out of the reserve compartment. If the reserve pilot chute exerts more pull force on the bridle than the main parachute, then the main parachute will unhook and the reserve pilot chute will deploy the reserve parachute normally.
The primary advantage of the Skyhook system over traditional RSLs is a higher reserve deployment after a cutaway. More altitude means more time for the skydiver to perform a safe landing, which is especially important because most malfunctions cause a significant loss of altitude. Since the Skyhook relies on a cutaway main, it offers no advantages in situations where the main parachute has not been deployed from the parachute container.
The Skyhook system can also cause complications. One such incident was captured on video, where a spinning main caused a Skyhook to severely twist the reserve. A standard RSL or manual reserve pull would have taken longer to deploy—but a reserve pilot chute would have pulled the reserve freebag upwards, rather than spinning and entangling it. Due to the increased risk of spinning malfunctions, the use of Skyhooks with high-performance canopies remains an active area of discussion.
An important component of a Skyhook system is the Collins Lanyard, which cuts away the left parachute riser in the event of the right parachute riser releasing. This is important because the Skyhook RSL is activated by the right hand side riser releasing.
The Skyhook was developed by the founder of United Parachute Technologies, Bill Booth. It was originally only available on Vector III parachute containers. Now it is being licensed to other manufacturers, Sunpath being the first licensee to market.