The thickness of air: Parachuting from fixed objects

parachuting bjsm

By A Westman, M Rosn, P Berggren, U Bjrnstig

Kindly invited by the Editor to comment on our paper “Parachuting from fixed objects: descriptive study of 106 fatal events in BASE jumping 1981-2006,” we would like to bring the attention of the readership to the thickness of air.

In parachuting jargon, a fall through the atmosphere is often incorrectly described as a “free fall,” distinct from flying an inflated parachute. However, a person starting to passively fall through the lower atmosphere will after approximately nine seconds reach a terminal vertical velocity of circa 200 km/h, i.e. about seven times her maximum running speed. Parachutists in terminal velocity can readily initiate and terminate motion (including rotation) by steering the flows of air around their bodies, e.g. stopping a somersault into a prone position by extending the arms and palms forward, deflecting the airflow like running water. Hence the name for sport parachuting from aircraft: Skydiving.

The vertical velocity range 0-200 km/h is called subterminal. In subterminal velocity, there is less airflow for the parachutist to deflect. A hydrodynamic analogy is the steerageway needed for a nautical vessel to be manoeuvred. Skydivers seldom experience the vacuous subterminal environment, since they usually exit into the horizontal airflow generated by the forward movement of the airplane. However, skydiving from a hot-air balloon may entail even less initial relative airflow than BASE jumping, since balloons move with the winds.

Current fixed object parachuting technology distinguishes between subsets of the subterminal velocity range, with one important division being made after approximately 3 seconds of falling, when a reefing device to retard the parachute opening becomes necessary to reduce deceleration forces, to protect both equipment and parachutist. The prevailing reefing system is called a slider. In the BASE jumping community, a safety concern has been raised regarding high subterminal velocity parachute deployment, i.e. between approximately 3 and 9 seconds of free fall delay from exit, when a slider is necessary but the strong airflow of terminal velocity not yet has been achieved. The concern is that the slider’s interaction with the lines and the momentary reefing of lower surface inflation may yield less consistent heading performance, with increased risk of off-heading opening and subsequent object collision.

Another technological consideration related to expected deployment airspeed is the size of the pilot chute, i.e. the small round parachute that is deployed into the airstream, where it anchors and subsequently extracts the ram-air wing parachute from the container. BASE jumpers commonly use at least four different pilot chute sizes for different deployment airspeeds: The lower the deployment airspeed, the larger the pilot chute, to ensure sufficient drag. However, too big of a pilot chute may cause deformation of the parachute pack job during inflation, with resultant risk of off-heading opening or parachute malfunction.

A varied level of detail available in the material frustrated our efforts to satisfactorily study adverse events related to high subterminal velocity parachute deployment. Future research concerning the seconds when the air thickens may be of value.

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