My friend is a U.S. Army paratrooper. Today, through an unfortunate series of events, he was jerked out of a C-17 traveling at 160 knots by his reserve parachute. First-hand accounts describe it as he was instantly gone.

Since he came through it relatively unscathed, I'm curious to know what level of deceleration he might have experienced.

By unscathed I mean his right side is all bruised to hell from whacking the the hatch on his way out, but he's alive.

  • $\begingroup$ Ask him how long it took... Though he may not necessarily remember.. $\endgroup$ – Manishearth Apr 28 '12 at 7:41
  • $\begingroup$ I've asked and I'm waiting for a reply. However, after the day he's had, he's trashed as he well should be. He had his static line attached which means he traveled between 10 and 30 ft before that pulled taught and deployed his main parachute. $\endgroup$ – Ben Apr 28 '12 at 7:59

The forces experienced by your friend will be the same as if he was free falling at 160 knots and opened his parachute. The fact he was in the plane when the parachute deployed makes no difference, because in both cases he is slowed by the parachute from a high speed relative to the air to whatever the speed of a parachute descent is. The rate of slowing will be determined by how much drag the deploying parachute creates.

According to http://en.wikipedia.org/wiki/Parachuting the forces experienced when a parachute opens are 3-4G. However there are two differences to your friend's situation. Firstly free fall is usually about 120mph and you friend was travelling at 160 knots, so the drag on the parachute, and therefore the decelleration your friend experienced will be higher. Secondly it was your friend's reserve parachute, not the main one, that opened. I don't know how the reserve chute differs from the main one, but it's entirely plausible the drag is different and I'd guess lower.

Because of the variables involved I don't think we can more than guess at the forces experienced by your friend, but the 3-4G of a normal parachute drop is a good starting point. I'm not surprised that your friend's colleagues described him as "instantly gone". I've seen a friend accelerate downwards at 1g when caving (he was on a safety line!) and my recollection is that one moment he was there and the next he was gone. A 4G deceleration would indeed seem virtually instant.

  • $\begingroup$ Very good point. When your friend has been sucked out of an airplane, the layman doesn't think about the vector, just that there was an unexpected sucking out of an airplane. So the lower limit is 3-4G and the upper limit is dependent on human endurance and duration of the event. In other words, shrug your shoulders and say "That must've sucked." Sounds glib, but you put it into perspective. Thank you. $\endgroup$ – Ben Apr 28 '12 at 8:29
  • $\begingroup$ This guy REALLY puts it into perspective: en.wikipedia.org/wiki/John_Stapp. $\endgroup$ – Ben Apr 28 '12 at 9:05

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