One or Two Reserves for paragliders?

We’ve covered reserves and throwing on many of the Mayhem Podcasts, but they came up specifically in the podcast with Russ Ogden and Tom Payne. One of our listeners, Daniel Schmitt broke it down really well and I wanted to share his thoughts on this incredibly important safety subject (thanks Daniel!):

If I remember correctly Russ in Episode 31- Russ Ogden, a Masters Class in Paragliding
said something like “You should fly with two reserves since it doubles your chances.”

In Episode 34- Tom Payne and Insights into our world, Tom mentioned something like: “Wait a minute, if the first one did not open you would probably be in so much trouble (rotation, fallen into the sail, etc.)  the chances are lower that the second one will not open either.”

Tom’s point is very valid but what does it mean to “double your chances anyway”? Maybe it seems obvious at first glance but it really is not. Let me explain:

Let us say, the probability that your reserve opens if you throw it, P(open), is 95% and the probability that it does not open is then P(not_open) = 5%. If your chance of the thing opening is 95% one cannot double (probabilities above 1 or 100% are not probabilities anymore in a mathematical sense).

If the second reserve throw was independent (like throwing it on a different day with no previous trouble) the probability that both of them do not open was P(not_open) * P(not_open) = (5%)^2 = 0.25%

But of course Tom has a point so maybe let us assume that the probability of the second reserve opening conditional that the first one did not, P(open|notopen) was 50% instead of P(open)=95%. In this case P(notopen|notopen) = 1 – P(open|notopen) = 50% and the probability of being fucked carrying two reserves and both of them failing, P(notopen)*P(notopen|notopen) = 2.5%.

I would argue that intuitively most people would agree that Russ intuitively meant something like that: “Half the chance of no reserve opening” with two reserves compared to one. Of course, if you think that P(open|notopen) is much lower than 50% carrying a second reserve seems less and less attractive.

I made a spreadsheet where everybody can play with the numbers/probabilities:

An excellent article on when to throw and how can be found here on Flybubble paragliding. WHEN IN DOUBT- GET IT OUT!!! Make sure to listen to Episode 58 with Acro master Theo De Blic which covers throwing in detail.

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