In article ,
says...

Finally, the conclusion includes the following words:

"A 5-point harness is superior to a 4-point harness in an accident
impact situation and also under conditions of negative-g. This is
especially so if the harness is slack."

It also goes on to say: "It is recommended that the 5th strap be
designed to avoid injury to the crotch of the pilot. This design of
strap should be fitted to new gliders and be retro-fitted (where
structurally feasible) to gliders in current use".

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The above seems pretty conclusive to me and in my syndicate glider we
had a fifth (crotch) strap fitted in the rear cockpit. We were told
that in the front cockpit there was not a proper hard point to which a
fifth strap could be fixed. Maybe with some minor structural
strengthening it could be, this thread has reminded me that we ought to
investigate it.

Tony Segal published a later article, which concluded six point
harnesses were superior to five point harnesses. It was published in
S&G, but I'm not were I can look at the article for the date.

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Eric Greenwell USA