Eric, from a pevious posting on r.a.s. I turned up the following:

Title of paper:
Four and Five Point Glider Seat Harnesses - Static and Dynamic Tests

Authors
Dr Anthony M Segal, Lasham Gliding Society, UK
Leslie P Neil, Senior Engineer Impact Protection, UK DERA
Graham A Reece, Instrumentation Engineer, UK DERA
Philip G Murtha, Impact Test Track Engineer, UK DERA
(DERA = UK Defence Evaluation and Research Agency)

This paper was presented to the gliding scientific organisation
OSTIV during their meeting at Bayreuth, Germany, in August 1999
and no doubt will be available in full from OSTIV
(OSTIV = Organisation Scientifique et Technique Internationale de
Vol a Voile).

The paper is 20 pages long but para 5b is worth quoting in full
(QRF = Quick Release Fastening, that is, the harness release box)::

"5 b) Observations After the Impact Test
When a 5 point harness was being tested, both with the harness tight
and with the harness slack, the lap straps remained in the
correct position over the hip bones. The QRF also stayed
in the correct position.

When a 4 point harness was being tested, both with the harness tight
and with the harness lose, the lap straps were seen to have moved up
over the abdomen until they were jammed tightly under the lower rib
margin.
The QRF had moved upwards until it was in the epigastrium
(the "pit of the stomach"). This is very serious, because severe,
even fatal injury may be caused to the internal organs in the upper
abdomen.

This is considered to be the most important finding of the entire
test.

Following the impact test, with a 4 point harness, the shoulder straps
were seen to be hanging loosely between the seat back and the pilot
dummy's shoulders. This was due to the upward movement of the
lap straps and the QRF."

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: "The 5th strap should be fitted to new gliders
and be retro-fitted (where structurally feasible) to gliders
in current use".

-------- end of quotes --------

Hope this helps

Ian Strachan