This may sound like a dumb question but it’s meant seriously!
The cockpit of a glider obviously does not offer much protection in case of an accident. Almost every auto has a bigger and stronger “crush zone” that can absorb some of the impact energy. luckily, this is partially compensated for by our use of a very good restraint system :
our 4 point or even a 5 point harness.
However this harness must be:
- correctly installed
- correctly constructed
- correctly used
in order to offer the maximum protection.
The Technical Control Commission of Rhineland (“TUEV Rhineland”) has examined this question extensively. Martin Sperber of the Air Transport Section has given a very informative talk on this subject, part of which is repeated below:
The more important half of the restraint system is the hip belt. It has to absorb most of the energy because most of the weight of the body lies between the lower border of the rib cage and the feet. The attach point of the hip belt has to be chosen such that the belt makes an 80 degree angle from a point over the center of gravity of the body backwards and upwards to the attach point. A person’s center of gravity is approximately half way between the hip joints. Many mistakes in belt placement were made in earlier gliders with respect to this simple fact because no one knew better. By intuition (or luck) the placement of seat belts in DG gliders has been correct from the beginning. It is also true that the seat pans of our gliders have a relatively steep angle for the thighs. This is very comfortable and reduces the tendency to slide under the hip belt (“submarine”) during a crash.
The attachment of the shoulder straps had to be improved a few years ago with respect to their horizontal separation. This must be chosen such that it doesn’t rub on the pilot’s neck but at the same time it can’t be too wide. If the shoulder belts are far apart it is comfortable but the pilot runs the risk of the straps slipping down off the shoulders and losing their effectiveness.
It’s hard to believe some of the incorrectly constructed belts on the market. Certainly the harness should not gradually become looser during flight as do belts from some manufacturers. The belt material used in such cases is simply too smooth. The seat belt must be able to be tightened by pulling up. In some types, one has to pull down between the thigh and the cockpit wall where there is little room and it is awkward to apply sufficient force.
Proper tightening of the straps:
Pull the seat belt as tightly as possible!
It holds your weight in inverted flight and more than your weight in turbulence. And the tightness protects you during a crash from sliding under the seat belt.
Tighten the shoulder straps considerably less.
Never pull the shoulder straps so tight that the seat belt is pulled up over the abdominal wall.
In this respect, the same mistake can often be seen: the shoulder straps are easier to pull on and they are pulled as tightly as the seat belt. This pulls the seat belt up over the soft parts or the belly. A pilot thus “protected” looks OK after a crash but may die a few hours later due to internal bleeding.
The researchers from the TUEV Rhineland asked the pilots at a large contest whether they had any problems with fastening their seat harnesses. Eighty percent of the pilots said they had no problems. Then they went along the start grid and found that the 20 % of pilots “with problems” were generally correctly belted in their seats. The other 80% sat comfortably in their cockpits but incorrectly buckled in with the central fastener up under the lower border of the rib cage. That can have bad consequences in a crash!
The 5-point Harness:
Apparently, many of the problems covered here could not happen with a 5-point harness. It always sits at the optimal place over the hips without sliding up. “Submarining” is impossible – seemingly a perfect construction. Then the force on the crotch strap was measured during a (simulated) crash and a load of 1800 pounds was found!!! Gentlemen, we (and our wives) will not stand for that on our “most precious parts”!
This leads to the startling conclusion by Martin Sperber of the TUEV Rhineland that, with respect to safety, the 5-point harness should not be used.
Naturally, in acrobatic flight, the 5-point harness offers more comfort because it prevents sliding to an unwanted position. One of the members of the German “Luftfahrt-Bundesamt” (FAA) examined the situation in detail during his free time. He made several acro flights with 4- and 5-point harnesses.
Result: a correctly installed and fastened 4-point harness gave equal restraint and protection as a 5-point harness.
In the case of a crash, the 4-point harness can also protect you from internal injuries.
Conclusions:
The installation of the attach points in all of today’s modern gliders may be correct.
The correct usage of the restraint system is up to the pilot. The seat belt must be maximally tight and the shoulder belts considerably less so.
P.S.: More discussion of this subject can be found in our article: “Safety Details.”
Another Trial
There was a long discussion in the newsgroup rec.aviation.soaring about the advantages of 4-point or 5-point harnesses.
Of course I wanted to know the different opinions and had come to the conclusion that most of the readers wanted to have the possibility of using a 5-harness.
During my recent visit to the factory in Bruchsal I discussed with my engineers the topic and made the suggestion, that we build an attach point for the fifth strap in the bottom of the glider.
I had another long conversation by phone with Martin Sperber of the “Technical Supervisor Administration” in Cologne. Some years ago he investigated many accidents which happened with sailplanes with built in 4-point and 5-point harnesses.
The results were very clear:
In contrast to Acrobatic Aircraft (like (the EXTRA 300 and so on) a 5-point harness cannot be recommended in sailplanes. When the fifth strap functions at all, it means that the pilot did not fix the other 4 harnesses correctly, and then there is a lot tension on the fifth strap, which can injure the pilot badly. To offer a 5-point harness by a manufacturer could result in a case of product liability – especially in the USA.
Sorry for that conclusion, but it is really true.
– friedel weber –
We do not want to force anybody, to agree with our statements.
That is why we publish another opinion here:
Betreff: Five point harnesses in gliders.
Von: Peter Saundby
Dear Mr. Weber
While it is true that a properly adjusted four point harness can give
protection up to + 40G at 250G/Sec for 0.2 Sec (Stapp 1961), it has
been generally accepted that a five point harness is more effective and
is less vulnerable to careless adjustment.
I have read the attachment from DG and remain unconvinced.
An important point is that although this paper correctly states that the
center of gravity of the average human body is between the hips, a body
under load does not behave as a rigid entity with forces acting though
the C/G. Humans are non rigid structures and the component parts will
follow their own paths within the limits of flexibility and strength,
rather like the links of a chain. In a crash deceleration, the upper
torso will swing forward on the fixed hips and impose loads on the
shoulder harness which in turn can distort the lap strap. I repeat that
the prime function of the fifth crutch strap is to stabilize the lap
strap and not to provide direct restraint. The high loads measured in
this strap arise from opposing the forces exerted by the mass of the
upper torso on the shoulder harness.
All this is not simply my personal opinion, I copy an extract from the
current leading textbook [Aviation Medicine, 2nd Edition 1994, Ernsting
& King, ISBN 0 7506 2275 X; page 171]
“Double lap and shoulder harness with a negative G strap (‘Five-point harness’)
The simple four point harness is much improved by the addition of a
negative G strap (figure 12.4(b)). This strap, also known as ‘lap belt,
tie down strap’ or ‘harness stabilizing strap’ rises from the seat in
the mid line between the legs to join the harness at a central quick
release point. It prevents distortion of the harness by forces imposed
on the torso and is extremely effective during aerobatics and aircraft
maneuvers that involve negative G, during vertical vibration in high
speed low level flight and under crash impact.”
See also the following references:
Aircraft Crash Survival Design Guide (1980) US Army Research and
Technical Laboratories USARTL-TR-79-22
Haley JL, (1982) Impact injury caused by Linear Acceleration:
Mechanisms, prevention and cost. AGARD/NATO Conference Report CP-322.1-5.
Von Guirke HE, & Brinkley JW, (1978) Impact Accelerations. In
Foundations of Space Biology and Medicine. Vol 11 Book 1 Chap 6. NASA.
Wilhelm Dirks has stated that “a 5-point harness is very dangerous”.
Because this is contrary to current text book statements and accepted
opinion in other countries, his evidence should be published as a
scientific paper in a refereed journal. If Martin Sperber has evidence
from accident investigation that contradicts accepted wisdom, that also
should be published as a scientific paper. Then the whole question would
be considered in a balanced manner, and at a later date, amendments
could be incorporated into JAR 22.
What is unacceptable is an emotional argument, not based on any
published evidence, intended to frighten pilots about vulnerable
components of their anatomy.
Any of this may be copied or repeated at your discretion.
Regards, Peter.
Please allow another comment from us:
Our knowledge mainly came from Martin Sperber and his investigations.
He says that at any case you must not compare the circumstances in Military Aircraft as well as in Acrobatic Aircraft with ours in sailplanes. The sitting position is completely other because we are not sitting on a “kitchen chair” but like in a “deck chair”. So the forces are quite different and our abdomen is very near to the 5th strap.
Nevertheless it is worth to consider again.
And what is to do for the manufacturer?
I personally would let decide our customers and would offer the 5-point-harnesses as an option.
But the last decision is for the Chief Designer!
A further comment to the seat belt material
Glaser-Dirks at one time used seat belts made of very smooth material which had a tendency to loosen up during flight. The belts we use today, made by Schroth do not have this problem. Belts are good for a limited number of years in any case.