Glider harnesses, 4 versus 5-point
Ian Strachan
5-point harnesses to all new gliders during build. This is to provide
more body restraint in the event of hard deceleration or a crash, and
particularly to prevent the body slipping under a four-point harness
(submarining) during deceleration and doing even more damage than the
basic deceleration would cause. My glider certainly has a five point
harness.
Also, during aerobatics good body restraint is essential, and in
negative-G manoeuvres, a 5-point harness is highly desirable if a lot of
aerobatics is to be carried out. In a two-seater there may be issues of
liability if the pupil were to slip out of an ill-adjusted 4-point
harness. Have you ever "fallen out of a loop" (I certainly have), and
if so, what sort of harness would you rather have!
In fact I have been surprised that for the above reasons a 5-point
harness has not been a mandatory fit during build. The extra cost of
the crutch strap would be not discernible if built in from the
beginning. Only last month I was advocating this to Louis Boermans, the
President of OSTIV, the international technical gliding organisation.
However, these ideas have been questioned in a recent article in what I
believe is DG's house newsletter "DG-aktuell" which someone faxed to me.
I think what is said is important enough to spark a discussion on this
newsgroup on this important safety issue.
The article starts off by pointing out that for proper restraint, any
harness has to be properly adjusted. A common error is to over-tighten
the shoulder straps, which pulls the lap strap up. The lap strap is the
one which takes most of the strain in an impact, and should be tight
(but not uncomfortable) and positioned low on the body so that there is
not a gap below it which will allow the body to slip below it in the
event of a severe deceleration.
No disagreement there. But the article then continues:
"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 crutch 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 XX of the Technical Control
Commission of Rheinland 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 Luftfahrt Bundesampt (LBA) examined the situation in more 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:
1. The installation of the attach points in today's modern gliders may
be correct.
2. The correct usage of the restraint system is up to the pilot.
3. The seat belt must be maximally tight and the shoulder belts
considerably less so.
4. The use of 5-point harnesses should be avoided."
END OF QUOTE.
My first-pass opinion is that there is a lot of valuable stuff here, but
regarding 5-point harnesses, the wrong conclusion is drawn from the
evidence.
I have spent many thousands of hours sitting on ejection seats in
military aircraft and it is no co-incidence that they all have 5-point
harnesses.
Outside the ejection seat environment, the crutch strap prevents the
body slipping forward in a high impact situation. The crucial point in
all safety assessments is "balance of risk". In certain weird
situations it can be better not to wear a safety harness at all, but we
all do because in the statistically more likely situations we know that
it will be better for us. When automobile seat belts first came in,
there was a vociferous lobby against them. But what do you do, today?
I recall that one of the British World War One flying aces was killed
because he used to deliberately undo his harness for takeoff and
landing. He was afraid of being trapped in the aircraft if it caught
fire on the ground. The inevitable happened, he had an engine failure
after takeoff, was thrown out of the aircraft on the forced landing, and
died. "Balance of risk", but he got it wrong, sadly.
I suspect the 4 versus 5-point harness situation is similar. If it is
true that a five point harness might give you a certain amount of
soreness in a sensitive area in the event of a crash, it might also save
your life. To put it crudely, would you rather have a sore crutch for a
short while, or be dead, or in a wheelchair for the rest of your life?
Actually I query the "sore crutch" argument anyway, because the 5th
(crutch) strap ensures that the load-carrying lap strap is in the right
position to take the load, and in itself may take little load unless the
crash is severe enough to be fatal anyway. But just think of what will
happen if you do "submarine", not only to your crutch, but your legs and
abdomen ... Doesn't bear thinking about!
The reason I post this is to spark comment from people with more
knowledge in this area than myself. It is also a classic way in which a
newsgroup discussion can contribute to world gliding in a constructive
way. I know that there are several flight surgeons who are also glider
pilots, and I know that glider crash testing has been carried out in the
form of controlled "drop tests" using instrumented cockpits and
manikins. It would be interesting to have inputs from people who have
worked in this area ....
--
Ian Strachan
i...@ukiws.demon.co.uk Bentworth Hall West
Tel: +44 1420 564 195 Bentworth, Alton
Fax: +44 1420 563 140 Hampshire GU34 5LA, ENGLAND
Bob Greenblatt
with have added two straps which anchor behind the pilot. They travel under
and then up around the thigh to attach to the buckle at the same point as
the lap straps. This arrangement serves to stop submarining, and does not
encroach on the "family jewels" making inflight relief (peeing) much easier.
Bob Greenblatt
ba...@csi.com
bobgre...@msn.com
Eric Greenwell
> For many years I have advocated, on safety grounds, the incorporation of
> 5-point harnesses to all new gliders during build. This is to provide
> more body restraint in the event of hard deceleration or a crash, and
> particularly to prevent the body slipping under a four-point harness
> (submarining) during deceleration and doing even more damage than the
> basic deceleration would cause. My glider certainly has a five point
> harness.
When I bought my ASH 26 E, I questioned both Waibel and Heide about the lack of
a 5th strap. Their reply was the steep slope of the seat pan underneath the
thighs prevented the pilot from submarining; in addition, the attach points for
the lap belt had been carefully chosen to keep the lap belt in the optimum
position over the hips. Because of the features, they said a 5th belt (or
"crotch strap" in the US) had no value. I believe the LS gliders have also done
this for many years.
I did use a 5th strap in my ASW 20 C, which did not have such a steeply sloping
seat pan. It kept the lap belt in position well, but was a real nuisance
otherwise.
--
>>Delete the "REMOVE" from my e-mail address to reply by e-mail<<
Eric Greenwell
Philippe Athuil
contact them etc...)
You can leave a message at p_at...@ix.netcom.com
Thank you
Philippe Athuil
Bob Greenblatt wrote in message <72v6ki$n1t$1...@ash.prod.itd.earthlink.net>...
dickmoc...@my-dejanews.com
"Bob Greenblatt" <bobgre...@msn.com> wrote:
> There is another solution - the 6 point harness. I, and several others I fly
> with have added two straps which anchor behind the pilot. They travel under
> and then up around the thigh to attach to the buckle at the same point as
> the lap straps. This arrangement serves to stop submarining, and does not
> encroach on the "family jewels" making inflight relief (peeing) much easier.
>
> Bob Greenblatt
> ba...@csi.com
> bobgre...@msn.com
this sounds like the ticket. Can you give more particulars? Or contact me at
972-435-6271.
Thanks
Dick Mockler
>
-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
Bert Willing
Bob Greenblatt wrote:
> There is another solution - the 6 point harness. I, and several others I fly
> with have added two straps which anchor behind the pilot. They travel under
> and then up around the thigh to attach to the buckle at the same point as
> the lap straps. This arrangement serves to stop submarining, and does not
> encroach on the "family jewels" making inflight relief (peeing) much easier.
We are thinking about the same solution for our Calif. Latch straps on the
Calif are anchored too far behind to be of a big value. Every time when we work
in a rotor, things become lousy ... Our first idea was the 5th belt, but we are
really concerned about injuries on an impact if it is this belt between your
legs which takes your body weight during a 15g+ deceleration - precious parts
will no longer be of interest in this case as you will probably be concerned of
the belt penetrating your brain ...
We also came to the conclusion that the addition of two latch straps, anchored
in a forward position, would solve the problem nicely and insure a well defined
position of the buckle.
I think that this is a typical problem of ships with a 15+ years design.
Nowadays, designers are not only concerned about performance, and they start
thinking about comfort and security right from the beginning of the concept.
And if you do things properly, you will be able to ensure the buckle being in a
defined position even with 4 belts.
--
Bert Willing
Caproni Calif D-6600
Visit the airfield of La Motte du Caire in the French Alps:
http://www.decollage.org/la_motte/
Walter Weir
"behind the pilot" and don't go between the legs they are just lap straps.
If they do go between the legs they are crotch straps.
Can you explain the routing in more detail?
Walter
Bob Greenblatt <bobgre...@msn.com> wrote in article
<72v6ki$n1t$1...@ash.prod.itd.earthlink.net>...
Armand A. Medeiros
experience in the ASK21 (5 point harness) and currently fly a Centrair
101ARW (4-point). My glider is not certified for aerobatics, loops and such.
I tried that once and didn't like it all that much. My preference is to stay
upright.
In the Centrair 101ARW, you sit very supine with the stick very near the
precious parts and your legs upright. The lap belt is at the lowest position
in the seat pan.
In my opinion, I would need to hit something so hard that it is assured the
nose would crush, break legs and such before I could ever possibly slide UP
the seat pan. That would be a very severe crash and probably cause the
fuselage to crush the rear of the cockpit as well. In just about any other
circumstance, I think (!) there is no way for me to slid forward and up out
of my seat pan.
In any case, using your own logic, statistics probably support the use of
either system provided the belt placement and design is correct. The tiny
tiny added statistical benefit to the 5th belt may be entirely seat
pan/cockpit design dependent.
JMHO
Armand
Ian Strachan wrote in message
<snip>
Bob Greenblatt
belt I guess they are indeed crotch straps. However they go over the top of
the thigh and don't interfere with peeing.
Here's the scoop. The belts I used are standard aircraft shoulder harness
belts avail from Sporty's and Wag Aero (among others). The anchor end is
simple web and one of those fold back buckles. I cut some slots in the
bottom of the back of the seat pan so the belt could go back and anchor on
the steel tube structure. The buckle end is a simple slotted metal fitting.
On the Pacific Scientific buckle arrangement, when the buckle releases, the
right waist belt remains attached. This is obviously no good as the new
right thigh belt must attach to this point. (You could step through the belt
to get in to the plae but it wouldn't allow an emergency exit.) Therefore
the buckle has to be
disassembled and the release plate modified. Very simple to do but hard to
describe here. I modified it so that when it releases, the right waist belt
releases, but the right shoulder belt does not. In this way, the buckle
remains attached to the shoulder belt. I've seen some modifications where
the entire buckle comes off. This is OK too as long as you don't misplace
it. The metal fitting
from the waist belts fits through the slot in the thigh belt. The waist
belts then plug normally into the buckle.
Bob Greenblatt
ba...@csi.com
bobgre...@msn.com
Wings
>On the Pacific Scientific buckle arrangement, when the buckle releases, the
>right waist belt remains attached. This is obviously no good as the new
>right thigh belt must attach to this point. (You could step through the
belt
>to get in to the plae but it wouldn't allow an emergency exit.) Therefore
>the buckle has to be
>disassembled and the release plate modified. Very simple to do but hard to
>describe here. I modified it so that when it releases,
Let's be careful here advising modifications.....seatbelt assemblies are
also typically certified parts and therefore cannot be legally modified by
owners or even mechanics without proper authority and then only if STC's or
field approvals are given. (Lap belts will normal have TSO approvals but
shoulder and 5th belts "normally" do not require TSO)
There are other options and methods that can be used as well as 5th belts
offered by the seatbelt manufacturers and approved repair facilities.
As a side note, you might even want to (or maybe you don't want to know)
check the seatbelts you already have and see if they have an expiration date
or if they even still have the approval tags in place....these are required
in normal certificated aircraft..
tim
Visit Our Web Site: http://www.glider.com/wings
Wings
--
Bob Greenblatt <bobgre...@msn.com> wrote in message news:731tbv$kc2
>On the Pacific Scientific buckle arrangement, when the buckle releases, the
>right waist belt remains attached. This is obviously no good as the new
>right thigh belt must attach to this point. (You could step through the
belt
>to get in to the plae but it wouldn't allow an emergency exit.) Therefore
>the buckle has to be
>disassembled and the release plate modified. Very simple to do but hard to
>describe here. I modified it so that when it releases,
Let's be careful here advising modifications.....seatbelt assemblies are
also typically certified parts and therefore cannot be legally modified by
owners or even mechanics without proper authority and then only if STC's or
field approvals are given. (Lap belts will normall have TSO approvals but
Tom or Linda Dixon
this set up for a couple of reasons. In my youth I raced sports cars in the
SCCA and dur to my great skills had a few occasions to test the straps in
hitting something head on and going end over end. In all of these I would
have submarined without the fifth strap. In none of these did I hurt the
jewels as the lap strap was tight and low on my hips. The fifth strap held
the lap belt in place as my body upper moved against the shoulder harness
and would have moved the lap belt out of position. Next, in my LS due to my
height and weight-kind of skinny- I sit a ways back and it is hard to keep
the lap belt tight and low even when I don't tighten the shoulder staps real
tight. Again, i find the fifth belt helps hold the lap belt in place.
Regards: Tom
JNBearden
In article <7302hn$bah$1...@nnrp1.dejanews.com>, dickmoc...@my-dejanews.com
writes:
>In article <72v6ki$n1t$1...@ash.prod.itd.earthlink.net>,
> "Bob Greenblatt" <bobgre...@msn.com> wrote:
>> There is another solution - the 6 point harness. I, and several others I
>fly
>> with have added two straps which anchor behind the pilot. They travel under
>> and then up around the thigh to attach to the buckle at the same point as
>> the lap straps. This arrangement serves to stop submarining, and does not
>> encroach on the "family jewels" making inflight relief (peeing) much
>easier.
>>
>> Bob Greenblatt
>> ba...@csi.com
>> bobgre...@msn.com
>>Bob:
>this sounds like the ticket. Can you give more particulars?
This is a sensitive subject as it involves alterations to, potentially, type
certificated aircraft and parts (as Tim Mara has accurately and properly
noted). And sorry about the length, but here goes.
Speaking hypothetically, if one were to order a set of shoulder harnesses from,
say, Aircraft Spruce or Wag Aero in the US--the kind with the long metal
slotted piece which fits over the metal tang of a conventional powered aircraft
seat belt--and secure the free ends of said shoulder harnesses to points behind
one's body (in the case of Shempp-Hirth by using the steel cross member behind
the seat or on LS types by employing the bolts which attach the forward
mounting points of the landing gear) or simply looped through the seat belt
anchor fittings (in the case of the ASW-24 and similar aircraft), you would
have the basic pieces. Hypothetically, the choice of what type of fitting to
order on the anchor end of the shoulder harnesses (i.e., triangle, belt with
adjuster, etc.) would be governed by how one might contemplate mounting the
harnesses in one's own cockpit.
Proceeding, again hypothetically, if one were to open up the standard German
center buckle (the type which releases four of five points when you twist the
center knob, and which has an American company's name covered by a German
company's stick-on label on the reverse) by prying up the round center button
and removing the socket head, reverse-threaded bolt under it, being careful to
note the position of all the pieces, one would see a pentagonal metal plate.
This plate is the "pusher" which actuates the levers which release the
individual seat, shoulder, and crotch buckle tangs from the center buckle. One
would observe that one side of this pentagon has a notch in it so that one
lever is not actuated when the plate pushes down, such that the center buckle
remains attached to one strap when the center knob is twisted (normally this is
one of the seatbelts).
If, hypothetically speaking, one were to lay this plate on a piece of paper and
trace around it, then flip it over and trace again (which yields a perfect
pentagon WITHOUT the notch), then paste the tracing onto a piece of metal of
similar strength and thickness (very important) and cut around it carefully
(making certain that the critical dimensions are the same as the original
except for the notch), and then replace the original plate with this new one
and reassemble the buckle, one would have a buckle which releases ALL FIVE
points (both shoulder harnesses, both seatbelts, and the crotch strap).
Note: the modification which Bob Greenblatt mentions--i.e., that the buckle
remains attached to one of the shoulder harnesses--is probably a good one.
Without this, the entire buckles comes free in one's hand. On the other hand, a
heavy buckle flailing around on the end of a shoulder harness might be a hazard
in itself so the choice is yours. In any event, the buckle can be freed by
actuating the separate little release on the buckle that allows the shoulder
harnesses to be released without releasing the seatbelts.
Note that this hypothetical modification applies ONLY to the newer type of
buckle where the tangs do not have "ramps" which could catch and bind in the
slotted fittings on the end of the shoulder harnesses, as some earlier buckle
types might.
Now if one were to sit in the cockpit and secure the conventional shoulder
harnesses normally, but thread a newly installed shoulder harness under each
thigh and then back over the lower torso to the center buckle and insert the
tang of the appropriate seatbelt through the matching shoulder harness metal
slotted piece before plugging into the buckle, and then cinch the seatbelts
tight and adjust the thigh straps (i.e., new shoulder harnesses) until they are
almost snug, then one might find that only a small amount of "submarining"
tightens the thigh straps against the inside of each thigh (i.e., on either
side of the sensitive parts of the male anatomy) and prevents further
movement--exactly like the leg straps of a parachute--without having to snug
the thigh straps down initially very much. Hypothetically speaking, one might
discover than the thigh straps themselves are hardly noticed in flight and do
not interfere at all with peeing.
One thing they would NOT do very well, however, is help hold the lap belt down
in the proper position. For this, you are better off with a 5th-point crotch
strap with its attendant risks.
While this is all hypothetical in a sailplane, it is common practice in race
cars and I know of at least one sailplane which was retrofitted with a standard
race car seatbelt/shoulder harness/thigh strap combination. Whose owner gave me
some great ideas but shall remain nameless.
I also know of at least one person has has gone to the trouble of sourcing
laser cut replacement plates for the German/American buckle who, if reading
this post, might wish to pass on the information. Otherwise, it's not much work
to fashion such a plate oneself. Speaking hypothetically, of course.
Unfortunately, liability being what it is in the world of aviation, I
understand that the American company whose name appears on the buckle will not
deal directly with individuals, even if such a replacement part were available
from them.
I, too, understand that the current Schleicher cockpits are designed to prevent
submarining without the necessity of 5- or 6-point harnesses, but for some
pilots the extra peace of mind is worth something. And the cost--in terms of
money, risk, and complication--seems minimal. Ex the legal issues, of course.
And for those who question whether they would, indeed, "submarine" under the
lap belt of, say, an ASW-20 type of cockpit, I have an autopsy report that
indicates otherwise and discusses the consequences thereof (crushed chest is
mentioned). And for those who are concerned about the injury involved with
submarining into a crotch strap in a crash, I have personally seen the
consequences of that: the attachment point of the 5th-point strap broke. I have
heard second-hand that at least one sailplane designer was dubious of the
crashworthiness of such a strap for precisely this reason, that the point where
the two fuselage halves are joined--where typically the belt is anchored--is a
weak point compared to other areas. So you might say that a crotch strap
potentially offers the worst of all worlds: pain, but no gain. On the other
hand, it's probably the best thing for rough ridge flying.
Be careful no matter what you do. I think the discussion of how to make
sailplanes more survivable is a healthy one, however, and I once again offer my
thanks to Gerhard Waibel for his advanced work in this area. At the time I
bought my ASW-24, this was a major factor in my decision.
Chip Bearden
ASW-24 "JB"
PAPA3
since been exhausted, and I no longer have access to the laser cutter.
However, using the sketching method described by Bob, you should be able to
take your sketch to any of the myriad engineer types who seem to populate
gliderports and get them to create a drawing. Then, see if you can find a
machine shop with a CNC laser cutter and voilla. The drawback is that the
cost is somewhat prohibitive under normal circumstances. In addition, it
was difficult to find the exact thickness of blank stock - I ended up
having to sand down the plate by hand in the end... It's probably just as
easy to cut, file, and drill manually.
Although there may be a liability issue in the aviation application of said
solution, I would have no qualms about trying to dig up the CAD drawings
for this if potential requesters would assure me that this was only to be
used on their airport golf-carts and other ground-bound, low speed
conveyances. Hypothetically assuming I had these drawings, that is . . .
Erik Mann
LS-4 (Experimental) "P3"
JNBearden <jnbe...@aol.com> wrote in article
<19981119203658...@ngol03.aol.com>...
sis...@my-dejanews.com
waist strap better for inverted flight. We found that snugging the crotch
strap then rolling inverted was painful and unnecessary, and we always left
the crotch strap connected but loose after that. The waist straps were
located well enough down the hips to be effective at 3.5 negative G's. The
fifth point we felt was just for submarining in a crash. Galen Fisher,
Sailplane Enterprises
Ian Strachan
Saundby, who has given me permission to relay the following opinion on
harneeses, which I quote below.
Peter is an ex RAF Flight Surgeon and qualified RAF pilot ("Flying
Doctors", we used to call them), and was the "Medical Officer Pilot" at
the UK test flying base at Boscombe Down for several years. He is a
keen glider pilot, instructor, current member of the BGA Executive
Committee, and is a member of the FAI Medical Commission. I am trying
to get him to come on to this newsgroup so that he can correspond
direct, but meanwhile here is what he has said to me on this issue:
"Extensive research papers exist on harness design and restraint,
starting with the classic work by Col. Stapp in the USA using rocket
propelled sled tracks. While restraint is primarily provided by the lap
strap, the function of the [fifth] crutch strap is to stabilise the lap
strap and oppose the pull of the shoulder harness. Examine the usual
four point harness when fitted to a pilot and the lap strap will not be
straight, being pulled upwards by the shoulder harness. In a crash there
is often a flexing distortion of the cockpit area, extending the
shoulder harness, and then under deceleration there is space for the
body to slide forward below the loose lap strap. Even without cockpit
distortion, there will be some extension of the shoulder harness under
load. With a properly fitted five point harness, the lap strap should
follow a straight line and provide maximum restraint.
I am aware of the high loads measured in a crutch strap by Martin
Sperber, but disagree with his conclusion. He implies that it is direct
contact with the perineal region of the dummy which is responsible for
the load, but I would have thought that this force arises from the mass
of the dummy tensioning the shoulder harness. If the force applied by
the shoulder harness is not opposed, it must result in an upward
distortion of the lap strap which will impair restraint.
Pilots worry that a crutch strap will injure their private parts, in
fact injuries are not found here with five point harnesses, even
following military fatal accidents when the forces have been very large.
But if pilots submarine below the lap harness, the control column may do
the damage that they fear.
Regards,
Dr. Peter Saundby
Medical Adviser British Gliding Association
END OF QUOTE
If anyone wishes to correspond directly, he is on email at
peter....@virgin.net
Bob Greenblatt
I completely concur.
I also wish to thank Chip for his excellent hypothetical description of how
one might consider such modifications.
As mentioned when using only 6 points, submarining is probably
(hypothetically) eliminated, but vertical motion such as that encountered in
extreme turbulence as in ridge flying is still possible. I would also like
to point out that with the hypothetical modifications Chip described, the
original fifth crotch strap can also be used making a 7 point harness.
Again as Chip mentioned with the buckle now attached to the right shoulder
harness it may flail around a bit. It can now extend out of the cockpit area
and bang on the fuselage side or the canopy if it is hinged on the side. I
have found that being careful getting in and out to avoid this is not a
problem; and, in my case far superior to getting settled in only to find
that the buckle is not attached to anything and is probably under or behind
me.
Bob Greenblatt
ba...@csi.com
bobgre...@msn.com
JNBearden
Bob Greenblatt's idea of using the crotch strap as a 7th point for ridge flying
and turbulence is a good one. Frankly, I didn't have the nerve to suggest a
SEVEN-POINT harness but I'm glad he did. If you want to have all your bets
covered, this is the way. The crotch strap should help retain the seatbelt in
the proper, low position while the thigh straps should help prevent the
submarining (which, in addition to crushing the chest, might, in conjunction
with the crotch strap, otherwise threaten the male pilot's private parts). So
the only negative I'm aware of is that the crotch strap is not as convenient as
thigh straps for peeing in flight.
Other points: In the Ventus I observed where the owner had installed a set of
race car harnesses (and which served as the model for my hypothetical example),
small slots had been cut out of the seat pan or seat back (can't recall which)
so that the thigh straps could be threaded through and looped around a steel
cross member that was positioned behind the pilot's lower back. The pilot
actually sat on the thigh straps when in the cockpit.
As I understand it (and I am not an expert), thigh straps serve only one
function and that is to restrain the pilot's body from moving forward in a
crash. Hence, the anchor points must be far enough back so that a longitudinal
restraining force can be applied by the straps in the event of an impact on the
nose of the sailplane. That point can be as far back as the tail of the
aircraft (to be extreme) but not so far forward that the straps fail to loop
around each thigh. Somewhere around the seatbelt anchor points is probably the
forward limit, but again I leave it to the experts.
Finally, anyone contemplating a modification to their seatbelt buckle as
discussed in my hypothetical example should be aware not only of the legal
issues, but of the practical ones as well. In particular, the thickness of the
"pusher" plate is, from a technical viewpoint, somewhat critical. If it is too
thick, it could result in pressure being applied to the release levers all the
time, leading to possible premature release of the seatbelts and/or shoulder
harness during a hard load. If it is too thin, the levers might not fully
release, with potentially embarrassing or even disastrous results. I imagine
that this would be an easy thing to check when installing a new plate if one
were inclined to do that sort of thing.
Chip Bearden
ASW-24 "JB"
Ian Strachan
regulations, including Dr Tony Segal who contributed to OSTIV
regulations and is referenced in them.
The Organisation Scientifique et Technicale de Vol a Voile (OSTIV) and
the European Joint Aviation Authorities (JAA) are key in this field, and
provide design standards. OSTIVARS are voluntary whereas JARs are
mandatory on JAA nations (which include all European nations plus some
others, the US FAA is an observer at JAA an has expressed a long-term
intention to bring world airworthiness standards into line).
In OSTIVARS (see below) there is an alternative of a crutch strap
(5-point harness) or straps (6-point) OR a specially-shaped seat pan
where the hips are lower than the seat pan slope forward of the hips.
So-called "submarining" is addressed, so people have thought about it.
I was flying an aircraft today with a 4-point harness and was very
conscious of the correct (ie, low) positioning of the lap strap. On
long soaring flights in non-turbulent conditions many pilots tend to
allow their straps to become loose (as do I). This brings into focus
the necessity of tightening the straps before landing, but how many
pilots do this on every flight? This is, in my opinion, a great
advantage of the fifth strap which, even if all the straps are
comfortably loose, will give you much better protection with only four
straps in a similar situation. Even better if the straps are tight, as
they should be for landing.
OSTIV Airworthiness Standards (OSTIVARS) state the following:
EXTRACT ABOUT PILOT PROTECTION
3.7.5.2 The inertia forces under which there must be a "high
probability of each occupants escaping serious injury" are 7.5G upward,
15G forward, 6G sideward, 9G downward.
4.2 Pilot Protection. Serious accidents often result in spinal
injuries, usually as in 4.21, 4.22 and 4.23 below, and may involve
submarining, as in 4.24 below.
4.21 Acute flexion injury of the neck by the latter being forced to
bend forwards and then backwards. For protection against this injury
there is now a standard for the provision of headrests.
4.22 Flexion injury of the upper part of the spine, by the latter
being forced to bend forwards. For protection against this injury the
shoulder-strap connection anchorage points should be located in a
sector, centred on the top of the shoulders extending from the
horizontal to 15 degrees below. Also lateral separation of the straps
should be between 150 mm and 200 mm at the top of the shoulders.
4.23 Longitudinal compression of the lower spine. For protection
against this injury, seatbacks should be shaped so as to maintain the
normal curvature of lumbar region of the spine. Rectangular parachutes
cavities should be avoided. In existing sailplanes the use of suitably
shaped spinal supports is recommended., such supports should be made of
suitable firm (not rigid) material. The use of flexible parachute packs
having soft lower edges fully supporting the spine is also helpful.
Airfilled bladders should not be used.
4.24 Submarining. This means the occupant sliding downward and
forward through the harness. Protection against this may be achieved by
the use of (a) or (b) below.
(a) Crotch strap or straps.
(b)(1) Suitably shaped seat pans, in which the forward part under the
occupant's thighs is adequately inclined, and the transition radius
between the forward part and the part under the occupant's hips does not
exceed approximately 15 mm.
(b)(2) Lap-strap anchorage points located significantly below the H
point (hip point, which is the intersection of the centre lines of the
torso and hips) in a sector centred on the latter extending from the
vertical to 20 degrees aft.
Lit. Segal A M: Protection from spinal injury.
Technical Soaring Vol 12 No 4
Sperber M: Restraint system in gliders under bio-medical
aspects.
Technical Soaring Vol 13 No 2
PS: Has anyone the email address of Martin Sperber? We really ought tpo
bring him into this thread, as his name is mentioned above and also by
Dr Peter Saundby in his expert contribution. Tony S is not on the
Internet, unfortunately, although I talked to him at Lasham yesterday.
Wings
--
Visit Our Web Site: http://www.glider.com/wings
Todd Pattist <pat...@DONTSPAMME.worldnet.att.net> wrote in message
>
>Chip - Is this "slotted piece" designed to connect with the
>waist belts on the "standard German center buckle (the type
>which releases four of five points when you twist the center
>knob, and which has an American company's name covered by a
>German company's stick-on label on the reverse)".
this would be a "Pacific Scientific" Buckle covered by a "Gadringer" logo
decal
>One other comment - My Ventus C attaches the waist belts and
>the crotch strap to the seatpan! They lift out with the pan
>when I remove it to access below. This has always bothered
>the heck out of me, and I wouldn't mind getting something
>like the U-turn thigh straps running back to a more solid
>attachment point, like the steel crassbars behind me, but
>undertaking changes in the restraint system is *not*
>something one undertakes lightly.
Ian Strachan
following I had from him may be of interest:
"The arrangement of the six point harness described is similar to that
which existed on the combined parachute and seat harness of the Mk 4
Martin Baker (ejection) seat. In that seat a loop attached to the side
location point passed under the thigh and [indirectly] took the upward
load on the lap strap. The lap strap passed through the bottom loop and
then entered the Quick Release Box (QRB).
In a 'six point' harness, the lap strap stabilisation is indirect and
less effective because of compression of the thighs, but there are
normally only four attachment points because two straps can share each
side attachment; and no structural change or re-stressing is required.
This design is less effective in providing negative 'G' restraint.
Regards, Peter."
JNBearden
Todd,
You write:
>I wonder whether the waist strap metal pieces (their tangs
>and attachments to the buckle) are designed to take the
>forces from the direction that would be applied by the
>slotted metal piece from the thigh straps. It seems
>possible that there would be a large forward load applied to
>the waist strap tangs by the thigh straps in a direction
>tangential to the buckle that the buckle and thigh straps
>might not be designed for. I also see the possibiity of
>some twisting forces on these tangs that might not be
>possible with the original 5 point attachments.
Good questions. On the installation that you describe, I think it would be more
of an issue for the buckle/seatbelt tangs than the shoulder harness/thigh
straps but obviously one should do a proper engineering analysis. I think the
biggest potential problem would be the buckle releasing unexpectedly on crash
impact due to deformation. I suppose there might also be the danger that the
buckle would not release properly after a crash but that wouldn't seem to be as
much of a problem.
I did have a hypothetical discussion with a parachute rigger about making a set
of thigh straps from a single piece of belt/webbing which would run from the
right anchor point under the right thigh, back over the right thigh and through
the slot in a conventional crotch strap fitting/tang, then back down and under
the left thigh to the left anchor point. This way the loads would be
transferred into the buckle assembly exclusively by the crotch strap for which,
one might assume, the assembly has been designed. And, of course, no
modification of the buckle would be required. The only real negative (apart
from the regulatory issue) is that this arrangement would be somewhat less
convenient for peeing. Not as bad as the crotch strap, of course. Adjustment
would not be as convenient but this is not a problem in one-owner sailplanes as
I understand that the thigh straps need adjustment/tightening very
infrequently.
>How do race car 6 point attachments work? Are there 6
>independent attachments to the buckle, or 4?
On the installation I saw, the thigh straps ended in loops of webbing through
which were passed the right and left halves of a conventional seatbelt (i.e., 4
attachments to the buckle). I did NOT like this approach because the buckle on
one side was prone to catching on the webbing loop. This is not, apparently,
much of a problem with race cars where rapid exit from the cockpit during a
spinout is never required, but our situation is much different. I have also
seen photographs of other types of harness systems where all six points seem to
come together in metal-to-metal fittings (i.e., 6 attachments). But I'm not an
expert. Perhaps someone else could comment.
>One other comment - My Ventus C attaches the waist belts and
>the crotch strap to the seatpan! They lift out with the pan
>when I remove it to access below. This has always bothered
>the heck out of me, and I wouldn't mind getting something
>like the U-turn thigh straps running back to a more solid
>attachment point, like the steel crassbars behind me, but
>undertaking changes in the restraint system is *not*
>something one undertakes lightly.
I know of at least one Shempp-Hirth driver who has added multiple layers of
Kevlar to the seat pan for exactly this reason. Please don't take this as
criticism, however. I'm not an expert in this area and I assume that this
design has met the necessary engineering tests and regulatory standards. In the
crash I referred to earlier (involving a Ventus A), the crotch strap anchoring
point broke (I recall that it went through the seat pan to a glassed-in anchor
on the bottom of the fuselage, or to a bulkhead) but the seatbelt attachment
points did not fail despite the fact that the impact was said to have been
about 45 degrees nose down.
And yes, undertaking changes to the restraint system is NOT to be taken
lightly, as I hope I have made clear.
Chip Bearden
ASW-24 "JB"
Ian Strachan
<pat...@DONTSPAMME.worldnet.att.net> writes
>>In the
>>crash I referred to earlier (involving a Ventus A), the crotch strap anchoring
>>point broke (I recall that it went through the seat pan to a glassed-in anchor
>>on the bottom of the fuselage, or to a bulkhead) but the seatbelt attachment
>>points did not fail
In a crash situation, if an attachment "fails", this may be inevitable
as part of the geometry of the crash and the direction of the loads. It
may be no fault of the design. In other load geometries and magnitudes,
the attachment will no doubt be intact afterwards.
In the case in question, in the process of "failing", the strap may have
taken body load, and fulfilled a useful purpose in the early time
history of the impact before the attachment point crumbled.
So called "graceful degradation" involves progressive but controlled
failures of structure with a view to preventing sudden disaster.
Gapagod
legs. Not the nicest though. I'd rather it hold for the duration of the whole
incident and have the cockpit walls absorb the energy.