Nimbus 4 Accident

Roy Bourgeois a écrit :
My my own experience (32 years soaring , 2000 hrs, active
CFI-G) leads me to doubt that the pilot reaches for the dive brake while
plummeting down in a dive. More likely, he over stresses by a too dramatic
pull out during which an un commanded dive brake pop out occurs that
destroys the wing. Because of the location of the dive brakes on the
Nimbus 2 and 4 (they are quite far inboard) - this creates loads that would
not be found on a 15m ship pulling the same G load.

It is not unlikely that speed be very close to VNE increasing and in
such a case most pilots would extend the airbrakes if not all. That's
what airbrakes are for.

The wisest manouever in such case would be to extend the airbrakes as
soon as the glider is nose down, that would avoid the need for too much
g-load on recovery.

Note that in the case of an uncommanded pop out the airbrakes do *not*
increase the loads on the wing because they *decrease* the g-load - only
for the same g-load factor the wing root bending stress is increased.
Therefore, if there is a pop-up and the pilot does not pull more to get
the same g-load again, there should be no increased risk.

--
Denis

R. Parce que ça rompt le cours normal de la conversation !!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la question ?

I have never flown a glider of more than about 20 metres span (ASW 17,
Jantar 2).

However, I am in the process of asking for other pilots' experience.

One such pilot's reply includes the following:

From your experience of big Nimbi do you think there is merit in Stan's
ideas?

"Only to the extent that you fly a big glider like a big glider, not like
a Spitfire. You adjust all your flying to the fact that you are flying
a big ship. For instance to adjusting your thermalling technique to
gradual bank angle changes rather than spirited use of controls to roll
in and out of thermals.

"I have always said about large span gliders, of whatever make, that you
regard them like flying a 747 Jumbo. Not like an agile 15m glider.

"For instance, in a very turbulent thermal such as what you often get
over power stations, I often am reluctant to put on over about 15
degrees of bank. Yes, fifteen degrees. I do not wish to be sucked in
to a "loss of control" situation like Ivans and Engen or the Levers.

Which particular brand of Nimbus do you fly?

"4DM. I am entirely happy with it but I fly it like the big glider that
it is."

I would like to add the following:

On the question of use of air-brakes, it would appear that they were not
opened in the case of the Spanish accident.

In the case of the Minden accident, there was correspondence on Rec.
Aviation Soaring when the report was published by others who had experience
of the inadvertent deployment of the brakes at high speed in turbulence;
the suggestion was that deployment may have been uncommanded.

In addition, Dick Johnson posted that with the Minden accident there may
have been a problem with the oxygen system undiscoverable after the crash,
such as oxygen tanks filled with Nitrogen (not entirely unknown I am
afraid).

It is also notable that whereas there seems to have been no attempt to bale
out with the Minden accident, with the Spanish accident both pilots did bale
out and deploy there parachutes, but unfortunately one of them had his
canopy foul of the wreckage which brought him down with it.

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.


"Bill" wrote in message
oups.com...

I posted the message below on the thread, "Nimbus 4DT accident 31 July
2000 in Spain." I am posting it again for better visibility.

Stan Hall presented his analysis of the Nimbus-4DB accident in Minden,
NV, 1999. Stan's analysis is scary, to say the least.

The article, Probing for the Smoking Gun, was reprinted in the Soaring
Association of Canada's free flight, 2/04. Go to the link below. Click
on free flight on the side bar. Go to free flight back issues - 2004 -
issue 2. Down load the PDF file.

http://www.sac.ca/

Bill Feldbaumer 09

In the case of the Minden accident, there was correspondence on Rec.
Aviation Soaring when the report was published by others who had
experience
of the inadvertent deployment of the brakes at high speed in turbulence;
the suggestion was that deployment may have been uncommanded.

I had an unintentional airbrake opening on my ASW24.
I asked other ASW 24 owners on our Yahoo group if anybody experienced the
same thing. Apparently I was the only one, so far.
Very scary indeed. I installed a lock to prevent it from happening again.
The flight was through many vertical gust for many miles.
On one very powerful one it let go.
Regards
Udo

I think we have to accept that there are occasions
when the finding of an accident report should be 'I
don't know but........'
In the case of the Nimbus 4 accident(s) there are so
many factors which 'may' have occurred that a definitive
conclusion is almost impossible, the best that anyone
can do is speculate, and this applies to most glider
accidents where there are no living witnesses. It is
very easy to attribute the cause to human factors and
in many cases this may be true but there is always
the possibility that there is an unknown factor. What
I have taken from these reports is that it would appear
that there is a point at which a situation becomes
irrecoverable and this point maybe reached quicker
in big wing gliders thatn in small ones and the response
must be to act to prevent the situation ever arising.
To me this means that care should be taken to never
spin or enter a spiral dive which may mean smaller
bank angles and higher airspeed in turbulent conditions
even if this means that some performance is lost.
There are plenty of examples of problems with aircraft
getting into irrecoverable situations, The DH Chipmunk
was just one such example. Modern aircraft, designed
for performance at the expense of handling such as
the SEPECAT Jaguar are know to be almost irrecoverable
if they depart from flight. In the case of the Jaguar
this was discovered during testing and the pilot had
the option of departing the aircraft, survived and
was able to tell the story. While not wishing to digress
the discussion too much we have a similar situation
involving the Puchaz where there have been several
fatal spin ins. It has been found in most cases that
the pilot must have mishandled the controls and this
may indeed be the case however this has to be speculative.
It is not beyond the realms of possibility that there
are some loading conditions and other circumstances
which make recovery from the spin impossible, we cannot
know for certain as there are no living witnesses to
these events. While this may be thought unlikely the
DH Chipmunk is an example of this and several pilots
died before the true problem was realised and anti
spin strakes were added. Test flying can never duplicate
every loading condition or minor difference in construction
and I believe it is very dangerous to assume, that
in the absence of any other cause, the pilot must have
been at fault.
Perhaps the lesson to be learned from all this is that
we know less that we think we do and 'exploring the
envelope' can bring us closer to disaster. It is a
matter of personal choice as to whether we accept this
risk.
In this context Stans assessment has cogent arguments
but must be speculative, we simply will never know.
Not knowing is propbably the most difficult thing that
we as human beings have to accept.

DAJ ASW17 401
At 00:00 08 July 2005, W.J. \bill\ Dean \u.K.\. wrote:
I have never flown a glider of more than about 20 metres
span (ASW 17,
Jantar 2).

However, I am in the process of asking for other pilots'
experience.

One such pilot's reply includes the following:

From your experience of big Nimbi do you think there
is merit in Stan's
ideas?

'Only to the extent that you fly a big glider like
a big glider, not like
a Spitfire. You adjust all your flying to the fact
that you are flying
a big ship. For instance to adjusting your thermalling
technique to
gradual bank angle changes rather than spirited use
of controls to roll
in and out of thermals.

'I have always said about large span gliders, of whatever
make, that you
regard them like flying a 747 Jumbo. Not like an agile
15m glider.

'For instance, in a very turbulent thermal such as
what you often get
over power stations, I often am reluctant to put on
over about 15
degrees of bank. Yes, fifteen degrees. I do not wish
to be sucked in
to a 'loss of control' situation like Ivans and Engen
or the Levers.

Which particular brand of Nimbus do you fly?

'4DM. I am entirely happy with it but I fly it like
the big glider that
it is.'

I would like to add the following:

On the question of use of air-brakes, it would appear
that they were not
opened in the case of the Spanish accident.

In the case of the Minden accident, there was correspondence
on Rec.
Aviation Soaring when the report was published by others
who had experience
of the inadvertent deployment of the brakes at high
speed in turbulence;
the suggestion was that deployment may have been uncommanded.

In addition, Dick Johnson posted that with the Minden
accident there may
have been a problem with the oxygen system undiscoverable
after the crash,
such as oxygen tanks filled with Nitrogen (not entirely
unknown I am
afraid).

It is also notable that whereas there seems to have
been no attempt to bale
out with the Minden accident, with the Spanish accident
both pilots did bale
out and deploy there parachutes, but unfortunately
one of them had his
canopy foul of the wreckage which brought him down
with it.

W.J. (Bill) Dean (U.K.).
Remove 'ic' to reply.


'Bill' wrote in message
oups.com...

I posted the message below on the thread, 'Nimbus
4DT accident 31 July
2000 in Spain.' I am posting it again for better
visibility.

Stan Hall presented his analysis of the Nimbus-4DB
accident in Minden,
NV, 1999. Stan's analysis is scary, to say the least.

The article, Probing for the Smoking Gun, was reprinted
in the Soaring
Association of Canada's free flight, 2/04. Go to
the link below. Click
on free flight on the side bar. Go to free flight
back issues - 2004 -
issue 2. Down load the PDF file.

http://www.sac.ca/

Bill Feldbaumer 09

Earlier, Bill wrote:

...Stan Hall presented his analysis
of the Nimbus-4DB accident in Minden,
NV, 1999...

One aspect of Hall's report with which I will take issue is what is
meant by a "45-degree bend" in the wing. The report shows a diagram
that depicts a 45-degree bend as the wing bend at which the pilot must
looks upwards 45 degrees from the lateral axis to see the wingtip.

I don't doubt that that might be what witnesses reported seeing. What I
do doubt is whether that is what the factory meant when they said that
under static test their wing deflected about 45 degrees (46.5, to be
exact) at 8g.

When I was doing deflection calculations for the wing spar in my HP-24
project, it seemed that the most useful measures of deflection were the
angular deflection at any point on the wing, and ultimately the total
accumulated angular deflection at the wingtip.

This figure compares 45-degree bends as defined by Stan Hall and by me:

http://www.hpaircraft.com/misc/nimbus_bend.GIF

I'm not privy to what definition of deflection the Schempp-Hirth
factory used in saying that their Nimbus wing would bend 46.5 degrees
at 8g. However, if their definition is similar to mine, and what
witnesses saw matched Stan's definition, it would mean that the
aircraft was seen at a loading substantially greater than 8g.

Also, I will submit that witnesses often over-report wing bending, and
that photos often seem to exaggerate it. What happens is that the
witness or camera viewpoint is rarely very close to the aircraft
longitudinal axis, and the oblique angle of the view tends to
"compress" the wings laterally, exaggerating the dihedral and wing
bend.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com

I have not flown a Nimbus 4D, but have several hundred hours experience in
its' early predecessor the Nimbus 2.

A couple of points that might be relevant to this accident:

1) With flapped gliders there is usually a positive flap limiting speed,
often way below Vne. If you are thermalling you are likely to be in a
positive flap setting, so if a loss of control occurs that leads to a spiral
dive, it is very easy to exceed the flap limiting speed and risk twisting
the wings off.

2) With large span gliders, opening the airbrakes causes the wingtips to
bend up. We fitted a second paddle to our Nimbus 2 airbrakes and I did the
test flight in which I was supposed to fly up to Vne (135 knots) with the
brakes extended. By 95 knots the wings were bending up so much that I
started to fear for the structure and didn't go any faster! I resolved that
should I ever lose control of the speed, I would slow it down by pulling g
rather than opening the airbrakes. It also had a tail-chute that could be
deployed in extremis.

I can only recommend that the first action in any loss of control situation
should be to select neutral or negative flap, and then sort things out.

Derek Copeland

Derek Copeland a écrit :

2) With large span gliders, opening the airbrakes causes the wingtips to
bend up. We fitted a second paddle to our Nimbus 2 airbrakes and I did the
test flight in which I was supposed to fly up to Vne (135 knots) with the
brakes extended. By 95 knots the wings were bending up so much that I
started to fear for the structure and didn't go any faster! I resolved that
should I ever lose control of the speed, I would slow it down by pulling g
rather than opening the airbrakes.

I don't understand your choice ! if there is a risk at high speeds, the
best choice to avoid it is to avoid these speeds, and that's what the
airbrakes are for, aren't it ? Pulling g's after loosing control is the
best way to break any aircraft...



--
Denis

R. Parce que ça rompt le cours normal de la conversation !!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la question ?

Denis a Nimbus 4 does not have airbrakes. It has spoilers. These are
two different animals alltogether

Earlier, wrote:

Denis a Nimbus 4 does not have airbrakes. It has spoilers. These are
two different animals alltogether

I'm not one to be a sticker about such things, but I generally observe
this system: If the boards hinge out of the wing surface like on a
Schweizer, I call them spoilers. If they slide out of slots in the wing
like on most European ships, I call them airbrakes.

Anyhow, the factory calls them airbrakes, specifically "Schempp-Hirth
type airbrakes." From the Schempp-Hirth Web site:

The wing (six pices for easier rigging)
has full span camber-changing flaps and
double-panel Schempp-Hirth type airbrakes
on the upper surface of the inboard panels.

The standard text on sailplane design, _Fundamentals of Sailplane
Design_ by Fred Thomas (translated by Judah Milgram), generally applies
the term "dive brake" to any flat nasty that comes out of a glider
wing. It does mention in passing that hinged panels are often referred
to as "spoilers," but that is the only use of the term.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com

A Nimbus 4 has airbrakes.
And as for different animals: A glider certified under JAR22 needs to have
means which limit a dive to 45 deg at vne. Whatever you call them, you can
use them to limit your speed and deploy at any speed up to vne.

--
Bert Willing

ASW20 "TW"


a écrit dans le message de news:
...
Denis a Nimbus 4 does not have airbrakes. It has spoilers. These are
two different animals alltogether