Nimbus 4 Accident

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

I'd say that his analysis is questionable, at least.
It sounds like written by somebody who is astonished that a 26m ship might
handle in some situations differently than a 15m ship, and that if there is
any accident evolving of this, the designer of the ship should be
responsible (and not improper pilot reactions).

And to the subject of pulling the airbrakes at vne pull-ups: If the max
g-load at vne is lower with airbrakes extended than without, it's written in
the manual. In any case, coming out of a spiral dive / spin combo with such
a ship, you are pretty sure that you will come _at least_ close to vne
and/or max g-load. A pilot pulling the airbrakes _and_ pulling hard up in
such a situation hasn't thought about such a situation beforehand (not
good), or has no idea what he is playing with (also no good).
--
Bert Willing

ASW20 "TW"


"Bill" a écrit dans le message de news:
...
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

I'd say Stan's analysis of this accident is right
on. He's the only one that has come up with a reasonable
sequence of events that could lead to the known results.
We hit a violent shear in the same area, the day before
this accident. It rolled our ASH-25 up on one wing.
If we hadn't taken immediate corrective action (aileron
& rudder against the roll) I could see us entering
a spiral dive. As we are learning, once the nose is
down in a big ship, you are headed for a moment of
truth.

JJ Sinclair


At 13:42 06 July 2005, Bert Willing wrote:
I'd say that his analysis is questionable, at least.
It sounds like written by somebody who is astonished
that a 26m ship might
handle in some situations differently than a 15m ship,
and that if there is
any accident evolving of this, the designer of the
ship should be
responsible (and not improper pilot reactions).

And to the subject of pulling the airbrakes at vne
pull-ups: If the max
g-load at vne is lower with airbrakes extended than
without, it's written in
the manual. In any case, coming out of a spiral dive
/ spin combo with such
a ship, you are pretty sure that you will come _at
least_ close to vne
and/or max g-load. A pilot pulling the airbrakes _and_
pulling hard up in
such a situation hasn't thought about such a situation
beforehand (not
good), or has no idea what he is playing with (also
no good).
--
Bert Willing

ASW20 'TW'


'Bill' a écrit dans le message de news:
.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

I wouldn't want to speculate about the sequence of
events in a specific accident but note the author's
comment:

'designers of new machines might consider the installation
of a jettisonable drogue chute in the tail as standard
equipment.'

IMHO this should not just be 'considered' - all new
high performance glider designs should have a well
engineered jettisonable tail chute or some other fuselage
sited speed limiting device. If they already had them
at least 3 lives we have discussed recently on RAS
would likely have been saved.

John Galloway




At 13:42 06 July 2005, Bert Willing wrote:
I'd say that his analysis is questionable, at least.
It sounds like written by somebody who is astonished
that a 26m ship might
handle in some situations differently than a 15m ship,
and that if there is
any accident evolving of this, the designer of the
ship should be
responsible (and not improper pilot reactions).

And to the subject of pulling the airbrakes at vne
pull-ups: If the max
g-load at vne is lower with airbrakes extended than
without, it's written in
the manual. In any case, coming out of a spiral dive
/ spin combo with such
a ship, you are pretty sure that you will come _at
least_ close to vne
and/or max g-load. A pilot pulling the airbrakes _and_
pulling hard up in
such a situation hasn't thought about such a situation
beforehand (not
good), or has no idea what he is playing with (also
no good).
--
Bert Willing

ASW20 'TW'


'Bill' a �crit dans le message de news:
.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

Bert Willing wrote:
It sounds like written by somebody who is astonished that a 26m ship might
handle in some situations differently than a 15m ship, and that if there is
any accident evolving of this, the designer of the ship should be
responsible (and not improper pilot reactions).

Bert Willing

ASW20 "TW"


Bert,

That's an odd comment. When someone like Stan Hall writes about
stability, control, and structures, I listen. Carefully.

I think the bottom line in his analysis is very clear. Open Class
performance comes with risks that need to be completely understood AND
there are flight regimes where the margin between recovery and disaster
is very, very thin.

Erik Mann
LS8-18 "P3"

Sorry, I don't care who he is, I care what he writes (and I read it
carefully).

He does make valuable points - a tail chute is not a stupid idea. His
comment on wing momentum is very good (and I don't think that people are
well aware enough of that), and I completely agree on the fact that if a
60:1 performance is breathtaking on a long glide, 60:1 performance in a
vertical dive coming out of a spin is life-taking.

What I disagree with is the underlying tune of the the disaster(s) being
designed into the ship :

"But I think one can legitimately ask if there was something inherent in the
design of the sailplane that led its occupants into a situation from which
there was no escape. My thesis is, there was."

Now if today somebody wants 60:1, he will have to go for 26m, and the
structure of these ships cannot be built otherwise - at leat not today. Even
the newest development, the Eta, will bite during the recovery from a spiral
dive as it has been shown (on purpose - those guys have some balls... and
parachutes). With such a ship, some situations are extremely dangerous
(situations which would be rather fun in 15m), and if you want to fly such a
ship, you better start to think first.

What I also think being pure speculations is the stiffness of the ailerons
due to flexing (and in this situation, you don't loose time fiddling with
ailerons anyway), and when and how the airbrakes have been deployed. There
is a chance that the pilot pulled the airbrakes before reaching vne and
pulled to hard, and there is a chance that he pulled them after having
exceeded vne and tried to pull up (and at a certain point beyond the v-n
diagramm, any glider will brake up) - we will never know at which speed and
at how many g's the wings came off. In this light, his comment

"What the AFM didn't say of course was, "if you exceed the maximum permitted
speed and open
the brakes be prepared to have the wings come off."

is plain stupid.

However, his last paragraph is very good.
--
Bert Willing

ASW20 "TW"


"Papa3" a écrit dans le message de news:
...


Bert Willing wrote:
It sounds like written by somebody who is astonished that a 26m ship
might
handle in some situations differently than a 15m ship, and that if there
is
any accident evolving of this, the designer of the ship should be
responsible (and not improper pilot reactions).

Bert Willing

ASW20 "TW"


Bert,

That's an odd comment. When someone like Stan Hall writes about
stability, control, and structures, I listen. Carefully.

I think the bottom line in his analysis is very clear. Open Class
performance comes with risks that need to be completely understood AND
there are flight regimes where the margin between recovery and disaster
is very, very thin.

Erik Mann
LS8-18 "P3"

I read all of this stuff with interest since I've spent most of my soaring
career in long wing gliders (18 years in an 20m ASW-17 and 5 in a 24.5m
Nimbus 3). While I respect Stan and his extensive resume - he acknowledges
that he has no experience flying the open class ships. I have not found
that the long wing gliders are any more or less susceptible to spiral
dives, spins or unexpected acceleration as opposed to 15m ships. If
anything, they are more stable tend to telegraph the stall pretty early and
spin pretty slowly. The Nimbus does have a remarkably short span
stabilizer which when stalled can cause pitch down - but this is pretty
rare and easily recovered from. I also confess that I do not understand the
second hand comment about "extreme rudder sensitivity" (all open class
pilots wish it were so!). It should also be noted that most of these break
ups seem to happen in the 2 place ships (or 2 place with engine versions)
where the fuselage is hanging more weight on the same wing that is fitted
in the single place pure glider model. While single place N3s and 4s have
crashed, I don't think any have come apart in the air. This tells me the
problem is more complex than just wingspan.

What is clear is that opening the dive brakes at high loads and high wing
flexion is the last step before disaster. It suddenly increases the bending
moment outboard of the brakes and pushes the wing to failure. This was
found in both the Spain and Minden accidents. Stan spends a great deal of
time talking about what happens to aileron loads at high wing flexion. It
would be more interesting to study what happens to the dive brakes when the
wings are flexed at 45 degrees. Do they pop open because of forces on the
control rods (or forces on the caps caused by the flexion)? I don't think
that there is any real data or experimentation on this issue (and I'm not
volunteering to be the first!). When the factory does their stress load
testing - I do not believe that they actuate the dive brakes and the issue
cannot be studied by merely stressing a wing with a dive brake rod
unconnected to the fuselage. Also, there is no scientific study as to
what pilots do in sailplanes when surprised by high angle dives and high G
pull outs. 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.

This is not to be critical of anybody - all of this is good for all of us.
So I thank Stan for his article.

Roy B.


tel: 508 798 8801
fax: 508 754 1943

Roy Bourgeois wrote:

I read all of this stuff with interest since I've spent most of my soaring
career in long wing gliders (18 years in an 20m ASW-17 and 5 in a 24.5m
Nimbus 3). While I respect Stan and his extensive resume - he acknowledges
that he has no experience flying the open class ships. I have not found
that the long wing gliders are any more or less susceptible to spiral
dives, spins or unexpected acceleration as opposed to 15m ships. If
anything, they are more stable tend to telegraph the stall pretty early
and spin pretty slowly. The Nimbus does have a remarkably short span
stabilizer which when stalled can cause pitch down - but this is pretty
rare and easily recovered from. I also confess that I do not understand
the second hand comment about "extreme rudder sensitivity" (all open class
pilots wish it were so!). It should also be noted that most of these
break ups seem to happen in the 2 place ships (or 2 place with engine
versions) where the fuselage is hanging more weight on the same wing that
is fitted
in the single place pure glider model. While single place N3s and 4s have
crashed, I don't think any have come apart in the air. This tells me the
problem is more complex than just wingspan.

What is clear is that opening the dive brakes at high loads and high wing
flexion is the last step before disaster. It suddenly increases the
bending
moment outboard of the brakes and pushes the wing to failure. This was
found in both the Spain and Minden accidents. Stan spends a great deal of
time talking about what happens to aileron loads at high wing flexion. It
would be more interesting to study what happens to the dive brakes when
the wings are flexed at 45 degrees. Do they pop open because of forces on
the
control rods (or forces on the caps caused by the flexion)? I don't think
that there is any real data or experimentation on this issue (and I'm not
volunteering to be the first!). When the factory does their stress load
testing - I do not believe that they actuate the dive brakes and the issue
cannot be studied by merely stressing a wing with a dive brake rod
unconnected to the fuselage. Also, there is no scientific study as to
what pilots do in sailplanes when surprised by high angle dives and high G
pull outs. 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.

This is not to be critical of anybody - all of this is good for all of us.
So I thank Stan for his article.

Roy B.


tel: 508 798 8801
fax: 508 754 1943
Wasn't the 4 breakup in the NZ world's a single place?

Frank

Yes - But the pilot flew at high speed into the primary wave rotor. Not
the N4's fault I think.

Roy

Roy A. Bourgeois

Papa3 a écrit :

That's an odd comment. When someone like Stan Hall writes about
stability, control, and structures, I listen. Carefully.

Even the best pilots may have accidents, and even a well known engineer
may write obvious stupidities. Obviously Stan Hall did make statements
that are completely false. He admits himself that he never flew an open
class glider, and that should have led him to be more prudent in his
analysis.


I think the bottom line in his analysis is very clear. Open Class
performance comes with risks that need to be completely understood AND
there are flight regimes where the margin between recovery and disaster
is very, very thin.

*That* is right. As it has already been discussed here, that the load
factor admissible with airbrakes out is much lower than without
airbrakes. This is obviously not easy when recovering from a spin not to
apply too much g-factor, especially above VNE or near the ground, but it
may save your live !


--
Denis

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