Avoiding Vne

Andreas,

You are obviously referring to my posts earlier.

If you read them again, you will see that when you get close to 200 knots
diving an ASW20C it does not matter whether you notice it or not, it is too
late (this is not my opinion or experience, it came from Schleicher).

The pilot was fairly inexperienced, and new to the type. This was true for
me in the past, how about you?

It is not unheard of for pilots recovering from a stall/spin to be too
enthusiastic about the recovery, and to finish diving too steeply for too
long. If you have ever had any experience as an instructor, you have
surely had a pupil do this to you, I certainly have (which is one reason
why we practice recoveries dual). I gather this is what happened with the
accident I commented about, I believe the whole thing started below about
2,000 ft. above the crash site.

If you have read the report of the Minden accident on 13th July 1999 you
will find much the same thing. There were other factors at Minden of
course, so were there with the ASW20C.

I think the date of the accident was 11th January, 1987.

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


"Andreas Maurer" wrote in message
...

Could anyone please tell me how one can get an ASW-20 to 200 kts
without noticing it? I've got a couple of hours in the 20, but I have
not found any situation where this could have been possible.

Bye
Andreas

Sorry about the wrong link to the Minden Nimbus 4DM accident on 99.07.13.

With regard to this accident:

The report makes it clear that the airbrakes were extended when the glider
broke up.

The brief report states:
"Detailed examination of witness marks and other evidence in the wreckage
established that the pilot extended the airbrakes at some point in an
attempt to slow the glider during the descent prior to the break-up.
Concerning limitations on use of the airbrakes, the AFM notes that while
airbrakes may be extended up to Vne they should only be used at such high
speeds in emergency or if the maximum permitted speeds are being exceeded
inadvertently. The manufacturer noted that the airbrakes function like
spoilers and have the effect of shifting the aerodynamic loads outboard on
the wings. The control linkages for the airbrakes and flaps are
interconnected so that when full airbrake deployment is achieved, the flaps
are extended to their full down limit."

The report does not give an estimate of the speed or 'G' loading at the
point when the brakes were extended, or comment on whether this was a
wise thing to do; it assumes that the deployment was deliberate.

There were postings to Rec. Aviation Soaring when the report was published,
from pilots with experience of the Nimbus 4 and similar models who had
experience of inadvertent deployment of the airbrakes. If the brakes
deployed inadvertently while the pilots were recovering from the dive, this
surely may have been the reason for the amount of bending seen; and for the
overload which led to failure. Presumably those investigating the accident
were not aware of these incidents when writing the report.

The probable cause of the accident is given as:
"The National Transportation Safety Board determines that the probable cause
of this accident was the pilot’s excessive use of the elevator control
during recovery from an inadvertently entered spin and/or spiral dive during
which the glider exceeded the maximum permissible speed, which resulted in
the overload failure of the wings at loadings beyond the structure’s
ultimate design loads."

Note that the authors of the report are unclear as to whether the machine
was spinning or in a spiral dive. If the departure started with a spin
entry, they do not give an opinion as to when the machine became unstalled.

The report does not say why they think the machine exceeded Vne, though this
seems a reasonable deduction from the witness reports. With the brakes out
and flaps down the machine could surely have broken up at a speed close to
but below Vne.

The recovery was described by a witness:
"A glider pilot who witnessed the in-flight break-up stated that his glider
was soaring about 1,000 feet below the accident glider when he observed the
accident glider in a high-speed spiral with a 45-degree nose-down attitude.
After two full rotations, the rotation stopped, the flight stabilized on a
northeasterly heading, and the nose pitched further down to a near-vertical
attitude."
Other witness reports were consistent with this.

Note that water ballast was not carried, the accident started at about 9,000
to 11,000 ft. msl., and the conditions were rough strong thermals, a "strong
day".

For authoritative figures and information, in particular limitations, you
have to go to the manufacturer's handbook etc. (Aircraft Flight Manual), and
to JAR-22 to which all modern European gliders are designed.

See:
Joint Aviation Authorities, Europe. http://www.jaa.nl/ ,
JARs – Section 1 – JAR-22 http://www.jaa.nl/section1/jars/445499.pdf .

Denis, you are very scathing. What do you think went wrong?

What would you have done?

Do you have any experience in the Nimbus 3 & 4 series? I don't.

Are you more experienced or better than the pilots who did not make it?

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


"Denis" wrote in message
...


W.J. (Bill) Dean (U.K.). wrote:

There have been several cases of certificated gliders overstressed in
stall/spin recoveries, some of them broke up.

(...)

You can also read about the Nimbus 4DM at Minden; 99.07.13 -
LAX99MA251 -
http://www.ntsb.gov/NTSB/brief.asp?e...09X01702&key=1 Nimbus
4DM - Minden - Two killed.


the link is actually
http://www.ntsb.gov/NTSB/brief.asp?e...12X19310&key=1

the conclusion is "The pilot's excessive use of the elevator control
during recovery (...) resulted in the overload failure of the wings at
loadings beyond the structure's ultimate design loads."

[the possibility of speed being over VNE or Vd is neither confirmed nor
being one of the causes of the wing failure according to the report]

This supposes that unfortunately the pilots did what Bill told : "pull
however hard is necessary" with the result that "At the ultimate load
limit, the deflection was 46.5-degrees, similar to the witness
observations of the wing deflection just prior to the break up."

Do you imagine you may safely "pull however hard you need" with your
wings bent at 45° up ??? I don't.

The report quotes also that the G limit for the Nimbus 4 at VNE is 3.5 g
*only* (compared to 5.3 g at Va) and the design "safety margin" is
between 1.55 to 1.75. Thus even on a plane in perfect condition, and if
the manufacturer made no mistake, it *will* break between 5.4 and 6.1 g
at VNE (even without airbrakes)

Remember that at that speed (285 km/h) you could pull about 16 g ! and
at Vd (324 km/h) more than 20 g...

Yes it was certificated, but certification does *not* guarantee you that
the glider will not break if you pull 20 g... !

Denis

Eric Greenwell wrote:

I should have said "I'm curious about why the G loading (airbrakes
closed) is lower at Vne than at Va". Perhaps it is to absorb gusts, or
to allow greater control deflections.

The only physical reason I see is that there is a torsional stress on
the wing, increasing with the square of the speed, and which might add
somewhere to the bending stress due to G loading.

Another reason is that the certification standards don't require the
same limitation in G-loads at VNE that at Va, and the flight manual
reflects that...

--
Denis

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

W.J. (Bill) Dean (U.K.). wrote:

There were postings to Rec. Aviation Soaring when the report was published,
from pilots with experience of the Nimbus 4 and similar models who had
experience of inadvertent deployment of the airbrakes. If the brakes
deployed inadvertently while the pilots were recovering from the dive, this
surely may have been the reason for the amount of bending seen; and for the
overload which led to failure. Presumably those investigating the accident
were not aware of these incidents when writing the report.

If airbrakes deploy inadvertently, the first effect (along with the very
high drag) will be a *decrease* in G-loading *and* bending moment), both
due to the loss of lift near the airbrakes. The increase of bending
would happen only after the angle of attack has been further increased
(voluntarily or not) to restore the initial G-loading with more lift on
the outer panels (instead of the airbrakes section), hence the higher
bending.

Denis, you are very scathing.

That is not my intention... all I want is to give my opinion when I
think something is said here that may lead to dangerous flying - such as
sentences like "don't exceed VNE, but no problem if you exceed permitted
G-loading".

What do you think went wrong?

What would you have done?

Do you have any experience in the Nimbus 3 & 4 series? I don't.

Are you more experienced or better than the pilots who did not make it?

I don't know them and I would not pretend to be better (there are no
good pilots, only old pilots...). And although I have some experience in
Nimbus 4D (more on ASH 25) I never experienced a spin recovery and I
hope I never will have to. Therefore I don't know what I would do in
such a situation. All I can say is what I think (sitting comfortably in
my chair) is the better thing to do, as I said in a previous post :

"If your speed is going to exceed VNE within this manoeuvre [pulling
up], you should stop or reduce pulling and apply full airbrakes. At any
dive angle up to 45° this prevents the glider to exceeding VNE, and you
have time to recover pulling gently (under 2 g's). This of course
supposes that there is sufficient ground clearance... "



--
Denis

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

Pete Zeugma wrote:

you pull as hard as you need, and its not impossible
to know either, thats why you should practice spin
recovery.

AFAIK spin training is prohibited in most, if not all, open class ships...

and presuming you have not blacked out by then also,
i start to grey out at about 7g from more than 5 secs
exposure at that level.

you would probably have blacked out about 9g

Yes, except grey out or black out happens only after a few seconds (the
time needed for the blood to fall into your lower boby). And it is very
dependant of the condition and trainig of the pilot.

But a glider wing breaks within milliseconds of overstressing !


--
Denis

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

On 3/29/04 12:34 PM, in article ,
"Denis" wrote:


...there are no good pilots, only old pilots....

You must always fly by yourself.


I never experienced a spin recovery and...[t]herefore I don't know what I
would do in such a situation.

With your (claimed) thousands of hours of flight experience?



Jack

Eric Greenwell wrote:

I should have said "I'm curious about why the G loading (airbrakes
closed) is lower at Vne than at Va". Perhaps it is to absorb gusts, or
to allow greater control deflections.

The only physical reason I see is that there is a torsional stress on
the wing, increasing with the square of the speed, and which might add
somewhere to the bending stress due to G loading.

Another reason is that the certification standards don't require the
same limitation in G-loads at VNE that at Va, and the flight manual
reflects that...

--
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 think (and my memory is hazy on this, so I stand to be corrected) it might
possibly have something to do with the gust case. Va I understand to be the
speed at which any one control can be fully deflected and not exceed the design
specifications; at Vne this is reduced to 1/3 of any one control.

'Rough air speed', normally the same as Va (but not always) is the highest
speed at which the glider can encounter an 'OSTIV Strong Gust' (10m/s
instantaneous shear) and stay within the flight envelope. At Vne the airframe
should be able to withstand a 'OSTIV Weak Gust' (2m/s anybody?) without
reverting to kit form. (As an aside, just think about that next time you
redline it across the airfield during the soaring day...)

I'm still chewing over why 'g' limits reduce between Va & Vne. I can picture a
typical flight envelope and this does indeed happen: whether this is a real
(structural/aerodynamic) limitation or just something the manufacturers put in
there to stop us going mad I don't know (probably the former).

Any budding aerodynamicists care to comment?

At 22:18 28 March 2004, Andy Blackburn wrote:
I may not be exactly right about this, but I believe
that most certified aircraft (including gliders) are
tested to, and must be able to hold together at, a
static G-load of roughly 150% of maximum. To meet this
requirement the wings are generally tested to destruction
(check out the DG website for a video of this procedure
for the DG 1000).



I think your definetly right Andy. My Ventus is in
the shop now replacing worn bushings in the aileron
circuits. Last year when getting very close to VNE
on final glide (you know me always coming in high
I had to hold a little right aileron into it so the
aileron would not 'start vibrating' which I interept
as incipent to flutter. Side note is I thought it was
only the bushings at the connect point on the aileron,
turns out I had wear on both ends and the 'slide' that
the tube rides on had worn half way through the tube'
Glad I had Bill check them out.

Cliff

At 22:18 28 March 2004, Andy Blackburn wrote:
One additional thought - I suspect that for older gliders,
wearing of bearings/bushings may degrade the flutter
margin faster than normal material aging degrades the
G-limit, so you might need to think differently if
you have a high-time glider.

Thoughts?


I think your definetly right Andy. My Ventus is in
the shop now replacing worn bushings in the aileron
circuits. Last year when getting very close to VNE
on final glide (you know me always coming in high
I had to hold a little right aileron into it so the
aileron would not 'start vibrating' which I interept
as incipent to flutter. Side note is I thought it was
only the bushings at the connect point on the aileron,
turns out I had wear on both ends and the 'slide' that
the tube rides on had worn half way through the tube'
Glad I had Bill check them out.

Cliff

On Mon, 29 Mar 2004 15:04:46 +0100, "W.J. \(Bill\) Dean \(U.K.\)."
wrote:

You are obviously referring to my posts earlier.
Yes.

If you read them again, you will see that when you get close to 200 knots
diving an ASW20C it does not matter whether you notice it or not, it is too
late (this is not my opinion or experience, it came from Schleicher).

No doubt about that (I'm still of the opinion that most 20 and 20C's
will break up due to flutter before they come close to 200 kts) - but
honestly: Flying far over Vne and not noticing it?

The pilot was fairly inexperienced, and new to the type. This was true for
me in the past, how about you?
We all were inexperienced once - but this is not the point.
How often did you fly 55 kts faster than Vne when you were
inexperienced with a new glider? Or screwed up a spin recovery so
badly that you ended up far over Vne (and this in a 20 that can be
kept under 100 kts in a properly executed spin recovery)?

Pilot error is pilot error - and in this case the pilot error was
huge. And huge pilot errors unfortunately kill a pilot most of the
time - even the smartest designs cannot prevent that.




Bye
Andreas