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