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

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

"Stefan" wrote in message
...

This is exactly the point: certificated gliders can always be recovered
from a spin without exceeding the limits, otherwise they wouldn't have
been certificated. The ETA wasn't certificated and broke up during a
test flight. It won't get certificated before this issue is fixed.