I have always understood that the first composite gliders were well
over-engineered, because the manufacturers were dealing with a new
technology and did not want any risk of failure. I have not heard of any
problems with old glass gliders due simply to age or high hours.

As designers and manufacturers have gained experience I imagine they have
been able to design and build to the limits rather than way past them, so
gaining performance and reducing cost. There are a number of cases where
older designs have had the limitations increased without any modification to
the airframe.

Perhaps one problem we now have is stall/spin recovery. The usual cause of
disaster from a stall/spin is hitting something (usually the ground) before
recovering to normal flight.

There is another problem, flying outside limits while recovering to normal
flight. This can happen with any type if the recovery is handled badly, I
have known of cases involving a Ka6E and more than one K13.

Modern types of glider are easy to fly, and normally reluctant to depart
into
an inadvertent stall/spin (and often difficult to make spin deliberately).
However, many of them can depart violently into a steep spin, and recovery
has to be immediate and correct or Vne will be exceeded. This is most
likely to happen when ballasted and thermalling hard in strong lift.

I know of three fatal accidents where it appears that the glider went
outside limits while recovering from what appeared to be an inadvertent
stall/spin recovery.

1. ASW20CL at Dunstable (I think more than 10years ago), the glider
reached a speed in the dive which made the dive irrecoverable; the C. of G.
may have been aft of limit.

2. Nimbus 4DM at Minden on 13th July 1999.

http://www.ntsb.gov/publictn/publictn.htm , then
http://www.ntsb.gov/publictn/2002/AAB0206.htm or
http://www.ntsb.gov/publictn/2002/AAB0206.pdf .

From the report it would appear that the glider departed into a stall/spin
while thermalling, and was overstressed in the subsequent dive while
recovering.

The brakes opened during the recovery, which reduced the 'g' limitations;
correspondence to Rec. Aviation Soaring indicates that some pilots have
experienced inadvertent deployment of the brakes on this type of glider.
There was also a suggestion that the pilots may have been incapacitated by a
problem with their oxygen system, thought there was no evidence to back
this.

3. Nimbus 4DM in Spain 1999.

Referred to in the report of the Minden accident above.

Quote: "the pilot stated they were in a turn when a heavy thermal caused the
glider to enter a steep descending spiral. The pilot could not recover the
aircraft from the spiral and the glider quickly exceeded Vne. The pilot
then reported that the right wing failed and he bailed out." The second
pilot was unable to bail out.

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


"Howard Franks" wrote in message
...

I have always assumed the recent (last 20 years) composite
gliders to be very robust, i.e. no issues flying right
up against placarded limitations at anytime (maybe
once or twice nipping over?).

Perhaps the Eta break-up is a sign that we are reaching
the structural/design limitations of the current materials
and the designers ability to optimise gliders for performance.
Are we at the point of diminishing returns where the
small increase in performance only comes at a far greater
risk of structural failure (similar to the Americas
Cup yachts).

This coupled with the recent Schempp-Hirth issues (agreed
manufacturing not design defects), and the AD restricting
GROB 103s is leading me to rethink just how tough these
things are.