Earlier, Stefan wrote:

...But it wasn't an accident. It
was a test flight to explore the limit,
and they found the limit, although a
bit earlier than they wanted, I guess.
That's exactly what test flights are for...

I'm not sure I completely agree with that assessment of the situation.

The way I understand it, the limits they were expecting to explore
were control and handling limits, not structural limits. The way I
read the Babel Fish translated report, they placed the aircraft into a
spin, executed a recovery, and were surprised by a structural failure
of the aft fuselage during the recovery.

I think that if they were really executing a structural test, they
probably wouldn't have had two people on board.

Going forward, it will be very interesting to follow the investigation
of why the structure failed. Did the maneuvers performed apply
loadings beyond the expected ultimate limit? Or did the structure fail
to provide the expected strength and stiffness?

And if the latter, were the stresses miscalculated or did the stucture
not perform as expected?

It's really far too early to speculate, but it will be interesting to
see if perhaps bending loads applied by the elevator plus bending and
torsional loads from the rudder combined to produce stresses in excess
of the expected maximums.

I hope that they were carrying a flight recorder, and that it (unlike
too many dented Cambridge Model 20s) yields usable data.

Anyhow, I'm glad the pilot and observer weren't injured badly. That's
what's most important.

Thanks, and best regards to all

Bob K.