On Saturday, February 8, 2014 12:35:58 PM UTC-8, BruceGreeff wrote:
Similar failure happened on a Nimbus 4DM some years ago - on a fast

final glide into Gariep.



Height was low, speed was high (well past Vmo) when they hit a powerful

thermal.

Wings took the shock, but one airbrake actuator failed under the shock

load. It had been damaged by "buzzing" over an extended period - the

caps were not held down properlyso the blade would chatter in it's box

on pull ups. This set up fatigue in the connecting rod which snapped

under the sudden whip motion on the wing.



Due to high speed, no spin departure.

The resulting yaw caused an immediate roll toward the open brake wing.

Pilot correctly assessed the situation, opened the other airbrake and

attempted to recover from the inadvertent barrel roll.

Attempt was unsuccessful, wings failed symmetrically at the outer panel

join, and then a couple of metres from the root - conveniently

preserving the failed actuator and the witness marks... Calculated

failure somewhere on the far side of 300km/h and enormous g.



Fuselage impact was a couple of seconds later - both pilots being

ejected many metres from the point of impact.

Being a DM the resulting fire destroyed much of the confetti that

remained. Somehow, enough of one of the three loggers remained for a

trace to be obtained.



Moral of that story is - check your airbrake actuators for wear and

fatigue - particularly on long flexible wings. And have some respect for

the numbers - they are there for a reason.

Very interesting report, we had a Nimbus 4DM come apart at Minden and both pilots were killed several years back. They got into a death spiral for unknown reasons..............wonder if one spoiler could have popped open to start the spiral? The accident report stated that both spoilers deployed when the ship was near vertical, but now I'm wondering if one was open before the other? I have seen instances where one spoiler came out caused by bending of its pushrod or flexing of drive unit in the root rib, but nothing broke! Would the accident investigator notice a bent rod or flexed root rib in a pile of fiberglass rubble?
JJ