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.

Time from initial failure to impact was maybe 30s. Think about how fast
you can close the distance to the ground next time you are at 250km/h
and 600 feet height.
That height is the distance you are travelling in three seconds (70m/s).

Bruce


On 2014/02/06 4:02 PM, JJ Sinclair wrote:
On Wednesday, February 5, 2014 8:42:13 PM UTC-8, OneTango wrote:
This is my best guess. I have no idea if it is correct.



What do you do next?



Use the spoiler handle to open the right spoiler to match the left.



Follow my practiced spin recovery technique and return to level flight with both spoilers fully open.



Decide whether to land or bail out.



If there is a place that I can land with both spoilers fully open. Do that.



If I cannot make it to a field with both spoilers open, quickly determine if it is possible (and if there is less sink) to fly with one spoiler open and one spoiler closed. If it is possible, decide whether to try it or bail out.



Why did the left spoiler pop open?



When the push rod that controls the spoilers locks past the detente, it is under compression and it is designed to bend slightly. Due to a manufacturing defect, the bent aluminum push rod rubs against the fiberglass where it passes through a wing rib (hole is not in exactly the right place or hole is not the right shape or size). Over time, the glass wing rib saws through the aluminum pushrod. At a certain point the pushrod buckles and breaks. The spoiler pops open.

Your scenario #1 actually did happen in a DG-400 at Truckee about 25 years ago. One spoiler needed to be adjusted slightly, so the (non mechanic) pilot gave the quick disconnect fitting a half turn in. Under positive G force the fitting was now trying to pull away from the ball, rather than being forced into to ball. At about 4000 feet AGL, one spoiler popped full open. The pilot correctly assessed the situation and pulled the other spoiler open. Having plenty of altitude, he decided to enter the pattern to the normal glider runway (19) at 2000 feet..........thinking that extra altitude is always good, right? The extra altitude meant he had to fly farter down-wind to get rid of his excess altitude, turned final into some wind and didn't make it back to the runway! Landing short of 19 at Truckee means you hit a cliff.

If you are faced with this situation (one spoiler open), Put the ship in the middle of the longest runway available!
JJ


--
Bruce Greeff
T59D #1771