Reading some of the more reasoned posts got me curious (a curse). Apart from a couple of delightful hours, more years ago than I care to remember, I have no knowledge (zero) of Tiger operations – so indulge me here. I am curious as to the 'why' a wing 'broke', also which wing went first (top or t'uther).
For the sake of discussion, lets take a small risk and make some assumptions; (a) the aircraft was correctly maintained and 'in good order'; (b) the pilot was of 'average' competency, medically serviceable and could manage 'basic' aero's. The weather is a given. I believe we could almost ignore a bird strike as being a 'wing breaker'. So what else could possibly cause a sudden event??
Some will no doubt recognise their words in the quotes below; so my question – is it remotely possible that the deployment of leading edge slats, if unlocked could create enough additional load (twisting ?) on a 'weak' spot to create the break ?. Looking at the drawings creates a doubt, there's some pretty robust material to 'break' built in.
It is important to lock the automatic slats (leading edge flaps) during aerobatic manoeuvres
Put simply, Auto slots (slats) work by the center of pressure moving forward until there is a lifting force on the slats rather them being held in position by slipstream. I think the reason for locking them is mainly to prevent them being damaged, after all, if it deployed on the wing about to stall that would delay the stall and hence reduce the likeyhood of a spin - though not significantly.
The slats on the Tiger's top wing can be locked shut, and indeed some are permanently wired shut. They are usually locked for aeros, as at high AoA one slat might deploy before the other. Indeed that might have been a factor in this accident, if he let the AoA get too high in the loop and one slat deployed and the other didn't, round you go into spin-land (unless you counter it with instant forward elevator and appropriate rudder, an instinctive reaction for an experienced aeros pilot,
In a perfect world both slats deploy at exactly the same AoA and there is no delay in deployment. In the real world there will be a difference in the AoA that provoke deployment wing to wing, and then a delay (also variable wing to wing) from closed to open, which become significant in the sort of accelerated stall that happens if you pull back too hard while climbing steeply (loop entry). It's a flick really, so it all happens very quickly compared to a level stall.
Consider pulling up into a loop, pulling too hard near the top, and one slat pops out. Both wings were near the stall and flow breakaway was starting, but now the slatted wing fully recovers so there is a roll component towards the unslatted wing which increases the AoA of that (downgoing) wing by quite a lot and it fully stalls. Of course its slat will then deploy but by then, assuming the pilot keeps pulling as the speed continues to decay, the damage is done and the aeroplane has flicked into a spin.
That is precisely why they should be locked before aeros!
As a commercial Tiger Moth Pilot, I can tell you that the slats are a real pain. The cables used to operate them stretch over time, especially if one is doing a lot of aeros allowing the slats to 'float'. This stresses both the slat mechanism and the wing, leading to all manner of maintenance headaches. We remove the slats as a matter of routine whenever we are having wings rebuilt.
Thanks for replies so far. asw 28-866 - If the slat cables stretched over time and caused the "floating" you describe, this suggests routine maintenance of the cables was lacking - not faulty cables. Most components, if not serviced to manufacturer's specified time limits will inevitably cause problems down the line. While the IAS stalling speed difference may be negligable, the CAS stalling speed is what counts. There is no doubt the slats act as an effective stall warning device.
Removal of a component for reason of servicing costs can lead to undesirable results. In 1992, a Tiger Moth spun in during the execution of a simulated engine failure shortly after take off at 300 ft. One pilot was killed. The accident investigation board criticised the operator for locking the slats in the permanently closed position (to save the bother and costs of servicing). Litigation quickly followed.
The Board of Inquiry stated the accident might have been avoided if the slats had been operating as they served as an effective signal of stall warning and a slower stall speed.
Just asking - ???