I regarded these manoeuvres as highly stressful to the empennage and reported the matter to the last inquiry. I still think it was probably a metal fatigue structural failure of the tailplane or other elements of the empennage that caused the crash.
Capt Fergus Ryan, retd
Narnia - I completely agree that some sort of empennage failure is the most likely cause, but to someone with some aerodynamics knowledge the reported specifics of this accident are a real mystery....
Basically, in classic aircraft such as the Viscount, pitch stability is achieved by having the tailplane produce a downforce i.e. a nose up pitching moment which in stable level flight is balanced by the C.G. being forward of the centre of lift. So any empennage failure (say loss of elevator and/or tailplane) would upset this balance and cause the aircraft pitch sharply nose down. Several accidents have highlighted this:
1) Vanguard G-APEC 1971, near Aarsele, Belgium: Rear pressure bulkhead failed due to corrosion & caused tailplane failure - dived near vertically into the ground.
2) Boeing 707 G-BEBP 1977, Lusaka: Fatigue failure of right tailplane on approach - dived into ground.
3) Viscount SE-FOZ 1977, on approach to Stockholm: severe icing on the leading edge of the tailplane caused the tailplane to stall and near vertical dive into a parking lot.
So - I can't see it being a simple empennage failure or we'd almost certainly just have seen a near vertical dive into the sea off the south Irish coast. I doubt such a failure would be controllable/recoverable even with the height they had at the point of initial upset.
At least one witness reported the nature of the initial upset to be that of a spin/spiral dive. The Viscount then appeared, according to various witnesses, to fly erraticaly at fairly low level - at least once flying low enough to "flatten the grass" according to one witness. Another witness reported seing the faces of the passengers looking out through the windows - so again that must be pretty low level. The obvious speculation here is that the pilots were fighting some sort of controllability issue, but the fact that the aircraft spent some time at very low level but amazingly didn't hit the ground at these points suggests suggests that the aircraft was under some sort of pitch control (perhaps marginal?).
Analysis of the recovered aircraft wreckage suggests a fairly low forward speed (less than 130 knots according to analysis of the engines & prop pitches) but a high vertical descent speed - with possibly one wing lower on impact with the nose being somewhere between level pitch and 45deg nose down. The only condition I can visualize that fits all this is a spin - which makes an interesting comparison to the witness statement regarding the initial upset.
So - to sum up: what failure would give an aircraft marginal control for an extended period and perhaps a tendancy to spin?