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lateral load pick-ups

bearfoil,

After arm ‘36g’, now to the lateral load pick-ups in the vertical stabilizer attachments. It took me a little while to understand how they failed. Please forgive me a somewhat lengthy ‘approach’ before I ‘land’ on them. According to BEA’s analysis, the V/S departed in a forward motion relative to the fuselage, under inertia forces as the forward motion of the airplane was decelerated by water forces acting on the lower parts of the airplane.

The inertia force on the V/S acts through the center of gravity of V/S plus rudder, in a direction opposite to the deceleration of the airplane, i.e. forward and essentially horizontal. It is opposed at the level of the three V/S attachments by rearward forces. The forward and rearward forces together result a pitch-down moment on the V/S, which is opposed by an upward or push-force at the front attachment, and a downward or pull-force at the aft attachment.

The forces exerted by the V/S on the fuselage are opposite to those just described: forward at the three attachments, downward (push) at the front attachment, and upward (pull) at the aft attachment. The forward attachment is thus pushed down on the rear pressure bulkhead beneath it, while the hoop-shaped fuselage frames under the aft attachment are less resistant to the pull forces exerted on it, and fail. The center attachment then gets twice that pullforce (because the moment arm is halved) and fails also. At the forward attachment, the back-up structure of the V/S front spar is weaker than the fuselage, so it fails. The V/S is now free to move forward, taking parts of the fuselage frames below the center and aft attachments with it which, held back at their fracture point, swing backwards.

The lateral load pick-ups at the center and aft attachments are placed in the extended plane of the respective V/S spars. It should be noted that the extension of the center spar passes in front of the pivot of the center attachment, whereas the extension of the aft spar, and thus the lateral load pick-ups attached to it, is aft of the pivot of its attachment. Therefore, when the frames together with the lower part of the attachments between them swing backwards, both pick-ups at the center attachment fail symmetrically in tension, while those at the aft attachment fail in compression.

Could the failure of the lateral load pick-ups be the result of lateral loads? Their function is to prevent or alleviate lateral loads on the male and female lugs of the six attachments. I cannot see a way in which they would fail in left/right symmetry, without lateral failure of the main lugs or a failure in the structure between left and right attachments.

Does this remove some of your doubts regarding BEA’s description of the tail separation?

HN39