Page:CAB Accident Report, TWA Flight 891.pdf/51

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(b) Gust: At the typical design cruising Speed VB and design speed for manmum gust intensity Vb, that is to say at the plane's speed of 261 and 326 knots (EAS), the wing use capable of standing, without breaking, vertical gust speeds not in excess, respectively, of 100 ft. per second (30.5 m/sec.) and. 75 ft. per second (22.? m/sec.).

These figures are Very high and give a convmcing demonstra- tion of the structure's margin of safety With respect to stresses due to gusts, even if the calculations concerning this inquiry were developed exclusively from the static aspect, without taking into concideration the dynamic effect of the gusts.

However, a further investigation for the evaluation of the dynamic effect of gusts on the wing of the model 1611.94 plane disclosed that the increase factor of the bending moment on the wing, due to said dynemm effect, is not very great, ranging as it does between 1.06 and 1.2, and that in any case it is no greater than that calculated for the preVious models 7119 and 10149-0, both of which have been tested extensively. In as much as it is shown by the foregOing that the breakdown of the sung by overstress from gust requires the concomitance of high flight speeds (not adnuss1b1e in a highly turbulent atmosphere) and gusts of extreme intensity, and in as much as in this case also the breaking of the wing should have occurred, in all probability, in the outer part of the wring as explained above, the hypotheSis of the breaking

of the sung by stress from gusts is believed to be wholly improbable.

1h.lt.1.3 Breakdown from excesSive diving speed

This condition has been conSidered in the event - which cannot be excluded a Eiori - that the plane, hailing gone out of control in rough air, exceeded its design div-mg speed VD a 326 knots EAS.

(a) Static overload - Under this condition, the structure that undergoes the greatest stress is not the wing but the fuselage (on the rear area) because of the depresswe force exerted on the horizontal tail surface (downward flexion). Actually, the breaking up of the fuselage and separation of the complete tail assembly occurred in flight; however, as the exennnation of the wreckage disclosed, the separation occurred after and not before the Wing broke away (see par. 11.2.3.1L). Proof of this is the consideration that, had the tail assembly become separated before, the plane would have dived abruptly, with the result that the Wing would have broken off by inverse flexion, which did not happen.

(b) %amic oVerload