NASA-LLIS-0524

Abstract

<strong>Description of Driving Event:</strong> <br>On 1-15-91 a failure occurred in the 8' Transonic pressure tunnel during testing of a model wing with attached flap. During the test run, the flap peeled away from the wing, broke free, and proceeded down the tunnel. Extensive damage resulted. <br>It is hypothesized that reuse of screws having repeated applications of a thread-locking compound caused the east most flap bracket screws to either lose or never obtain proper preload. Under test conditions the screws loosened, then the east most flap bracket was lost, load was shifted to the flap fitting, the flap fitting sheared, and failure occurred. <br>Of special significance is the fact that a thread-locking compound is always used on the screws that attach the flap brackets to the wing. These screws were reused for all tests and no special effort was made to clean them. The flap bracket screws had received an application of a thread-locking compound approximately thirty minutes prior to final installation in the wing, resulting in dried, but undisturbed, thread-locking compound on the screw threads. Further, an additional coating of a thread-locking compound was applied to the flap bracket screw threads just prior to their actual installation. Torque values shown on drawings were given for a preload of 50% or proof load based on dry threads. It is impossible to predict the actual torque on the bracket under the conditions indicated. <br>The thread-locking compound corporation has provided conflicting information. One individual stated that screws should not be reused after having a thread-locking compound applied; further, a tap should be run into the female threads to clean them prior to a new screw being installed. Other representatives said they had no information about reusing screws. To achieve prescribed torque values, however, prudence would seem to dictate a more conservative approach to the use of a threadlocking compound as well as to the cleaning of threads.