Wing flexibility of the B-29 as well as wing-tip vortices caused concern, and the mechanisms for attachment required modifications. The first hookup of both F-84s with the B-29 occurred on the 10th flight on 15 September 1950. The longest flight with all connected was on 20 October 1950, and lasted for 2h 40min. All of these flights were accomplished with manual control of the F-84 aircraft. Republic received an additional contract to continue the experiments by incorporating an automatic flight control system. Meanwhile, as the modifications proceeded, additional test flights were made, including night flights. The automatic flight control modifications were ready for testing in March 1953, and a number of hookups were made with only one or the other of the F-84s while attempting to sort continuing electrical issues. On 24 April 1953, over Peconic Bay, New York State, the left-hand F-84 hooked up and the automatic system was activated. The F-84 immediately flipped over onto the wing of the B-29 and both crashed with loss of all five crew and the F-84 pilot.