Over a period of six months, a total of 24 test flights were conducted. Most of the complex electronics were not fitted to the first aircraft, so these flights were all concerned with the basic flying qualities of the aircraft which, according to the test pilots involved, were outstanding. Speeds of Mach 1.12 and sustained low-level flights down to 200 ft were achieved above the Pennines. Undercarriage vibration problems continued, however, and only in the final few flights, when XR219 was fitted with additional tie-struts on the already complex landing gear, was there a significant reduction in them. The last test flight took place on 31 March 1965.