Test flights revealed problems initially believed to be tail flutter. During high-speed flight approaching Mach 0.68, especially during dives, the aircraft's tail would begin to shake violently and the nose would tuck under (see Mach tuck), steepening the dive. Once caught in this dive, the fighter would enter a high-speed compressibility stall and the controls would lock up, leaving the pilot no option but to bail out (if possible) or remain with the aircraft until it got down to denser air, where he might have a chance to pull out. During a test flight in May 1941, USAAC Major Signa Gilkey managed to stay with a YP-38 in a compressibility lockup, riding it out until he recovered gradually using elevator trim. Lockheed engineers were very concerned by this limitation but first had to concentrate on filling the current order of aircraft. In late June 1941, the Army Air Corps was renamed the U.S. Army Air Forces (USAAF), and a total of 65 Lightnings were finished for the service by September 1941 with more on the way for the USAAF, the Royal Air Force (RAF), and the Free French Air Force operating from England.