third −8F made its first flight and joined the test program. During the flight tests, Boeing discovered a buffet problem with the aircraft, involving turbulence coming off the landing gear doors interfering with the inboard flaps. Boeing undertook an evaluation of the issue, which included devoting the third test aircraft to investigating the problem. The issue was resolved by a design change to the outboard main landing gear doors. In early April 2010, Boeing identified a possible defect in one of the upper longerons, a main component of the fuselage. According to Boeing, the parts, manufactured by subcontractor Vought Aircraft Industries, were, under certain loads, susceptible to cracking. Boeing said that the issue would not affect flight testing, but other sources stated that the problem could impact the operating envelope of the aircraft until it was fully repaired. Two other issues found during testing were oscillation in the inboard aileron and a structural flutter, which had not been resolved as of 2010. Combined, these problems slowed flight testing and used up almost all the margin in Boeing's development schedule. On April 19, 2010, the second flight-test aircraft was moved from Moses Lake to Palmdale to conduct tests on the aircraft's engines in preparation for obtaining a type certification for the aircraft. The remaining aircraft in the test fleet were scheduled to move to Palmdale during May. It was reported on June 3, 2010, that an engine on the second 747-8F was struck by a tug during a ground move. The engine cowling was damaged, but there was no damage to the engine itself. After repairs, the aircraft moved into fuel-efficiency testing. It was announced on June 14, 2010, that the 747-8 had completed the initial phase of flight-worthiness testing and that the Federal Aviation Administration had given Boeing an expanded type-inspection authorization for the aircraft. By the end of June 2010, the three 747-8Fs that composed the flight-test program had flown a total of over 500 hours and had completed hot-weather testing in Arizona. In June 2010, Boeing determined that a fourth −8F aircraft was needed to help complete flight testing. It was decided to use the second production aircraft, RC503, to conduct the non-instrumented or minimally-instrumented tests, such as HIRF and Water Spray Certifications. The aircraft, painted in delivery customer Cargolux's new livery, first flew on July 23, 2010. On August 21, 2010, a 747-8F proved the variant's capability