The possibility was studied that there might be some unknown phenomenon at work that could cause a reduction in structural stiffness with age. Such a reduction in stiffness could result in a reduced flutter speed and lead to flutter. In 2004, Airbus conducted GVT in support of its MRTT program. The testing was conducted on an aged A310 aircraft (MSN 523) that had accumulated over 28 000 flight hours. This test aircraft had the same design of VTP and rudder as the occurrence aircraft. GVT results found that fin bending and rudder rotation frequencies of the MRTT test aircraft were consistent with those obtained during the original A310-300 certification. No indication was found to suggest that stiffness had reduced with age.