... When will the TGS Mode be Needed? ... Expectations for when HST might enter two-gyro mode are somewhat uncertain because of the limited amount of lifetime data available for the types of mechanical gyros HST uses. A model has been developed that includes random gyro failures as well as a gyroscope wear-out function. ... The model based on HST flight experience predicts that two-gyro mode will be entered in spring 2006, part way through Cycle 14. ... Gyro 1 - in use, 2.1 years of operation . ...