Airworthiness Directives; MD Helicopters, Inc. Model MD900 Helicopters

This document supersedes AD 2008-18-52 and publishes in the Federal Register an amendment adopting Emergency Airworthiness Directive (AD) 2008-22-53 which was sent previously to all known U.S. owners and operators of MD Helicopters, Inc. (MDHI) Model MD900 helicopters by individual letters. This AD requires turning ON both Vertical Stabilizer Control System (VSCS) switches and turning OFF the autopilot (AP/SAS) switch; pulling certain AP/SAS circuit breakers; installing a placard near the AP/SAS master switch; installing an airspeed limitation placard on the instrument panel; and making changes to the Rotorcraft Flight Manual (RFM). This AD is prompted by 4 occurrences in which the left VSCS adapter tubes failed on the MD900 helicopters and the vertical stabilizer became uncontrollable, resulting in reduced yaw control of the helicopter. There were no injuries, but in one occurrence, due to the high speed quick yaw, the helicopter lost a window and a door. The actions specified by this AD are intended to prevent loss of yaw control and subsequent loss of control of the helicopter.