Airworthiness Directives; MD Helicopters Inc. Model 500N and 600N Helicopters

This document publishes in the Federal Register an amendment adopting Emergency Priority Letter Airworthiness Directive (AD) 99-25- 08, which was sent previously to all known U.S. owners and operators of MD Helicopters Inc. (MDHI) Model 500N and 600N helicopters by individual letters. This AD requires, within the next 5 hours time-in- service (TIS) or before further flight after December 31, 1999, whichever occurs first, inspecting the thruster control cable conduit cap (cap) for corrosion or a crack. This AD also requires, within the next 100 hours TIS or before further flight after February 19, 2000, whichever occurs first, inspecting the cap at a specified area of the forward and center thruster cables for corrosion or a crack. If an unacceptable crack is found, replacing the unairworthy thruster cable with an airworthy thruster cable is required. This amendment is prompted by the discovery of stress corrosion cracks on an MDHI Model 500N helicopter. The actions specified by this AD are intended to prevent failure of the cap causing a fixed thruster condition and subsequent loss of normal anti-torque directional control of the helicopter.