AD NUMBER: PL2008-18-51
MANUFACTURER: MDHI
SUBJECT: Airworthiness Directive PL2008-18-51
SUMMARY: This Emergency Airworthiness Directive (AD) is prompted by reports that two Model MD900 helicopters experienced broken Vertical Stabilizer Control System (VSCS) adapter tubes. In one case, the helicopter experienced uncommanded yaw, resulting in loss of a window and adoor. The same part-numbered adapter tube is used on the Model 500N and 600N helicopters Yaw Stability Augmentation System (YSAS). Some of the VSCS and YSAS adapter tubes were not manufactured to the required specifications and can fail.. This condition, if not corrected, could result in loss of yaw control and subsequent loss of control of the helicopter. We have reviewed MDHI Service Bulletins SB500N-040 for the Model 500N helicopters and SB600N-047 for the Model 600N helicopters, dated August 15, 2008, with YSAS adapter tubes installed. We have also reviewed Service Bulletin SB900-109, dated August 15, 2008, for the Model MD900 helicopters with VSCS adapter tubes installed. These service bulletins specify de- energizing the YSAS or the VSCS and installing a placard that limits the airspeed. The service bulletins specify removing the placard when the YSAS and VSCS adapter tubes are replaced with airworthy parts.The manufacturer anticipates having airworthy YSAS and VSCS adapter tubes available within the next 30 to 60 days. The FAA anticipates requiring that the YSAS and VSCS adapter tubes be replaced with the airworthy parts as a terminating action to the requirements of this AD.

FMI: www.faa.gov/regulations_policies/airworthiness_directives/