AD NUMBER: PL2008-18-52
MANUFACTURER: MDHI
SUBJECT: Airworthiness Directive PL2008-18-52
SUMMARY: This superseding Emergency Airworthiness Directive (EAD) is prompted by two incidents of impaired directional control. This impaired directional control occurred after the Vertical Stabilizer Control System (VSCS) circuit breakers were pulled in accordance with EAD 2008-18-51. EAD 2008-18-51 was issued on August 20, 2008 and requires several actions related to the VSCS for the MDHI MD900 helicopter and related to the Yaw Stability Augmentation System (YSAS) for theModel 500N and 600N helicopters. EAD 2008-18-51 was prompted by reports that two MD900 helicopters experienced broken VSCS adapter tubes. In one case, the helicopter experienced an uncommanded yaw, resulting in loss of a window and a door. The same part -numbered adapter tube is part of the Model 500N and 600N helicopter's YSAS. Some of the VSCS and YSAS adapter tubes were not manufactured to the required specifications and can fail. Since the issuance of EAD 2008-18-51, we have discovered that pulling the circuit breaker is causing impaired directional control, which could result in loss of control of the helicopter. Therefore, this EAD supersedes 2008-18-51 and requires turning OFF the VSCS or the YSAS instead of pulling the circuit breakers and installing placards that limit airspeed to 100 KTS or VNE, whichever is less. For the Model MD900 helicopters, limiting flight to VFR, prohibiting use of the autopilot, and making changes to the Emergency Procedures and Airworthiness Limitations sections of the RFM are also required. For all of the helicopter models, replacing the adapter tubes with airworthy adapter tubes that have a date stamp is also required. The adapter tubes must be replaced within 45 days and is terminating action for the requirements of this EAD.

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