AD NUMBER: PL2008-18-52
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.