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.
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