AD NUMBER: 2007-19-09R1

MANUFACTURER: Turbomeca Arriel 2B1 Turboshaft Engines
SUBJECT: Airworthiness Directive 2007-19-09R1

SUMMARY: The FAA is revising an existing airworthiness directive (AD) for the products listed above. This AD results from mandatory continuing airworthiness information (MCAI) issued by an aviation authority of another country to identify and correct an unsafe condition on an aviation product. The MCAI describes the unsafe condition as:

Since the issuance of AD 2007-0126 Turbomeca has released modification TU157 which consists in modifying the pressure relief valve of the HMU by introducing a damping device into the valve. Introduction of this device has demonstrated to decrease the pressure fluctuations in the system, therefore reducing significantly the risk of wear of the delta-P diaphragm fabric. This will delete the need for a periodical replacement of the delta-P diaphragm before overhaul of the HMU. The modification TU157 is therefore considered as the terminating action for this AD. 

This AD is issued to prevent the loss of automatic control mode coupled with the deteriorated performance of the backup mode, which can lead to the inability to continue safe flight, forced autorotation landing, or an accident.
 

FMI: http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/CC587B3B9603988B8625773600490DA3?OpenDocument