MANUFACTURER: Turbomeca Astazou XIV B and XIV H
Turboshaft Engines SUBJECT: Airworthiness Directive 2010-11-10
SUMMARY: 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:
Investigation of an uncommanded in-flight shutdown (IFSD) revealed
that a third stage turbine wheel rupture was not contained by the
turbine casings. The released portion consisted of a turbine blade
together with the rim piece immediately below the blade. The rim
piece was bounded by two adjacent axial slots and a fatigue crack
that had developed between the holes in which the slots terminate.
The slots and holes, which are closed by riveted plugs, were
introduced by modification AB 173 in order to improve the vibration
characteristics of the turbine wheel. Modification AB 208 brings an
improvement to modification AB 173 by changing only the riveting
detail. SN 283 72 0805 provides instructions for re-boring the
holes at overhaul or repair in order to improve their surface
condition. A manufacturing process modification has been introduced
to improve the surface condition of these holes in third stage
turbine wheels. Wheels subject to the improved manufacturing
process have S/Ns outside the range specified in Table 1. Although
there is only one known event, and although it resulted only in an
uncommanded IFSD, with no damage to the aircraft, the possibility
exists that additional events may occur, potentially involving
damage to the aircraft.
The FAA is issuing this AD to prevent uncontained failures of
the third stage turbine wheel, which could result in damage to the
helicopter.
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