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Old 12th Sep 2015, 09:05
  #393 (permalink)  
Avionista
 
Join Date: Nov 2006
Location: Scotland
Posts: 53
I have been reading the FAA AD designed to prevent the type of incident which occurred to the BA 777 at Las Vegas. I found the following parts of the document interesting:


Request

Two commenters, General Electric Company and The Boeing Company, requested that we remove the “UnsafeCondition” paragraph from the AD, and reword the Summary section to resemble the Summary section of AD 2002-04-11. The commenters stated that, by their analyses, cracks in the weld joint would not develop into an uncontained failure. The commenters stated that HPCR 8-10 stage spools, P/Ns 1844M90G01 and 1844M90G02, be inspected by an enhanced inspection, similar to those parts covered in AD 2002-04-11.

Answer

We do not agree. AD 2002-04-11 was issued because of additional focused inspection procedures that had been developed by the manufacturer. Because cracks were discovered on one HPCR 8-10 spool between the 9-10 stages in the weld joint, this unsafe condition is likely to exist or develop in other products of the same type design. The unsafe condition could result in failure of the HPCR 8-10 stage spool, uncontained engine failure, and damage to the airplane. We determined that this unsafe condition requires mandatory repetitive inspections for cracks. We did not change the AD.



Well done the FAA for 'sticking to its guns'.


Inspections of the HPCR 8-10 Stage Spool

(f)(1) At the next piece-part exposure of the HPCR 8-10 stage spool after the effective date of this AD, perform a fluorescent penetrant inspection (FPI) and eddy current inspection (ECI) of the weld joint between the 9-10 stages of the HPCR 8-10 stage spool for cracks.

(2) Thereafter, perform repetitive FPIs and ECIs of the weld joint between the 9-10 stages of the HPCR 8-10 stage spool for cracks at every piece-part exposure of the HPCR 8-10 stage spool.


Presumably, this type of inspection can be performed only during major engine overhaul, with the engine removed from the aircraft? If so, will the authorities now need to consider more frequent engine overhauls?


What seems to be missing is any move to find a permanent fix for the problem such as a redesign/manufacture of a HP turbofan which does not have an inherent weakness.
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