NTSB Identification: ENG08IA051
Scheduled 14 CFR Part 121: Air Carrier operation of US AIRWAYS INC (D.B.A. US Airways Inc)
Incident occurred Friday, June 27, 2008 in Phoenix (PHX), AZ
Probable Cause Approval Date: 04/12/2011
Aircraft: BOEING 737, registration: N514AU
NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft incident report.
In June 2008, after the incident airplane was towed, ground personnel noted that the left hand (LH) main landing gear (MLG) was leaking shock strut fluid from a crack in the wall of the outer cylinder. A visual examination of the outer cylinder revealed a crack located approximately 26” below the shock strut air valve. The outer cylinder had accomplished 3 ground air ground (GAG) cycles after completion of the December 2007 overhaul, and approximately 5836 cycles since the prior overhaul in 2004.
The subject outer cylinder developed fatigue cracking in the wall which resulted in leakage of shock strut fluid. The fatigue cracking initiated at and near the ID surface, and propagated through the wall toward to OD until the leakage was detected and the cylinder was removed from service. Crack initiation was due to base metal damage which took the form of over tempered martensite and chemical attack/pitting. The over tempering reduced the strength of the material and caused reduction of shot peen induced residual compressive stresses. The combination of these factors results in the affected material having reduced resistance to fatigue cracking. The ID surface displayed base metal chemical attack which most likely developed during a plating operation on the sealing surface during the 2004 overhaul. The pitting features act as stress raisers, thereby serving as favorable crack initiation sites. The observed ID surface conditions appear to have been created during the overhaul in 2004, with the fatigue cracking occurring once the cylinder was returned to service. The cracking in the cylinder wall was not detected during the 2007 overhaul. Spectrochemical analysis and hardness testing confirmed that the cylinder was fabricated from the specified material, in the required heat treat condition. The cylinder exhibited slightly elevated bulk hydrogen levels, likely caused by the multiple plating and stripping operations during the 2004 overhaul. The condition may have provided a minor contribution to the propagation of the fatigue crack after initiation had occurred.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: The failure of the MLG was due to cracking in the cylinder wall resulting from a fatigue mechanism which initiated at, and near the inner diameter (ID) surface, with propagation through the wall toward the outer diameter(OD) surface. Crack initiation was due to base metal damage which took the form of over tempered martensite (OTM) and chemical attack/pitting of the base metal. Full narrative available
Index for Jun2008 | Index of months