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On July 15th, 2009, at about 0830 Eastern Standard Time, US Airways maintenance personnel observed hydraulic fluid leaking from a pin hole on the forward side of the outer cylinder of the left hand (LH) main landing gear (MLG) of airplane N506AU, a Boeing 737-700. The airplane was located at Charlotte/Douglas International Airport (CLT), was not loaded at the time and there was no other damage to the airplane.
The subject gear was sent to AAR Landing Gear Services (LGS) where an ultrasonic inspection found crack indications approximately 31 inches from the open end of the outer cylinder. The main landing gear was disassembled and a subsequent magnetic particle inspection found a long, sharp indication through the chrome plating in the working inner diameter (ID) section of the outer cylinder. The subject outer cylinder was sent to Boeing Material and Process Technology for fracture analysis.
TESTS AND RESEARCH
Visual examination of the forward side of the cylinder revealed that two crack indications were present on the O.D. surface. The cracks were located approximately 31 inches from the open end of the cylinder and were oriented approximately 45 degrees from the centerline.
Examination of the I.D. surface of the cylinder with a boroscope found a crack indication present on an area matching the location of the cracks on the O.D. surface. A section containing the crack was excised from the cylinder to allow accurate magnetic particle inspection (MPI) and Barkhausen test. MPI confirmed the presence of a crack approximately 1.95” in length as well as the presence of several microcracks in the same general area. No cracks were found on the aft I.D. surface of the cylinder. A Barkhausen test was then performed, the whole I.D. surface on the forward side of the cylinder was found to have high Barkhausen readings. High Barkhausen readings are normally found on surfaces that have gone through some form of overheating which alters the residual stress condition. No abnormal Barkhausen readings were found on the I.D. surface of the aft area.
The aforementioned section of the cylinder was then cut along its length, separating the forward section and the aft section. The area of the forward section containing the cracks was again magnetic particle inspected to highlight the relationship of the Barkhausen readings and
MPI indication. The forward section was further cut to smaller sections to retrieve a piece that contained the primary crack. The primary crack was opened and macroscopic evaluation of the fracture surface revealed that the crack was the result of three separate smaller cracks that had linked together. All three cracks appeared to initiate from the I.D. surface and propagated towards the O.D. surface. The two on both ends of the primary crack had propagated all the way through to the surface and represented the two cracks that were observed on the O.D. surface of the cylinder. The middle crack had propagated for approximately 0.16” and had not reached the O.D. surface.
The fracture surface was cleaned and examined using a scanning electron microscope (SEM). SEM examination revealed that all three cracks that had linked together originated from multiple sites along the I.D. surface. These sites have morphologies consistent with a heat induced cracking mechanism. Beyond the multiple initiation sites, the morphology of the crack indicated that propagation was primarily due to fatigue. A couple of microcracks that were located adjacent to the primary crack were opened and also examined using the SEM. These too were found too have initiated and propagated in a similar manner as the primary crack.
The chrome plating of the forward section containing the primary crack was removed so that the condition of the base metal surface could be evaluated. Nital etching revealed the presence of numerous indications of overtempered martensite (OTM) which were circumferentially oriented. These indications were consistent with grinding burns that resulted from grinding/polishing which typically follows chrome plate application. Additionally, heat induced microcracks were also found present on some of the OTM bands.
Hole drilling measurements were performed to determine the residual stress at both forward and aft surfaces of the working I.D. of the cylinder. The data from the aft side indicated that this surface had been shot peened. The data from the forward side with hole A being taken on an OTM band, and hole B being taken in an area with no OTM. Both data suggested that the forward I.D. surface was also shot peened, however, the heat that was generated during abusive grinding/polishing of the chrome plate has lowered the compressive stress. In the OTM regions, the heat effectively removed the compressive layer and introduced a layer of high tensile stress. This condition lowers the fatigue resistance of this material to an unacceptable level.
Electron Probe Microanalysis (EPMA) and hardness testing confirmed that the cylinder material met the requirements specified in the engineering drawing.
It was reported that the subject outer cylinder had been through three overhauls, the first two being performed by US Airways. It was further reported that during the first two overhauls, US Airways did not perform any rework on the working I.D. of the cylinder. AAR performed the last overhaul and it was reported that at that time, corrosion was found and removed from the working I.D. section of the cylinder in accordance with the component maintenance manual (CMM). The oversized I.D. surface was then chrome plated and ground to the design diameter per standard operating procedures manual (SOPM) 20-42-03.
AAR last overhauled the LH MLG in December 2008, and it was installed on the subject aircraft in April 2009. The subject gear has accumulated a total of 40,639 cycles (379 since last overhaul) and a total of 59,837 hours (606 since last overhaul).