Incident Discovery
On July 15, 2009, at approximately 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 main landing gear (MLG) of airplane N506AU, a Boeing 737-700. The airplane was at Charlotte/Douglas International Airport (CLT) and was not loaded. No other damage was found.
Inspection and Disassembly
The landing gear was sent to AAR Landing Gear Services (LGS). An ultrasonic inspection detected crack indications approximately 31 inches from the open end of the outer cylinder. After disassembly, magnetic particle inspection revealed a long, sharp indication through the chrome plating in the working inner diameter (ID) section. The outer cylinder was then sent to Boeing Material and Process Technology for fracture analysis.
Fracture Analysis
Visual examination of the forward side of the cylinder showed two crack indications on the outer diameter (OD) surface, located about 31 inches from the open end and oriented approximately 45 degrees from the centerline. Boroscope examination of the ID surface found a crack indication matching the location of the OD cracks. A section containing the crack was excised. Magnetic particle inspection confirmed a primary crack approximately 1.95 inches long, along with several microcracks in the same area. No cracks were found on the aft ID surface. Barkhausen testing showed high readings on the forward ID surface, indicating overheating that altered residual stress. No abnormal readings were found on the aft area.
Further sectioning revealed that the primary crack resulted from three separate smaller cracks that had linked together. All three initiated from the ID surface and propagated toward the OD surface. Two had propagated through to the OD surface, while the middle crack had propagated about 0.16 inches without reaching the OD surface.
Scanning electron microscope (SEM) examination showed that all three cracks originated from multiple sites along the ID surface with morphologies consistent with heat-induced cracking. Beyond the initiation sites, crack propagation was primarily due to fatigue. Adjacent microcracks showed similar initiation and propagation.
Metallurgical Findings
Chrome plating was removed from the forward section containing the primary crack. Nital etching revealed numerous indications of overtempered martensite (OTM), which were circumferentially oriented. These indications were consistent with grinding burns resulting from grinding/polishing typically performed after chrome plate application. Heat-induced microcracks were also found on some OTM bands.
Hole drilling measurements assessed residual stress on both forward and aft ID surfaces. The aft side showed evidence of shot peening. On the forward side, measurements taken on an OTM band (hole A) and in an area without OTM (hole B) indicated that the forward ID surface had also been shot peened. However, heat generated during abusive grinding/polishing of the chrome plate had lowered the compressive stress. In OTM regions, the heat effectively removed the compressive layer and introduced a layer of high tensile stress, reducing fatigue resistance to an unacceptable level.
Electron probe microanalysis (EPMA) and hardness testing confirmed that the cylinder material met engineering drawing requirements.
Overhaul History
The outer cylinder had undergone three overhauls. The first two were performed by US Airways, during which no rework was done on the working ID section. The last overhaul was performed by AAR in December 2008. During that overhaul, corrosion was found and removed from the working ID section in accordance with the component maintenance manual (CMM). The oversized ID surface was then chrome plated and ground to the design diameter per standard operating procedures manual (SOPM) 20-42-03. The gear was installed on the aircraft in April 2009. At the time of the leak discovery, the gear had accumulated 40,639 cycles (379 since last overhaul) and 59,837 hours (606 since last overhaul).