Boeing 737-3B7 N506AU incident near Charlotte (Clt), North Carolina

Probable cause

The failure of the left hand main landing gear was caused by heat induced cracks resulting from abusive grinding/polishing of the chrome plate that was applied during the last overhaul.

NTSB analysis

On July 15th, 2009, US Airways reported that the Left Hand (LH) Main Landing Gear (MLG) from the incident airplane was found to have a hydraulic fluid leak through a pin hole on the forward side of the outer cylinder. The leaking gear was sent to AAR Landing Gear Services (LGS) where ultrasonic inspection found crack indications approximately 31 inches from the open end of the outer cylinder. The main landing gear was disassembled and 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 in Seattle Washington for fracture analysis.

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 ID of the cylinder. AAR performed the

last overhaul and it was reported that at that time, corrosion was found and removed from the working ID section of the cylinder in accordance with the component maintenance manual (CMM). The oversized ID surface was then chrome plated and ground to the design diameter.

AAR overhauled the LH MLG in December 2008, and it was installed in the incident airplane in April 2009. The subject gear accumulated a total cycles of 40,639 (379 since last overhaul) and a total hours of 59,837 (606 since last overhaul).

A crack measuring 1.95” was present on the working ID on the forward side of the outer cylinder. Several microcracks of varying lengths were also found present on this region. The primary crack was made up of three cracks that had linked together. The cracks originated from heat induced cracks that were present on the ID surface and propagated primarily by fatigue towards the outer diameter (OD) surface of the cylinder. The microcracks that were present on the region originated and propagated in a similar manner. The heat induced cracks were the result of abusive grinding/polishing of the chrome plate that was applied during the last overhaul.

Text from the NTSB. Open the NTSB docket for ENG09IA009 for the full record.