The UK’s Air Accidents Investigation Branch (AAIB) released a final investigative report on a Ryanair Boeing 737-800 that lost one of its nose wheels during takeoff. Investigators determined that heat-induced cracking originated three years before, during post-chrome-plate grinding.
Boeing 737 nose landing-gear axles were the subject of a 2014 service letter addressing heat damage that occurred during original manufacture. The manufacturer recommended that the inner cylinders be inspected at first overhaul, either by Nital etch inspection—which requires stripping and reapplication of the chrome plating—or by a non-destructive Barkhausen inspection.
The operator directed the maintenance provider to conduct the recommended inspection, which found no evidence of original manufacture heat damage. However, the problem presented again when, subsequent to the Nital etch inspection, investigators say abusive grinding occurred during the replating process. Subsequent inspections failed to detect the overheating, and the part was returned to service.
While a root cause for the abusive grinding could not be determined, investigators concluded that the fatigue could have been avoided if the maintenance provider had conducted a Barkhausen inspection instead, since the non-destructive test alleviates the need to strip and reapply plating.
According to the report, the effectiveness of the Barkhausen inspection is dependent on having a quality reference standard for calibration purposes, and the manufacturer does not make the standards available to third-party maintenance providers. “If the aircraft manufacturer were to make and supply more of the new reference standards, or supply MROs with the detailed instructions to make them, then this could increase the effectiveness of the Barkhausen inspections worldwide,” the AAIB says.
As part of the investigation, the manufacturer “loaned” one of its reference standards to Ryanair’s maintenance provider, which used it to conduct subsequent inspections of all landing gears potentially affected by abusive grinding. As of the report’s publication, nine of the 12 aircraft had been inspected and found in satisfactory condition.