DOUGLAS DC8 near Cincinnati (CVG), OH — 2010-01-07

Probable cause

Failure of the NLG cylinder occurred by intergranular rupture; coincidental to a chrome plating crack. The failure origin occurred along the inner surface of the cylinder and then propagated by stress corrosion cracking to the outer surface. A secondary crack with the same features as the primary crack was found diagonal to the primary crack. Both cracks initiated by intergranular cracking due to grinding burns caused by abusive grinding and propagated by stress corrosion cracking. Hydrogen embrittlement did not contribute to the intergranular cracking.

NTSB narrative

On January 7, 2010 Astar Air Cargo reported that during of a DC-8-73 hydraulic fluid leaking from a small crack at the upper end of the nose landing gear (NLG) cylinder. The following day, during taxi to the maintenance hangar, a loud bang was heard after a turn. Further inspection revealed that the NLG cylinder had fractured. The NLG was overhauled in April 2007 by AAR Landing Gear Services (LGS) in Miami Florida. The NLG was installed on the incident airplane on January 5, 2010 and had accumulated one ground air ground (GAG) cycle prior to the failure. The NLG assembly was removed and replaced and the failed cylinder was submitted to the Boeing Huntington Beach Failure Analysis Laboratory for failure analysis. At the time of the incident it was reported that the airplane had accumulated 86,911 flight hours and 32,212 cycles. The NLG cylinder failed due to intergranular (IG) cracking at the inner surface of the cylinder coincidental to a chrome plating crack, then propagated by stress corrosion cracking. The plating crack was determined to be cleavage fracture and could have occurred during the grinding of the chrome plating to size. The fracture surfaces of the primary and secondary crack, each exhibited a combination of dark gray and orange-brown discoloration that extended to 0.95 and 0.85 inches, respectively, and towards the outer surface of the cylinder. A short distance from the outer surface of the cylinder, there was no observable discoloration on the fracture surface of the primary crack. The discoloration indicated that the fracture occurred over an unknown period of time. Etch inspection of the inner surface of the cylinder revealed evidence of circumferential grinding burns. Light circumferential scratches were also observed however their origin could not be determined.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Inspection of NLG prior to taxi could have identified the crack.

NTSB coding

• Aircraft structural failure · Pushback/tow-engine not oper

Evidence available

• 2 docket documents

Docket documents2

• Structures 7 - AAR Landing Gear Services Engineering Reportreport
• Structures 7 - Boeing Fracture Analysis Reportreport