NTSB Factual Report

On September 22, 2002, at 1852 central daylight time, a McDonnell Douglas DC-9-32, N941N, owned and operated by Northwest Airlines as flight 1842, was substantially damaged when the right main landing gear collapsed during taxi at the Minneapolis-St. Paul International/Wold-Chamberlain Airport, Minneapolis, Minnesota. The 14 CFR Part 121 passenger flight was taxiing for takeoff and was bound for the Charlotte/Douglas International Airport, Charlotte, North Carolina. There were no injuries to the 81 passengers or 4 crewmembers.

The airplane departed from gate D3 and was taxiing on taxiway A to runway 30L when the right main landing gear collapsed. The airplane came to rest on taxiway A abeam gate F10. The right main landing gear outer-cylinder was found fractured into three pieces about 19 inches from the upper end of the cylinder. The fracture surface was visually examined and a dark spot was observed on the forward side the fracture surface of the cylinder. The cylinder was then removed from the aircraft and shipped to the National Transportation Safety Board's Materials Laboratory in Washington, D.C., for further examination.

The airplane, serial number 47450, was manufactured in 1969 and had accumulated 85,323 hours time in service, which included 74,121 landing cycles. The right main landing gear had accumulated a total of 105,099 landing cycles. The operator reported that the right main landing gear was last overhauled on April 22, 1995, and had accumulated 11,275 landing cycles in 18,446 hours time in service.

Laboratory examination of the fracture surface revealed that the fracture originated from the dark spot that was previously observed. This dark spot was examined using various methods and was found to have several areas with differing fracture features. The first region, centered on the fracture origin, showed intergranular fracture features. The first region was surrounded by a thin band, region 2, that exhibited transgranular fracture features that resembled fatigue progression in steel materials. Region 2 was also surrounded by a third region that again exhibited intergranular fracture features. Another band, region 4, surrounded region 3 and exhibited fatigue progression features. Outside of these regions, the fracture faces exhibited features consistent with overstress fracture.

A metallographic section of the upper surface of the fracture was made. The section was cut and polished perpendicular to the fracture face so that the polished surface intersected region 1. Multiple sulfide inclusions were found in the sectioned sample. Similarly, a section was made of the lower surface of the fracture, and a 0,012-inch long line of inclusions was found. This line of inclusions was oriented perpendicular to the fracture region and parallel to the outer surface of the cylinder. The inclusions were determined to be composed mainly of oxides of aluminum and calcium with others containing sulfides of iron and manganese.

Scanning electron microscope examination of the metallographic samples determined that the material was consistent with the material specified on engineering drawings for the part. Additionally, hardness testing indicated that the strength levels were also consistent with the engineering specification.

Additional testing was performed under the direction of the NTSB on samples from the landing gear of N941N and N8986E. N8986E is a McDonnell Douglas DC-9-31 that was damaged when its landing gear collapsed during landing at MSP (NTSB accident number CHI02FA148). The tests performed included dissolved hydrogen content, residual stress, tensile properties, notched tensile strength and sustained load tests. The results of the testing are summarized below.

Hydrogen content ranged from 1 to 5 ppm for both cylinders. Each cylinder was tested at three locations near the fracture.

Both cylinders had compressive stress layers at the outer surfaces consistent with shot peened surfaces. The residual stress profiles showed maximum compressive stresses of about 200 to 250 ksi transitioning to residual tensile stresses between 0.01 and 0.02 inch below the surface. Both cylinders were tested at two locations, one near the dark spot at the origin of the fracture and another remote from the dark spot. No significant differences were noted between locations.

Tensile tests of material from both cylinders met engineering drawing and specification requirements for tensile strength, yield strength, elongation and reduction in area.

Notched tensile strength was about 325,000 psi for both specimens. This property is not controlled by drawing or specification but is used in the sustained load tests.

Specimens from both cylinders passed 200 hour sustained load tests without failure.

Samples of both cylinders were tested for cleanliness by magnetic particle inspection. No significant inclusions were detected and both exceeded the requirements for cleanliness.

Parties to the investigation were the Federal Aviation Administration, Northwest Airlines, The Boeing Company, Air Line Pilots Association, and the Aircraft Mechanics Fraternal Association.

NTSB Probable Cause

The fatigue failure of the main landing gear strut due to inclusions in the material, residual stresses, and dissolved hydrogen content.