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On June 3, 2002, at 1928 central daylight time (cdt), a Douglas DC-9-31, N8986E, owned and operated by Northwest Airlines, sustained substantial damage when the right main landing gear (RMLG) collapsed during landing rollout on runway 12L (8,200 feet by 150 feet, concrete) at the Minneapolis-St. Paul International Airport (MSP), Minneapolis, Minnesota. The flight was being operated under the provisions of 14 CFR Part 121 as Northwest Airlines flight 877. Visual meteorological conditions prevailed at the time of the accident. The captain, first officer (FO), 2 flight attendants, and 66 passengers reported no injuries. The flight departed Louisville International Airport (SDF), Louisville, Kentucky, at 1836 eastern daylight time and was on an active instrument flight rules (IFR) flight plan.
According to the captain's written statement, the FO was the flying pilot during the accident flight. The captain reported that the instrument landing system (ILS) runway 12L approach was flown on autopilot until approximately 1,000 feet above ground level (agl) when the airplane was below the overcast cloud layer. The remainder of the approach was hand-flown by the FO and the aircraft touched-down approximately 1,000 feet from the end of the runway. The captain reported, "The touchdown was smooth and on centerline. Both mains [main landing gear] touched the runway at the same time." The captain stated that the FO applied reverse thrust after the nose landing gear made contact with the ground. The captain reported that he took over control of the aircraft at approximately 60 knots and, "Immediately after my initial brake application the aircraft dropped sharply to the right and we began skidding down the runway with a continuous warning horn. I continued braking and the aircraft came to a stop on centerline with a significant right lean."
According to the FO's written statement, "All aspects of the approach were normal and I disconnected the autopilot at 1,000[feet] msl [above mean sea level] when I reported seeing the runway and calling 'landing'. The final portion of the approach was normal and the touchdown was a smooth roll on with both mains touching down together." The FO reported, "I began normal braking below 100 knots and received a normal 80 knots callout from the captain at which time I began reducing the thrust reversing and at the 60 knot call completely stowed the reversers. As this was being done the captain told me he was joining me on the brakes and that he had control of the aircraft at which time I told him he had control of the aircraft. As this change was occurring I felt the brakes give an initial release followed by a hard tilt to the right with a continuous warning horn and I instinctively rode the controls with the captain until the aircraft came to a complete stop on centerline after about a 1,500 foot slide."
The captain was the holder of an airline transport pilot certificate with an airplane multi-engine land rating. The captain was type rated for the Douglas DC-9 aircraft. The captain's last medical examination was conducted on May 15, 2002, and he was issued a first-class medical certificate with no limitations or restrictions.
According to company records, the captain had a total flight time of 8,043 hours, of which 884 hours were in the DC-9. The captain was reported to have flown 177 hours in the last 90 days and 8.0 hours in the last 24 hours. The captain's last currency checkride was satisfactorily completed in a Douglas DC-9 aircraft on January 01, 2002.
The FO was the holder of an airline transport pilot certificate with an airplane multi-engine land rating. The FO was not type rated for the Douglas DC-9 aircraft. The FO's last medical examination was conducted on July 24, 2001, and he was issued a first-class medical certificate with no limitations or restrictions.
According to company records, the FO had a total flight time of 1,844 hours, of which 1,844 hours were in the DC-9. The FO was reported to have flown 227 hours in the last 90 days and 8.0 hours in the last 24 hours. The FO's last proficiency check was satisfactorily completed in a Douglas DC-9 simulator on July 28, 2001.
The aircraft was a Douglas DC-9-31, serial number 47402. The Douglas DC-9-31 is a low-wing monoplane of all-metal semi-monocoque construction. The DC-9-31 has fully cantilevered wings, a T-tail empennage, and is powered by two Pratt & Whitney JT8D-7B engines, each producing 14,000 lbs of thrust. The accident airplane was configured to accommodate a maximum of 100 passengers and an 8-person flightcrew. The Douglas DC-9-31 has a certified maximum takeoff weight of 108,000 lbs and a maximum zero fuel weight of 87,000 lbs.
The accident airplane was maintained by compliance with a Federal Aviation Administration (FAA) approved continuous airworthiness program and accumulated a total time of 75,241.57 hours at the time of the accident.
A weather observation station located at MSP recorded the weather approximately 11 minutes after the accident as:
Observation Time: 1939 cdt
Wind: 090 degrees magnetic at 9 knots
Visibility: 9 statute miles
Sky Condition: 1,200 feet agl overcast
Temperature: 13 degrees Celsius
Dew Point: 11 degrees Celsius
Pressure: 29.89 inches of mercury
The solid state flight data recorder (FDR), Fairchild model F1000, serial number 01787, was removed from the accident aircraft and sent to the National Transportation Safety Board's (NTSB) laboratory in Washington, D.C., for readout and evaluation. No anomalies were noted during the touchdown and rollout until the RMLG collapsed. Landing vertical g-force peaked at 1.176, a value that is within the acceptable limits for the aircraft. Landing lateral acceleration peaked at 0.056 g, a value that is within the acceptable limits for the aircraft.
The NTSB FDR Factual Report is included with the docket material associated with this investigation.
WRECKAGE AND IMPACT INFORMATION
The aircraft came to rest 5,220 feet from the runway 12L threshold and 10 feet right of the runway centerline. The outer-cylinder of the right main landing gear strut had fractured into two sections approximately 2 inches above the designed-fuse section of the strut assembly. The lower portion of the right main landing gear (including the wheels, brakes, and hub assembly) was impacted up into the right inboard flap assembly. The outboard 1/2 of the right wing had scraping damage through the lower wing skin and into the main spar and surrounding wing structure.
The captain reported that he decided that an evacuation of the aircraft was not necessary after being notified by the control tower that there was no fire and/or smoke observed coming from the airplane. Airport crash/rescue confirmed there was no fire and/or smoke coming from the airplane and instructed the captain to shutdown both engines and the auxiliary power unit (APU). The passengers and flightcrew departed the airplane via a portable stairway located at the forward galley left-side entrance. The passengers were transported from the accident site in buses provided by Northwest Airlines. According to airport crash/rescue records, all passengers and flightcrew personnel were clear of the airplane at 2010 cdt. The flightcrew and 66 passengers reported no injuries as result of the accident and/or post-accident activities.
TESTS AND RESEARCH
The right main landing gear strut assembly was disassembled at a Northwest Airlines maintenance facility. The fractured outer-cylinder was sent to the NTSB Materials Laboratory Division in Washington, D.C., for examination.
According to the NTSB Materials Laboratory Factual Report, the outer cylinder (part number 5925371-502) of the RMLG was fractured through the transition area above the fused section, with the fracture located approximately 19 inches from the upper end of the cylinder. The report states the overall fracture "initiated on the forward side of the cylinder in a 1.5 inch machined radius located just below the forward projecting arm for the forward trunnion fitting. This area was located about 2.5 inches above the reduced diameter structural fuse area of the outer cylinder."
The factual report noted the fracture initiation point "lies in the parting plane for the cylinder forging. The dark spot [fracture initiation point] was orientated transverse to the gear about 28 7/8 inches from the lower end of the cylinder." The fracture initiation point was a "semi-elliptical shape measuring 0.1889 inches wide and extending 0.110 inches into the cylinder." The fracture initiation point was sectioned for additional examination. According to the factual report, multiple inclusions and intergranular cracks were uncovered within the examined sample. The inclusions were mostly oxides of aluminum and calcium and some inclusions contained small sulfides of iron and manganese.
The report noted there were fracture features that were consistent with overstress separation "emanating from the dark spot [fracture initiation point] with mostly ductile dimple formations and some cleavage like features."
The complete NTSB Materials Laboratory Factual Report is included with the docket material associated with this investigation.
The outer-cylinder (part number 5925371-502, serial number FA117) had a total flight time of 71,665.47 hours, consisting of 67,467 cycles. The outer-cylinder accumulated 17,866 cycles, over 21,546.47 flight hours, since the last component overhaul. Eastern Airlines completed the last overhaul in 1988 and the next overhaul was projected to be completed December 2002. Northwest Airlines reported normal overhauls are completed every 20,000 cycles or 10 years.
The accident aircraft was released back to a representative of Northwest Airlines on June 5, 2002. The fractured main landing gear strut was released back to a representative of Northwest Airlines on August 26, 2002.
Parties to the investigation included the FAA, Northwest Airlines, The Boeing Company, Air Line Pilots Association (ALPA), and Aircraft Mechanics Fraternal Association (AMFA).