CHICAGO, IL, USA
N766NC
McDonnell Douglas DC-9-51
A DC-9-51 sustained a left outboard main (#1 main) wheel and tire assembly separation on takeoff. That wheel impacted and dislodged the airport's perimeter wall and impacted a vehicle on a nearby road. The wall panels impacted another vehicle. No injuries were reported. The flight landed without incident at its intended destination. That tire assembly was found pressurized and its hubcap was found intact. Its axle nut was found intact under the hubcap. The interior portion of the #1 axle was examined and its internal threads were found intact. The anti-skid transducer adapter was found loosened inside the axle's threads and backed out 4 1/2 turns. A gap was designed to exist between the adapter and the nut. The axle nut was found to contact the adapter when the adapter was backed out to the position as found. The tire was changed on October 1, 1999. The mechanics installed the wheel in accordance with manual 32-40-1. That manual was reviewed and its steps did not caution mechanics to check for the proper depth of the adapter. DC-9-20 through 40 series airplane's anti-skid transducer assemblies were initially secured in position through safety wire. Service Bulletin (SB) 32-111 was issued to incorporate a set screw method in lieu of safety wiring the anti-skid transducer in place. This airplane's SB 32-111 modification was incorporated prior to its delivery to the airline. Subsequent to the incident, the manufacturer issued temporary service bulletins to the affected airplane's 32-40-01 maintenance manuals, cautioning mechanics to check the depth dimension of the transducer adapter with reference to the applicable technical data.
On October 14, 1999, about 1355 central daylight time, a McDonnell Douglas DC-9-51, N766NC, operated as Northwest Airlines (NWA) flight #1412 to Detroit, Michigan, piloted by an airline transport rated Captain and an airline transport rated First Officer, sustained a left outboard main (#1 main) wheel and tire assembly separation on takeoff from runway 22L at Chicago Midway Airport (MDW), Chicago, Illinois. The left inboard main (#2 main) wheel and tire assembly remained attached. The 14 CFR Part 121 commercial flight operated on an IFR flight plan. Visual meteorological conditions prevailed at the time of the incident. The 2 flight crewmembers, 1 person in the cockpit jumpseat, 4 flight attendants, and 98 passengers were uninjured. The airplane's separated wheel and tire assembly impacted, damaged, and dislodged panels that make up MDW's southwest perimeter wall. That wheel and tire assembly also impacted a vehicle westbound on 63rd Street. The dislodged perimeter wall panels impacted another westbound vehicle on 63rd Street. The driver of the first vehicle was taken to the hospital for observation. The driver and two occupants of the second vehicle reported no injuries. The flight originated at the time of the incident and landed at Detroit Metropolitan Wayne County Airport, near Detroit, Michigan about 1604 eastern daylight time without further incident. An on-scene inspection revealed an impact mark consistent with the inboard cone bearing deformed cage's shape and roller bearing spacing, on the threshold bar marking defining the start of runway 4R's usable runway. The impact mark was five feet northwest of runway 4R's centerline. The bearing's race, marked with part number (p/n) 71450, was found 308 feet southwest of runway 4R's threshold. The bearing's cage was found 373 feet southwest of runway 4R's threshold. Twenty-three tapered roller bearings were found spread on the runway's prepared surface up to 360 feet southwest of the cage's location. The bearing components were found coated with a grease like substance and revealed no pre-impact anomalies. See appended wreckage diagram. An impression, approximately 26 feet long, was found in the grass area between 4R's prepared surface and a service road that parallels 63rd Street. The heading of the impression was 215 degrees. That impression heading led to the airport's perimeter wall. In line with that impression was a channeled fence post. The post was found dislodged from its frangible coupling at its base. Two wall panels were found dislodged from their channels. A vehicle, which traveled westbound on 63rd Street, was found with dents on its passenger side front quarter panel, hood, front bumper, and on its passenger side rear quarter panel. The front and the rear impression damage areas were semi-circular in shape. An airport incident report showed that a second vehicle, which also traveled westbound on 63rd Street, was found with damage to its front bumper from a section of the airport's perimeter wall. A tire and wheel assembly, p/n 956-0859 marked serial number (s/n) 00895, was found resting south of 63rd Street's eastbound lanes. The tire exhibited discoloration from substances transferred to its surface. The tire had abrasions on its outboard surface and abrasions and cuts on its inboard surface. The tire was found pressurized and was deflated for safety. The assembly's hubcap was found intact. The hubcap was removed which revealed a yellow colored axle nut, in-place, and safety wired. The axle nut held an axle nut spacer/retainer in-place. The safety wire had to be cut to free the spacer/retainer. The inboard flat surface of the axle nut was found distorted with areas having a polished appearance. The spacer/retainer, marked EM3241-01048/SB32-228, was found in-place. The spacer/retainer was found holding the outboard cone bearing grease seal's tangs in its slots. The spacer/retainer anti-rotation pin was found with a groove on its inner surface and the interior surface of the spacer/retainer had areas having a polished appearance. The grease seal and outboard cone bearing were found with no anomalies. The spacer/retainer anti-rotation pin, when engaged inward in the keyway machined into exterior surface of the axle's 9 o'clock position when viewed on end, prevents the rotation of the axle nut by lockscrews fastened to the face of the spacer/retainer. The airplane was examined upon landing at Detroit Metropolitan Wayne County Airport, near Detroit, Michigan, by Federal Aviation Administration (FAA) inspectors. The examination revealed no damage to the exterior portion of the #1 axle, fuselage, left flap, #1 engine cowl, and to the left fly door. The interior portion of the axle was examined and the internal threads were found intact. The anti-skid transducer adapter was found loosened inside the axle's threads and was found backed out from its specified seated position. The FAA inspector rotated the adapter 4 1/2 turns in a clockwise direction to its correct position. The #1 tire and wheel assembly rolled and rotated in the direction of loosening or counterclockwise rotation while the airplane is in forward motion on the ground. The anti-skid transducer adapter and the axle nut used the axle's same internal threads. A gap was observed in the DC-9's maintenance manual figures between the adapter and the nut when the adapter is installed at its specified position. The axle nut was found to contact the anti-skid transducer adapter when the adapter was backed out to the position it was found in after landing at Detroit. The airplane's maintenance records were reviewed. The incident tire was changed on October 1, 1999, in Minneapolis, Minnesota. The airplane accumulated 22:45 flight hours and 20 cycles between October 1, 1999, and October 14, 1999. The two mechanics' statements state that the tire assembly was installed in accordance with maintenance manual 32-40-1. The Northwest Airlines DC-9 maintenance manual, "Main Gear Wheel and Tire Assembly - Maintenance Practices", 32-40-1 was reviewed. The manual's section 3B directed the wheel and tire installation procedures. This section's 17 installation steps were reviewed and the steps did not note, caution, warn, or list checking the security and depth of the anti-skid transducer adapter within the axle for proper installation. The manufacturer's maintenance manual 32-40-1 was reviewed along with the United States Air Force's and United States Navy's equivalent DC-9 technical data. The three manuals list 16 installation steps and also did not note, caution, warn, or list checking the security and depth of the anti-skid transducer adapter within the axle for proper installation. Manufacturer's Service Bulletin (SB) 32-111 states that on DC-9 20, 30, and 40 series airplanes, that the anti-skid transducer was secured through a "lockwire" connection between the anti-skid transducer and the axle nut lockscrew. Manufacturer's Service Bulletin (SB) 32-111 was issued to stop safety wiring the anti-skid transducer to the axle nut lockscrew for an alternative set screw method of securing the transducer's rotation. The reasoning listed in SB 32-111 is "...to reduce the time required to change wheels and tires." This airplane's SB 32-111 modification was incorporated prior to its delivery to the airline. Subsequent to the incident, the manufacturer issued temporary service bulletins to the affected airplane's 32-40-01 maintenance manuals, cautioning mechanics to check the depth dimension of the transducer adapter with reference to the applicable technical data.
The landing gear's wheel separation due to the improper installation of the wheel by company maintenance personnel using incomplete maintenance steps, and the maintenance steps not listed in the manufacturer's manual. Factors were the airport perimeter wall and the vehicles.
Source: NTSB Aviation Accident Database
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