HUNTINGTON, WV, USA
N479AU
FOKKER F28-4000
DURING THE LANDING ROLL AFTER TOUCH DOWN, THE FLIGHT CREW OF THE COMMERCIAL PASSENGER JET NOTICED A VIBRATION THAT INCREASED IN SEVERITY, AS THE AIRPLANE DECELERATED. THIS WAS FOLLOWED BY THE AIRPLANE TILTING AND YAWING LEFT. THE PILOT-IN-COMMAND APPLIED RUDDER AND BRAKING, AND STOPPED THE AIRPLANE NEAR THE CENTER OF THE RUNWAY. EXAMINATION OF THE LEFT MAIN LANDING GEAR (MLG) REVEALED THE UPPER TORQUE HAD FAILED, WHICH ALLOWED THE SLIDING STRUT TO PIVOT, AND THE SUBSEQUENT SEPARATION OF BOTH LEFT MLG WHEELS. A TOLERANCE CHECK OF THE UPPER AND LOWER TORQUE LINKS REVEALED THE CLEVIS LUG AND THE PINTLE PIN CLEARANCES WERE OUT OF SPECIFICATIONS. SEVERAL SMALL FATIGUE CRACKS WERE OBSERVED IN THE VICINITY OF THE TORQUE LINK FRACTURE. INSPECTION CRITERIA REQUIRED THE MLG AND TOLERANCES TO BE CHECKED EVERY 1,150 HOURS, DURING THE 'B' CHECK. THE LAST INSPECTION OF THE LEFT MLG OCCURRED 475 HOURS PRIOR TO THE ACCIDENT. THE AIRPLANE RECORDS REVEALED A RECENT HISTORY OF LANDING GEAR VIBRATIONS.
On March 4, 1995, at 1405 eastern standard time, a Fokker F28-4000, N479AU, operated by USAir, received minor damage when both wheels of the left main landing gear separated during the landing roll at the Tri-State/Milton J. Ferguson Field (HTS), Huntington, West Virginia. The Airline Transport rated pilot, 3 other crewmembers, and 17 passengers were not injured. Visual meteorological conditions prevailed, and an instrument flight rules (IFR) flight plan had been filed for the flight conducted under 14 CFR Part 121. The flight originated at Pittsburgh International Airport (PIT), Pittsburgh, Pennsylvania, as USAir Flight 1028, to HTS. According to the First Officer's (FO) written statement, the flight from PIT to HTS was uneventful, and he was at the controls during the approach and landing. Immediately after the main gear touched down, the FO noticed a vibration that increased in severity as the airplane decelerated. He further stated: ...Suddenly the aircraft tilted and yawed to the left. Immediately the Captain announced that he had control of the aircraft. I felt him make appropriate rudder and braking inputs as I relinquished control. I continued to monitor the aircraft during deceleration. The aircraft came to a stop on the runway. The airplane was stopped in the center of runway 12, and the crew and passengers deplaned via the airplane airstair door without injury. According to a Federal Aviation Administration (FAA) Inspector, tire marks from the left main landing gear (MLG) extended for approximately 4/10 of a mile down the runway. At that point the rubber marks stopped and metallic scraping on the runway continued for another 750 feet. Examination of the airplane revealed that both wheels of the left MLG assembly were separated from their axles. The outboard wheel rolled off the end of the runway, while the inboard wheel remained within 20 feet of the strut. An inspection revealed that the upper torque link of the MLG assembly had failed, which allowed the lower sliding strut and wheel assembly to pivot. The lips of the outboard and inboard wheel retaining nuts were failed, which allowed the wheels to separate from their axles. The upper and lower torque links, upper and lower hinge pins, and the center hinge pin were examined and tolerances measured. All tolerances were within the minimum and maximum prescribed, except as follows: Lower Torque Link Clevis Lug Joint - Sideplay was .013 inches beyond design limits. The diametrical was .001475 inches beyond the allowable in-service wear limits, between the MLG sliding member bushing and the lower torque link pin. Main Fitting Forward Pintle Pin Joint - Diametrical was .0002 inches beyond the in-service wear limit given for the bushing I.D. Main Fitting Side Stay Attach Lugs - Sideplay was .0005 inches beyond design limits, and diametrical was .0006 inches beyond in-service wear limits given for the bushing I.D. The digital flight data recorder (DFDR) was removed from the airplane, read out by USAir, and then sent to the National Transportation Safety Board for readout and evaluation. The NTSB readout of the DFDR indicated that the airplane touched down at approximately 113 KIAS, on a magnetic heading of 115 degrees. The maximum vertical acceleration during the touchdown was a plus 1.22 Gs, and the initial maximum longitudinal acceleration was a negative 0.24 Gs. The magnetic heading increased to approximately 116 degrees after touchdown, and then varied between 114.4 and 115.4 degrees as the airspeed decreased to 87 KIAS. The magnetic heading increased to 118.0 degrees, then back to 117.3 degrees, where at an airspeed of 69 KIAS, the airplane rolled about 6 degrees left wing down. The 31 prior takeoffs and landings recorded on the DFDR were reviewed. The examination revealed that the maximum vertical acceleration experienced during those flights occurred during the eighth landing prior to the incident landing, and consisted of a 1.46 G initial touchdown peak. The heading variation during all prior landings was less than 2 degrees either side of the nominal rollout heading. According to the NTSB Metallurgist's Factual Report, it stated: ...The fracture face had a brittle appearance and contained numerous areas of small relatively flat, thumb nail shaped regions located adjacent to the surface of the radius. Magnified examination of these areas revealed crack arrest markings typical of fatigue cracking...Secondary fatigue cracks, intersected by the fracture were located on both sides of the primary crack...Further examination of the radius in the upper torque link revealed a number of additional small cracks... The left MLG assembly had accrued 19,533 hours, and 24,787 cycles since new; and 8,288 hours, and 9,345 cycles since last overhauled. The failed torque link had accrued 21,290 cycles since new, and 3,121 cycles since overhauled. The life limit for the torque link was mandated at 45,000 cycles. The torque link was overhaul time limited to 18,000 cycles between overhauls. Several time based maintenance inspections had been performed on N479AU. The most frequent inspection of the MLG torque links occurred during the B check, every 1,150 hours. The last B check occurred on December 12, 1994. The airframe had accrued 475 hours since the last inspection. According to the USAir maintenance records, the following entries concerning MLG vibrations were noted since the last B check: January 27, 1995; Crew reported aircraft vibrated upon landing. Found right gear side stay with excessive play. January 30, 1995; Removed and replaced right main landing gear and side stay. February 11, 1995; Crew reported a vibration in the right main gear on landing. Inspection revealed cut in inboard tire, and hydraulic fluid on gear assembly. Replaced number three and four wheel assemblies. February 13, 1995; Crew reported vibration on right main gear on landing. Performed center joint end float adjustment on left MLG, and adjusted from .011 inches down to .001 inches. USAir issued a post accident Campaign Directive to inspect the entire USAir F28-4000 fleet for proper clearances in the areas of the MLG. This included the torque link apex joint and, the upper and lower torque link clevis end joints. The inspection revealed that 13 of 34 torque link apex joints were beyond the allowable end-float clearance of 0.002 inch. According to a Fokker document which listed all torque link failures in the F28 Series since 1976, the failure of the upper torque link was the first reported occurrence in the F28-4000 Series. As a result of previous failures of the upper and lower torque links in the F28-1000 Series, a Fokker message was issued advising operators to install shimmy dampers on the 1000 Series MLG assemblies. As a result of the incident, USAir has adopted a more frequent inspection program of the MLG torque link apex joint.
THE FATIGUE FRACTURE OF THE LEFT MAIN LANDING GEAR UPPER TORQUE LINK.
Source: NTSB Aviation Accident Database
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