NTSB Identification: CHI00IA018.
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Scheduled 14 CFR (D.B.A. United Airlines)
Incident occurred Thursday, October 28, 1999 in CHICAGO, IL
Probable Cause Approval Date: 06/03/2002
Aircraft: Boeing 757-222, registration: N575UA
Injuries: 68 Uninjured.
NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.
The pilot reported, "We encountered substantial vertical control flutter FL310-350 .78-.80 Mach with left, center, right autopilot and no autopilot. Flutter stopped when IAS was decreased. Descended to FL310, 250 kts., .75 Mach and had no more flutter. The flight diverted to O'Hare Airport and made an uneventful landing. Maintenance inspections revealed excessive free play in the elevator flight controls. The allowable limit is 0.340 inch and 0.50 inch was measured. The free play was corrected in accordance with the current maintenance manual. The bushings, the left center power control actuator (PCA), two reaction links, and four ilder links were replaced. During a subsequent test flight, vertical oscillations were still present. Further maintenance action included replacing all elevator hinge bearings and four PCA's. The third flight test was successful and the airplane was returned to service. Inspection of two reaction link rod ends revealed axial and radial wear which exceeded allowable limits. Inspection of the 16 hinge bearings revealed 3 bearings had axial and radial wear which exceeded limits and had moderate to severe corrosion. On October 11, 2001, the FAA issued Airworthiness Directive (AD) 2001-20-11 that was applicable to all B-757 series airplanes to "prevent unacceptable airframe vibration during flight, which could lead to excessive wear of elevator bearings and result in reduced controllability of the airplane…" The AD required, "Before further flight after the free-play checks, lubricate the bearings in the elevator PCA load loop and hinge line." Lubrication of the hinge bearings had not been required in a previous AD. The new AD required recurrent inspections every 18 months instead of the previous requirement of 4,000 hours, or 3,000 hours for some models. On January 28, 2001, Boeing issued a maintenance manual revision that corrected improper instructions for elevator free-play checks. The improper procedures in the maintenance manual had been in effect from September 20, 1997, to January 28, 2001.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: the in-flight vibration of the airplane's elevator due to excessive freeplay in the elevator control system. Factors included the corrosion of the elevator PCA reaction link bearings and the elevator hinge bearings, excessive axial and radial wear of the elevator PCA reaction link bearings, the inadequate procedures for lubricating the elevator hinge bearings, and the incorrect procedures for measuring elevator free play. Full narrative available
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