Incident Description
A pilot reported encountering substantial vertical control flutter while operating a Boeing 757 series airplane. The flutter occurred at altitudes between Flight Level 310 and Flight Level 350, at Mach numbers ranging from 0.78 to 0.80. The phenomenon was observed with the left, center, and right autopilot systems engaged, as well as with no autopilot. The flutter ceased when the indicated airspeed was reduced. The flight subsequently descended to Flight Level 310 at 250 knots and Mach 0.75, after which no further flutter was experienced. The flight diverted to Chicago O'Hare International Airport and made an uneventful landing.
Maintenance Findings and Corrective Actions
Post-incident maintenance inspections revealed excessive free play in the elevator flight controls. The allowable limit for free play was 0.340 inch, but a measurement of 0.50 inch was recorded. The free play was corrected in accordance with the current maintenance manual at that time. To address the issue, the following components were replaced: the bushings, the left center power control actuator (PCA), two reaction links, and four idler links.
During a subsequent test flight, vertical oscillations were still present. Further maintenance action included replacing all elevator hinge bearings and four PCAs. A third test flight was successful, and the airplane was returned to service.
Inspection of two reaction link rod ends revealed axial and radial wear that exceeded allowable limits. Additionally, inspection of the 16 hinge bearings found that three bearings had axial and radial wear exceeding limits and exhibited moderate to severe corrosion.
Regulatory Actions
On October 11, 2001, the Federal Aviation Administration (FAA) issued Airworthiness Directive (AD) 2001-20-11, applicable to all Boeing 757 series airplanes. The AD was intended "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, lubrication of the bearings in the elevator PCA load loop and hinge line. This lubrication had not been required in a previous AD. The new AD also mandated recurrent inspections every 18 months, replacing the previous requirement of 4,000 hours (or 3,000 hours for some models).
Maintenance Manual Revision
On January 28, 2001, Boeing issued a maintenance manual revision that corrected improper instructions for elevator free-play checks. The improper procedures had been in effect from September 20, 1997, to January 28, 2001.