Boeing 757-222 N575UA

Summary

On 29 October 1999 at about 02:32 local time, a Boeing 757-222 registered N575UA was involved in an incident near Chicago, Illinois, United States. 68 people were on board and nobody was injured. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

29 October 1999 at 02:32

Classification

Incident

Location

Chicago, Illinois, United States

Coordinates

Not recorded

Aircraft

Boeing 757-222

Registration

N575UA

Category

Airplane

Engines

2

Operator

Not recorded

Operating rule

Part 121: Air Carrier

Purpose of flight

Unknown

Phase of flight

Not recorded

Route

Herndon → San Francisco

Aircraft damage

Not recorded

Weather

VMC

Light

Night

NTSB number

CHI00IA018

Nobody was injured.

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.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

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.

Quoted verbatim from the NTSB record.