CHI00IA018

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.

History of flight

On October 28, 1999, at approximately 2132 central daylight time, a Boeing B757-222, N575UA operated by United Airlines (UAL) as Flight 225, experienced an airframe vibration and diverted to O'Hare International Airport, Chicago, Illinois, for a precautionary landing. The 14 CFR Part 121 revenue flight had departed Dulles International Airport, Herndon, Virginia, at 1943 eastern daylight time with San Francisco International Airport, San Francisco, California, as the destination airport. While in cruise flight, a series of airframe vibrations occurred. The airspeed was reduced to 250 knots and the decision to divert to O'Hare was made. The landing was uneventful. None of the 2 pilots, 4 flight attendants, or 62 passengers were injured. Visual meteorological conditions prevailed and the flight was on an IFR flight plan.

After the incident, the Captain made the following write-up in the airplane's maintenance logbook:

"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. Two similar write-ups Oct. 17 and 21. Called SAMC [Systems Aircraft Maintenance Control]. They said they did not know what was wrong and that we might have some loose parts in the tail. Dispatch indicated Lt. to Mod. turbulence ahead over Nebraska and Colorado. Their advice was to divert to ORD. I agree and we diverted to ORD."

The First Officer reported the incident in the following manner:

"We left IAD on time at approximately 19:45. The departure and climb-out were normal. However, shortly after leveling out at FL 350 we felt a moderately strong vibration and were advised by the flight attendants that the entire cabin had felt it as well.

The most accurate way to describe it would be to say that it felt as if something in the tail section was moving, causing the front section of the plane to oscillate very rapidly up and down.

We disconnected the center autopilot and descended to FL 310, slowing to approximately mach .78. The vibration stopped temporarily, returning a few minutes later. We tried the left autopilot, and the vibration occurred with it as well, and did not stop until we descended and slowed later. [The Captain] requested that I handfly while he got on the radio with SAM, SFO Engineering, and Chicago Dispatch. We called a UAL pilot from the back to come up to the cockpit and help us out. At this point we were passing abeam ORD on our flight planned route. After [the Captain] talked with maintenance in San Francisco and was advised we might have 'a loose part in the tail,' and considering possible moderate turbulence past Denver, we elected to divert to Chicago. ATC gave us an immediate right turn and vectored us to ORD, giving us priority handling without declaring an emergency. We requested emergency equipment standby for the landing, which turned out uneventfully. We released the equipment upon rollout, and after debriefing maintenance at the gate, we exchanged planes and continued on into San Francisco."

During an interview, the Captain related the events concerning the in-flight vibration incident. He reported that the departure and climb out from Dulles were normal. The airplane leveled off at 35,000 feet with the center autopilot (AP) engaged. About 20 to 25 minutes after departure and south of Pittsburgh, the airplane experienced flutter (Captain's word). He reported that he clicked the center AP off and slowed the aircraft. He descended to 31,000 feet and stabilized at 31,000 feet.

He reported that he increased the airspeed with the left AP engaged. A short period later the flutter came back. He did not know what speed the airplane was flying. He reported that the flutter came back for 5 to 10 seconds. During that time, the Captain reported, he turned the left AP off and engaged the right AP. The flutter continued. He turned the right AP off and flew the airplane by hand. The flutter continued. He reported that when he reduced airspeed the flutter stopped. He reported that he thought the airspeed was controlling the vibration. He reported he had reduced the airspeed to about .78 mach or about 250 kts.

He reported that the vibration was felt throughout the airplane, but that it was not a violent vibration. He reported the vibration was a vertical vibration caused by the elevators.

He reported the flight attendants and the dead heading pilot felt the vibration. He characterized the vibration as what you would feel when taxiing over the bridge at O'Hare Airport. He felt the aircraft was completely sound.

He reported that he checked the flight manual, but could not find a procedure that applied to the situation.

He reported that he checked the aircraft logbook for previous write-ups. He found that the write-up from 8/21/99 seemed to be similar.

He contacted SAMC for their assistance. He described the problem and SAMC said they would check with engineering and get back to them. The Captain reported that it took SAMC about 20 to 30 minutes to call back.

SAMC and Dispatch contacted the flight. SAMC informed the Captain that there might be "...loose parts in the tail," and that some bushings or bearing might have more play than normal. During the three-way discussion between the Captain, SAMC, and Dispatch, it was decided that the flight should divert into Chicago.

During the interview, the Captain was asked if he had any theories concerning the vibration. He responded that he thought it was related to the airspeed. When the aircraft was slowed, the vibration stopped.

During the interview, the Captain was asked if he would do the same thing under similar circumstances. He responded that he would. He reported that there were no specific procedures for this type of occurrence, and that he would rely on his experience and airmanship.

During the interview, the Captain was asked why he tried all three AP's. He responded that he suspected the vibration was related to the AP. He was making a quick check of each AP since they are stand-alone systems.

During the interview, the Captain was asked if he had considered landing at Dulles since the flutter occurred within the first 20 to 25 minutes of a flight that lasted approximately 1.5 to 1.8 hours. He responded that he did not think it was necessary.

Aircraft information

The airplane was a twin engine Boeing B-757-222, serial number 26689, manufactured in 1993. The airplane seated 196 and its maximum gross weight was 241,000 pounds. The engines were Pratt and Whitney PW2037 turbofan engines that delivered 37,000 pounds of thrust each. The last inspection was on November 16, 1998, and was part of the continuous maintenance inspection program. The airplane had accumulated 3,160 hours since the last inspection and had a total time of 23,631 hours.

Tests and research

UAL Maintenance and Test Flights

A maintenance inspection was performed on N575UA at the UAL maintenance hangar at O'Hare Airport. The free play of the elevator was found to be out of limits. It measured 0.5 inch and the allowable limit was 0.340 inch. Parts that were replaced included bushings, the left center power control actuator (PCA), two reaction links, and four idler links.

The first test flight was conducted on October 31, 1999. The airplane failed the test flight since it still exhibited a vertical vibration throughout the airplane. The logbook test pilot narrative write-up read as follows:

"ACFT fluttered noted at FL350 and .81M with CTR AP engaged, disengaged CTR AP, selected L & R AP's fluttering continued, turned AP's off and fluttering continued. Descended to FL310 and reduced speed to 250 kts and fluttering subsided. Fluttering occurred in pitch motion, all trim indications normal per crew. [name deleted]