N359UA

HISTORY OF FLIGHT

On December 13, 2001, at 1203 central standard time, a Boeing 737-322, N359UA, operated by United Airlines (UAL) as flight 578, declared an emergency due to a reported "rudder malfunction" during decent to the Chicago O'Hare International Airport (ORD), Chicago, Illinois. Visual meteorological conditions prevailed at the time of the incident. The 14 CFR Part 121 domestic passenger flight was operating on an instrument rules flight plan. The 2 pilots, 4 flight attendants, and 93 passengers reported no injuries. The flight departed from the Lambert-St. Louis International Airport, St. Louis, Missouri, at 1129, en route to ORD.

The first officer stated that while descending through 9,300 feet mean sea level (MSL) the airplane began to bounce and slowly bank to the left, which led her to believe that they were experiencing wake turbulence. The A autopilot was engaged at the time when the airplane began a correction back to the right while continuing to bounce. The airplane rapidly banked 10 degrees when the captain said that he was disengaging the autopilot and autothrottles. The first officer described the acceleration rate of the bank as when the airplane is turned with the speed brakes deployed to the flight detent. Due to the yawing and bouncing sensation, the first officer instructed the flight attendants via public address system to be seated immediately. The first officer reported the airplane was now in about a 45-degree right bank.

The first officer further stated that the captain was very obviously opposing the roll with left rudder pressure and left aileron. The captain said something to the effect, 'I think it's a rudder problem.' The first officer stated that the A and B system hydraulic quantity and pressure gauges indications were normal. The first officer began assisting the captain by applying left aileron and left rudder control pressure and noticed that the airspeed decreased from 250 knots indicated airspeed (KIAS) to 238 KIAS. In response, the first officer pushed forward on the control column to increase the airspeed in an attempts to increase control effectiveness. As the airspeed increased, the effect of left control inputs began to move the airplane to a more comfortable attitude.

The captain appeared to regain control of the airplane about 8,200 feet MSL. At this point, the captain still had both hands on the control wheel with significant left aileron input. The first officer increased engine power to maintain 250 KIAS and 8,000 feet MSL. The captain asked the first officer to declare an emergency. The first officer reported a rudder malfunction, 100 souls on board, and about 10,300 pounds of fuel. The first officer initiated the quick reference checklist (QRC) for an uncommanded rudder, but the reference actions in the QRC did not provide any relief to the amount of the left aileron and left rudder pressure required to maintain control of the airplane. The first officer stated that there was no change in hydraulic pressure and quantity indication upon completion of each checklist item.

ORD Approach Control cleared flight 578 to 4,000 feet MSL and issued vectors for the runway 27L downwind. They were then cleared to descend to 2,500 feet MSL and 210 KIAS. At 220 KIAS, the captain called for flaps 2 which was then selected by the first officer. The captain then said that it "looks like a slam dunk." The captain utilized the speed brakes and called flaps 5. Air traffic control turned the flight on a base leg and cleared it to 2,100 feet MSL and 180 KIAS. The captain stowed the speed brakes. About 195 KIAS, the captain called for flaps 10 and gear down final descent checklist. At this time, they were issued a final vector and cleared for the ILS 27L approach. They flew a flaps 30 approach and landing to runway 27L. The first officer stated that the captain was stabilized per UAL criteria well before 1,000 feet above ground level (AGL). The captain commented, between 500 feet and 1,000 feet AGL, how it was becoming increasingly difficult to control the airplane with left rudder pressure and left aileron. At 500 feet AGL, the first officer assisted the captain on the control per his request.

The airplane landed on the centerline and touchdown zone of runway 27L. The speed brakes automatically deployed upon wheel spin up. The captain used some reverse thrust together with the autobrakes, began to slow the airplane. The captain and first officer attempted to maintain the directional control of the airplane through the landing roll with the use of rudder input, differential braking, and asymmetrical thrust. The first officer stated that it became increasingly difficult as the airplane slowed. About 100 KIAS, the first officer heard a loud ping or thud when the rudder simultaneously locked out all control input causing the left pedal to push against the first officer's foot.

The captain stated that during landing roll, about 100 KIAS, they heard a banging noise and got a full right scale deflection of the rudder pedals. The rudder pedals freed up and seemed back to normal shortly after coming to a stop and setting the parking brake. The airplane was then towed to the gate.

FLIGHT RECORDERS

Federal Aviation Regulation 121.359(h) Cockpit Voice Recorders, states:

"In the event of an accident or occurrence requiring immediate notification of the National Transportation Safety Board under part 830 of its regulations, which results in the termination of the flight, the certificate holder shall keep the recorded information for at least 60 days or, if requested by the Administrator or the Board, for a longer period. Information obtained from the record is used to assist in determining the cause of accident or occurrences in connection with investigations under part 830. The Administrator does not use the record in any civil penalty or certificate action."

The cockpit voice recording of the flight was not retained. The CVR and digital flight data recorder (DFDR) circuit breakers were not pulled out by either the flight crew or maintenance personnel following the landing. Data from the DFDR relating to the incident flight was able to be retrieved by the Safety Board.

A second DFDR was installed to record a series of control sweeps performed by the Airworthiness Group during the investigation.

The DFDR recording of flight 578 shows that the airplane achieved a right 26-degree roll; a left 65-degree control wheel deflection followed by right control wheel deflections. The rudder pedal deflection achieved a maximum value from left 10 degrees to right 10 degrees, which was followed by a right 1.5 degree deflection with corresponding aileron deflections ranging from left 6.0 degrees to right 1.0 degree. The pedal then remained deflected to the left over a range from 2.0-0.25 degrees with corresponding left rudder deflections ranging from 2.5-0.25 degrees until the airplane reached an altitude equal to about ORD airport elevation at which point both rudder and elevator deflections began to occur to the left and right center.

METEOROLOGICAL INFORMATION

Flight 578 was a designated UAL Aircraft Communication Addressing Reporting System weather flight. At 1146, the maximum pressure altitude recorded during the flight was 26,990 feet with an outside air temperature (OAT) of -35.8 degrees C. At 1200, the maximum eddy dissipation rate was recorded as 0.35, which equates to moderate turbulence. An OAT of -3.3 degrees Celsius (C) was also recorded at this time.

The ORD Automated Weather Observing System, recorded at 1156: wind 250 degrees at 8 knots; visibility 10 statute miles; sky condition 1,500 feet overcast; temperature 6 degrees C; dew point 3 degrees C; altimeter setting 29.93 inches of mercury.

TESTS AND RESEARCH

The Airworthiness Group performed visual inspections and testing of the rudder and lateral control system while following, but not limited to, guidance provided in service letter 737-SL-27-110-A, Unexpected Roll and Yaw Event Troubleshooting. Visual inspections of the flight control system, functional inspections, and testing were performed before any component removal.

Cockpit inspection revealed that the captain's left rudder pedal and first officer's right rudder pedal made intermittent contact with plastic housing located between the pedals. Nuts, drilled rivets, and other foreign objects were found in the foot wells. The housings were removed exposing the underlying cockpit floor and a portion of the rudder pedal arms. The cockpit floor aft of the pedals, the surrounding structure, and pedal arms were covered with lint/dirt ranging in thickness upwards of 1/2 inch. About 60 percent of the underlying flooring possessed a continuous area of reduced lint/dirt.

Inspection of the fuselage nose area forward of the nose landing gear revealed a wood-handled brush with wire bristles inventoried by UAL as Brush-Wire 1-1/2" 3 Row 8"L Stainless Steel, Part Number BR770. The brush, along with a fragment of twisted safety wire, drilled rivets, a nut, and plastic retainers were beneath the first officer's rudder pedals. The first officer's pedal arms contained scuff marks on the paint surface and gouge marks which penetrated the paint surface and metal. Further inspection with the aid of a borescope revealed the presence of displaced white paint on the structure aft of the pedal arms. No dents were noted in the vertical structure surrounding the pedal arm. The UAL party representative stated that work relating to an airworthiness directive was performed weeks prior to the incident. The brush, along with a new brush and the first officer's rudder pedal arms, were sent to the National Transportation Safety Board's Materials Laboratory for further examination.

According to the Materials Laboratory Factual Report, which is included in the docket of this report, the examination of the left rudder pedal arm i...