CHI02IA050

The scheduled domestic passenger carrying flight of a Boeing 737-322 achieved a maximum altitude of about 27,000 feet with an outside air temperature of about -36 degrees Celsius. About 30 minutes after departure and while on a visual descent to the destination airport, the flight encountered wake turbulence while descending through 9,300 feet mean sea level for visual approach to the destination airport. 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 captain applied left aileron and left rudder to oppose the roll and eventually the first officer began to assist the captain in his control inputs.

The digital flight data recorder showed that the airplane achieved a right 26-degree roll; a left 45-degree control wheel deflection followed by right control wheel deflections. The rudder pedal deflection achieved a maximum value of left 6.5 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 degrees. The pedal then remained deflected to the left over a range from 2.0-0.25 degrees with corresponding left rudder deflections ranging from 2.0-0.25 degrees until the airplane landed. The first officer reported that A and B hydraulic quantity and pressure gauge indications were normal and there was no change in hydraulic pressure and quantity indication upon completion of each uncommanded rudder checklist item. The captain stated that during landing roll, about 100 knots indicated airspeed, they heard a banging noise and got a full right deflection of the rudder pedals. The cockpit voice recorder recording of the flight was not retained by the operator following the incident.

Inspection of the cockpit nose area forward of the main landing gear revealed a wood-handled wire brush. 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 pedal arms were sent to the National Transportation Safety Board's Materials Laboratory for further examination. Examination of the first officer's rudder pedal arms and brush revealed that in the vicinity of the bolt heads revealed only light score marks scattered around the edge of the boss, consistent with tool usage. The right pedal arm contained one score mark on the rear edge of the boss. The score mark was circumferential for approximately 90 degrees of arc, centered at the horizontal line. It was more significant above the horizontal and tapered to nothing below it. Examination of the left, right , top, and bottom faces of the brush handle revealed no indications of any crush damage. Functional testing and macroscopic visual examination of the rudder power control unit revealed no anomalies. One estimate of the maximum rudder forces that can be exerted for various positions of rudder pedal range from 246 - 424 lbs depending upon rudder pedal position and distance from back of seat. Several like brushes were loaded in the lengthwise direction and also placed in various obstructive positions within the pedal arms. Brushes placed in obstructive positions with a pilot applying rudder pressure estimated that force he applied to the rudder pedals to fracture the brushes was 15-20 pounds. Brushes loaded in the lengthwise directions broke at loadings from a maximum of 80 pounds and below. A flight training simulator was back driven at UAL's Flight Training Facility using FDR data from the incident flight with both flight crew members aboard. The simulator structure at the rudder pedals had the same platform/shelf structure as the incident airplane. The rudder pedals set to the same adjustment as reported by the first officer, and the brush was inserted with the application of left rudder pedal pressure to hold the brush in place due to the resting platform being shorter than the actual airplane. The brush position for the best strength and holding power was noted when the brush was orientated such that the wire bristles were facing downward and towards the rudder control arms and the convex portion of the brush handle facing upwards with the other end against the vertical wall of the pedal box. Further pressure on the rudder pedals was not added due to a concern of causing damage to the simulator.

A search of the Federal Aviation Administration’s service difficulty reporting system relating to "Boeing Rudder Controls" was performed for a time period from January 1, 1997 to February 1, 2002. One occurrence shows that maintenance could not find any fault with the airplane and that the rudder hydraulic system components were replaced. A second occurrence shows that after the accomplishment of engineering callouts relating to rudder hydraulic system components, no defects were noted. A third occurrence noted the replacement of the PCU.

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 wh