United Airlines Boeing 737-322 incident

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.

Verbatim NTSB analysis and probable cause from the NTSB dataset