I thought B737 pilots may find this (taken from the NTSB site) of interest:
NTSB Identification: CHI02IA050
Scheduled 14 CFRPart 121 operation of Air Carrier United Airlines (D.B.A. United Airlines)
Incident occurred Thursday, December 13, 2001 at Chicago, IL
Aircraft:Boeing 737-322, registration: N359UA
Injuries: 99 Uninjured.
This is preliminary information, subject to change, and may contain errors. Any errors in this report will be corrected when the final report has been completed.
On December 13, 2001, at 1203 central standard time, a Boeing 737-322, N359UA, operated by United Airlines 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 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 reported in a written statement, "... At approximately 9,300', during the descent the aircraft began to bounce and slowly bank to the left, which led me to believe we were experiencing wake turbulence. With the A autopilot still engaged the airplane began a correction back to the right while continuing with the bouncing sensation. The airplane rapidly achieved approximately 10 degrees of bank when the captain announced that he was disengaging both the autopilot and the autothrottles. The rate of bank accelerated in a fashion similar to when one turns the airplane with the speed brakes deployed to the flight detent. As a result of the yawing and bouncing sensation I instructed the flight attendants to 'be seated immediately' via the PA. The airplane was now in an approximate 45 degree bank to the right."
"The captain was very obviously opposing the roll with both left rudder pressure and left aileron. The captain stated something to the effect 'I think it's a rudder problem'. I then looked at the A and B system hydraulic quantity and pressure gauges and saw normal indications. I then began assisting the captain on the controls with both the left aileron and left rudder pressure. At this point I noticed the airspeed was diminishing from 250 KIAS to about 238 KIAS. I then pushed forward on the control column to increase the airspeed in hopes of increasing control effectiveness. As our airspeed increased the effect of left control inputs began to move the airplane to a more controllable attitude. The captain appeared to regain control of the airplane at approximately 8,200'."
"At this point the captain still had both hands on the control wheel with significant left aileron input. As a result I initiated an increase in power to maintain 250 KIAS and 8000'. The captain then asked me to declare an emergency. I stated that we were declaring an emergency due to a rudder malfunction. I stated we had 100 souls on board and approximately 10,300 pounds of fuel and I would get back to ATC as time permitted. I then initiated the QRC for the 'uncommanded rudder' checklist. The reference actions in the checklist did not provide any relief to the amount of left aileron and left rudder pressure required to maintain control of the aircraft."
"Upon completion of each item on the checklist I tried to verify the hydraulic pressure and quantity indications to see if there was a change. I did not see any change on either. The captain requested that I let ATC know that we needed 27L. I did so. I asked the captain if I should tell the purser to prepare the cabin for evacuation. The captain agreed. I then called the purser and stated 'we have declared an emergency due to a rudder problem, prep the cabin for evac, you have approximately five minutes'. The purser acknowledged my instructions."
"Approach then cleared us to 4000' and vectored us to downwind. We were then cleared to 2500' and 210 knots. At 220 KIAS the captain called for flaps 2, I did so. The captain then stated 'looks like a slam dunk'. He then utilized speed brakes and called for flaps 5. ATC then turned us on a base leg and cleared us to 2100' and 180 knots. The captain then stowed the speed brakes. At approximately 195 KIAS the captain called for flaps 10 and 'gear down final descent checklist'. At this time we were given a final vector and cleared for the ILS 27L. I armed the approach mode after identifying the ILS frequency. It became obvious that the captain's flight director was not giving accurate localizer guidance when the captain discovered his course was still set to 220. He then selected 270 and joined the localizer and glideslope for 27L. I contacted the tower just inside the marker and stated again that we were an emergency aircraft and would like the trucks. Tower acknowledged and cleared us to land."
"We flew a flaps 30 approach and landing to 27L The captain was stabilized per UAL criteria well before 1000' AGL. Between 1000' and 500' AGL the captain commented on how it was becoming increasingly difficult to control the airplane with left rudder pressure and left aileron. At 500' I joined the captain on the controls per his request."
"The airplane landed on the centerline in the touchdown zone on 27L. The speed brakes automatically deployed upon wheel spin up. The captain utilized some engine reversing and the autobrakes began to decelerate the aircraft also. Together the captain and I attempted to maintain directional control of the aircraft throughout the landing rollout with the use of rudder input. It became increasingly difficult to do this as the airplane slowed. About when I called '100 knots' I heard a loud ping or thud when the rudder simultaneously locked out all control input causing the left pedal to push against my foot."
"The airplane was brought to a stop just short of the Tango highspeed taxiway..."