Narrative:

During climb southwest of allentown, PA, rudder apparently began uncommanded excursions. No feedback was felt in rudder pedals. Pilot control inputs were not effective in dampening yaw and roll of aircraft. My impression was that my control inputs -- especially rudder -- were not effective. Aircraft motions resembled dutch roll. However, aircraft was at a low altitude for dutch roll, all controls were pwred, and yaw damper was engaged. Autoplt was not engaged. While we were attempting to control the uncommanded aircraft yaw and roll and determine just what was happening, aircraft speed and altitude were allowed to vary. ATC was notified of control problem and resultant excursions. Yaw damper inputs do not feed back into rudder pedals, making it difficult to determine extent of inputs, but they seemed to greatly exceed the 2-3 degree authority/authorized the yaw damper is allowed. The yaw damper was turned off and B hydraulic control to the rudder was turned to standby eliminating power the yaw damper might use. Normal control returned. QRH checked for other crew action. ATC notified. Flight continued to orlando. Maintenance reported that water had shorted out the yaw damper and caused problem. This might explain other reported B737 control problems. Callback conversation with reporter revealed the following information: the reporter said that the first officer was flying when they felt the aircraft yaw. The movement was unusual and the captain took control with the movement becoming more pronounced. He said that at first he felt that he had made a mistake by taking control because the yaw was progressive. While he was discussing the situation the first officer, against the captain's wishes, shut off the yaw damper. This procedure is standard practice for uncommanded rudder inputs in the B737, but the reporter said that he was resistant because of his long experience in the B727 where the yaw and dutch roll moments are divergent. The crew then completed their checklist for this condition and continued to destination. Maintenance personnel found that the cover on the yaw damper unit in the forward electronics compartment was displaced allowing water from the previous night's rain to leak into the electrical system. This apparently caused the yaw damper to displace the rudder causing the rate gyro to signal a correction which would then cause another displacement according to information the reporter obtained from maintenance personnel.

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Original NASA ASRS Text

Title: AN ACR B737 FLC EXPERIENCED UNCOMMANDED RUDDER INPUTS. THESE CTL DISPLACEMENTS WERE APPARENTLY CAUSED BY WATER GETTING INTO THE YAW DAMPER UNIT IN THE FORWARD ELECTRONIC COMPARTMENT.

Narrative: DURING CLB SW OF ALLENTOWN, PA, RUDDER APPARENTLY BEGAN UNCOMMANDED EXCURSIONS. NO FEEDBACK WAS FELT IN RUDDER PEDALS. PLT CTL INPUTS WERE NOT EFFECTIVE IN DAMPENING YAW AND ROLL OF ACFT. MY IMPRESSION WAS THAT MY CTL INPUTS -- ESPECIALLY RUDDER -- WERE NOT EFFECTIVE. ACFT MOTIONS RESEMBLED DUTCH ROLL. HOWEVER, ACFT WAS AT A LOW ALT FOR DUTCH ROLL, ALL CTLS WERE PWRED, AND YAW DAMPER WAS ENGAGED. AUTOPLT WAS NOT ENGAGED. WHILE WE WERE ATTEMPTING TO CTL THE UNCOMMANDED ACFT YAW AND ROLL AND DETERMINE JUST WHAT WAS HAPPENING, ACFT SPD AND ALT WERE ALLOWED TO VARY. ATC WAS NOTIFIED OF CTL PROB AND RESULTANT EXCURSIONS. YAW DAMPER INPUTS DO NOT FEED BACK INTO RUDDER PEDALS, MAKING IT DIFFICULT TO DETERMINE EXTENT OF INPUTS, BUT THEY SEEMED TO GREATLY EXCEED THE 2-3 DEG AUTH THE YAW DAMPER IS ALLOWED. THE YAW DAMPER WAS TURNED OFF AND B HYD CTL TO THE RUDDER WAS TURNED TO STANDBY ELIMINATING PWR THE YAW DAMPER MIGHT USE. NORMAL CTL RETURNED. QRH CHKED FOR OTHER CREW ACTION. ATC NOTIFIED. FLT CONTINUED TO ORLANDO. MAINT RPTED THAT WATER HAD SHORTED OUT THE YAW DAMPER AND CAUSED PROB. THIS MIGHT EXPLAIN OTHER RPTED B737 CTL PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT THE FO WAS FLYING WHEN THEY FELT THE ACFT YAW. THE MOVEMENT WAS UNUSUAL AND THE CAPT TOOK CTL WITH THE MOVEMENT BECOMING MORE PRONOUNCED. HE SAID THAT AT FIRST HE FELT THAT HE HAD MADE A MISTAKE BY TAKING CTL BECAUSE THE YAW WAS PROGRESSIVE. WHILE HE WAS DISCUSSING THE SIT THE FO, AGAINST THE CAPT'S WISHES, SHUT OFF THE YAW DAMPER. THIS PROC IS STANDARD PRACTICE FOR UNCOMMANDED RUDDER INPUTS IN THE B737, BUT THE RPTR SAID THAT HE WAS RESISTANT BECAUSE OF HIS LONG EXPERIENCE IN THE B727 WHERE THE YAW AND DUTCH ROLL MOMENTS ARE DIVERGENT. THE CREW THEN COMPLETED THEIR CHKLIST FOR THIS CONDITION AND CONTINUED TO DEST. MAINT PERSONNEL FOUND THAT THE COVER ON THE YAW DAMPER UNIT IN THE FORWARD ELECTRONICS COMPARTMENT WAS DISPLACED ALLOWING WATER FROM THE PREVIOUS NIGHT'S RAIN TO LEAK INTO THE ELECTRICAL SYS. THIS APPARENTLY CAUSED THE YAW DAMPER TO DISPLACE THE RUDDER CAUSING THE RATE GYRO TO SIGNAL A CORRECTION WHICH WOULD THEN CAUSE ANOTHER DISPLACEMENT ACCORDING TO INFO THE RPTR OBTAINED FROM MAINT PERSONNEL.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.