Narrative:

When I was a first officer, I was told that a 'good captain' checks both the system a and system B stby hydraulic switches systems during the flight control preflight check. As it was explained to me, I wanted to make sure both switches worked. However, I discovered a better reason. It's the only way for a flight crew to ensure that the rudder power control unit (PCU) is working correctly, mainly the rudder centering mechanism. The way to do this is by checking system a then checking system B. If the aircraft has a bad rudder PCU, it will experience a 'shuttering' vibration that can and will be felt throughout the aircraft (esp in the aft section). This happens when the system B switch is placed back to the normal (closed guarded) position. In the last month, I have found three company aircraft with bad PCU's. I have talked to numerous captain's and have found only two that check both switches systems. Nobody realized that if there was a vibration the aircraft should be turned over to mntnc. I have heard reports from flight attendants that experienced these 'shuttering' vibrations and informed their capts, only to be told this is a normal vibration. It's not. We need to put the word out to pilots about a possible mishap and problem. I recommend a change to the fom stating that during the captain preflight control check both, not 'one' switches/systems be checked. And if a 'shuttering' vibration is felt, mntnc should be notified. Callback conversation with reporter revealed the following information: the reporter stated the test is performed using the rudder hydraulic switches 'a' and 'B' and actually tests the rudder centering mechanism looking for a rudder shudder lasting 2 to 3 seconds. The reporter said the test is not in the flight operations manual as the company would consider it being an unapproved test. The reporter stated the company believes the reporter to be over sensitive to the rudder problem.

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Original NASA ASRS Text

Title: A B737-300 CAPT RPTS FINDING DEFECTIVE RUDDER POWER UNIT CENTERING MECHANISMS USING RUDDER 'A' AND 'B' SWITCHES.

Narrative: WHEN I WAS A FO, I WAS TOLD THAT A 'GOOD CAPT' CHKS BOTH THE SYS A AND SYS B STBY HYDRAULIC SWITCHES SYSTEMS DURING THE FLT CTL PREFLIGHT CHK. AS IT WAS EXPLAINED TO ME, I WANTED TO MAKE SURE BOTH SWITCHES WORKED. HOWEVER, I DISCOVERED A BETTER REASON. IT'S THE ONLY WAY FOR A FLT CREW TO ENSURE THAT THE RUDDER POWER CTL UNIT (PCU) IS WORKING CORRECTLY, MAINLY THE RUDDER CENTERING MECHANISM. THE WAY TO DO THIS IS BY CHECKING SYS A THEN CHECKING SYS B. IF THE ACFT HAS A BAD RUDDER PCU, IT WILL EXPERIENCE A 'SHUTTERING' VIBRATION THAT CAN AND WILL BE FELT THROUGHOUT THE ACFT (ESP IN THE AFT SECTION). THIS HAPPENS WHEN THE SYS B SWITCH IS PLACED BACK TO THE NORMAL (CLOSED GUARDED) POSITION. IN THE LAST MONTH, I HAVE FOUND THREE COMPANY ACFT WITH BAD PCU'S. I HAVE TALKED TO NUMEROUS CAPT'S AND HAVE FOUND ONLY TWO THAT CHK BOTH SWITCHES SYSTEMS. NOBODY REALIZED THAT IF THERE WAS A VIBRATION THE ACFT SHOULD BE TURNED OVER TO MNTNC. I HAVE HEARD RPTS FROM FLIGHT ATTENDANTS THAT EXPERIENCED THESE 'SHUTTERING' VIBRATIONS AND INFORMED THEIR CAPTS, ONLY TO BE TOLD THIS IS A NORMAL VIBRATION. IT'S NOT. WE NEED TO PUT THE WORD OUT TO PLTS ABOUT A POSSIBLE MISHAP AND PROBLEM. I RECOMMEND A CHANGE TO THE FOM STATING THAT DURING THE CAPT PREFLT CTL CHK BOTH, NOT 'ONE' SWITCHES/SYSTEMS BE CHECKED. AND IF A 'SHUTTERING' VIBRATION IS FELT, MNTNC SHOULD BE NOTIFIED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE TEST IS PERFORMED USING THE RUDDER HYDRAULIC SWITCHES 'A' AND 'B' AND ACTUALLY TESTS THE RUDDER CENTERING MECHANISM LOOKING FOR A RUDDER SHUDDER LASTING 2 TO 3 SECONDS. THE RPTR SAID THE TEST IS NOT IN THE FLT OPS MANUAL AS THE COMPANY WOULD CONSIDER IT BEING AN UNAPPROVED TEST. THE RPTR STATED THE COMPANY BELIEVES THE RPTR TO BE OVER SENSITIVE TO THE RUDDER PROBLEM.

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.