Narrative:

At FL350, upon leveloff, with aircraft on autoplt, the aircraft started a climb/descent 200-500 FPM (indicated) on the ivsi. First officer disconnected autoplt and hand flew aircraft and also retrimmed aircraft. Just after he retrimmed aircraft, I noticed that the trim indicator on the pedestal was moving both backward and forward approximately 1/4 inch. This continued throughout the flight. About 1/2 way through the flight, flight attendant wxxw came up and told us that she had heard a knocking noise as well as a vibration on departure, climb out and at cruise. Flight controls were normal and felt normal. I went back to the aft part of the cabin, and I too felt the vibration and also heard a slight knocking which is not normal. Discussed that with dispatch and maintenance control, gave maintenance all the information I had. They said it might be the cable attached to the trim indicator, so we pressed on. Not long after, the flight attendant wxxw came up to the flight deck and informed us that the knocking and vibration was getting stronger. The trim was starting to trim again while the first officer was hand flying the aircraft. I made a call to dispatch and informed them of the information just stated, and I decided that I was going to declare an emergency because I wasn't sure what would happen when we start to configure the aircraft for landing. As we started configuring the aircraft, we noticed that as first officer trimmed the aircraft as flaps and gear were extended the trim indicator would move towards the nose down position. I asked first officer if he felt any nose down pressure, he said 'no.' (example: trimmed flaps 15 degrees/gear down trim was at 5.8 -- trim went back 5.1.) fire equipment was standing by, but was not needed after landing.

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Original NASA ASRS Text

Title: A DC9-30 IN CRUISE AT FL350 DECLARED AN EMER DUE TO ERRATIC PITCH TRIM INDICES MOVEMENT WITH NOISE IN TAIL COMPARTMENT CAUSED BY LOOSE INDICE CABLE.

Narrative: AT FL350, UPON LEVELOFF, WITH ACFT ON AUTOPLT, THE ACFT STARTED A CLB/DSCNT 200-500 FPM (INDICATED) ON THE IVSI. FO DISCONNECTED AUTOPLT AND HAND FLEW ACFT AND ALSO RETRIMMED ACFT. JUST AFTER HE RETRIMMED ACFT, I NOTICED THAT THE TRIM INDICATOR ON THE PEDESTAL WAS MOVING BOTH BACKWARD AND FORWARD APPROX 1/4 INCH. THIS CONTINUED THROUGHOUT THE FLT. ABOUT 1/2 WAY THROUGH THE FLT, FLT ATTENDANT WXXW CAME UP AND TOLD US THAT SHE HAD HEARD A KNOCKING NOISE AS WELL AS A VIBRATION ON DEP, CLBOUT AND AT CRUISE. FLT CTLS WERE NORMAL AND FELT NORMAL. I WENT BACK TO THE AFT PART OF THE CABIN, AND I TOO FELT THE VIBRATION AND ALSO HEARD A SLIGHT KNOCKING WHICH IS NOT NORMAL. DISCUSSED THAT WITH DISPATCH AND MAINT CTL, GAVE MAINT ALL THE INFO I HAD. THEY SAID IT MIGHT BE THE CABLE ATTACHED TO THE TRIM INDICATOR, SO WE PRESSED ON. NOT LONG AFTER, THE FLT ATTENDANT WXXW CAME UP TO THE FLT DECK AND INFORMED US THAT THE KNOCKING AND VIBRATION WAS GETTING STRONGER. THE TRIM WAS STARTING TO TRIM AGAIN WHILE THE FO WAS HAND FLYING THE ACFT. I MADE A CALL TO DISPATCH AND INFORMED THEM OF THE INFO JUST STATED, AND I DECIDED THAT I WAS GOING TO DECLARE AN EMER BECAUSE I WASN'T SURE WHAT WOULD HAPPEN WHEN WE START TO CONFIGURE THE ACFT FOR LNDG. AS WE STARTED CONFIGURING THE ACFT, WE NOTICED THAT AS FO TRIMMED THE ACFT AS FLAPS AND GEAR WERE EXTENDED THE TRIM INDICATOR WOULD MOVE TOWARDS THE NOSE DOWN POS. I ASKED FO IF HE FELT ANY NOSE DOWN PRESSURE, HE SAID 'NO.' (EXAMPLE: TRIMMED FLAPS 15 DEGS/GEAR DOWN TRIM WAS AT 5.8 -- TRIM WENT BACK 5.1.) FIRE EQUIP WAS STANDING BY, BUT WAS NOT NEEDED AFTER LNDG.

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.