Narrative:

While in cruise with the autopilot engaged; we experienced a continuous heavy tail vibration. Our speed at the time was .77 mach; at an altitude of 32;000 feet. We contacted the flight attendant in the aft end of the aircraft to ascertain what the cabin crew was experiencing. The flight attendant said that there was a continuous heavy vibration and shaking in the aft galley; extending all the way up to mid-wing area. The first officer and I discussed and evaluated the situation; and determined the safest course of action was to descend into more dense air and reduce airspeed; suspecting a flight control problem. We contacted ATC; requesting fl 300 and reducing speed to .75 mach. The vibration continued in frequency and intensity. We continued descent to fl 260; speed 270 kts; at which point the vibration decreased to continuous light vibration from the tail section. We had dispatched with yaw damper #2 inoperative; and we suspected at this point we may be experiencing a problem with yaw damper #1.in cruise; upon realizing a flight control problem; we used the QRH for any possible assistance. The only info provided there was to fill out the [standardized company] vibration form; found in the fom. Maintenance control also requested that I disengage the autopilot and put in control inputs to further assess the area of concern. I denied that request in the best interest of my crew and passengers. Thus far there is no known cause for the vibration. [I am] awaiting word from the fleet manager concerning maintenance's findings. Company radio was very unreliable when we needed it most. Due to the seriousness of the situation; typing our problem via ACARS would have been time consuming and would take one pilot out of the loop regarding the safe management of the aircraft.maintenance's request that I turn off the autopilot and enter 'some control inputs' at cruise altitude was discomforting; to say the least. I refused their request; in the interest of safety for my crew and passengers. Please advise maintenance control that we don't 'test fly' an aircraft with passengers and crew on board. We were hoping for some further direction on alleviating our problem from the QRH. We found the information provided inadequate and of no assistance. We suggest adding to the QRH suggested airspeeds; machs; and altitudes that may possibly reduce flight control vibration.we kept ATC informed of our flight control situation and they provided assistance as required. We informed dispatch and maintenance control of our situation via company radio. Dispatch asked us if we were diverting. We informed them that we would continue to our destination unless the situation deteriorated. ATC; along with dispatch; coordinated priority handling and we proceeded with no delays. Descent; approach; and landing were normal.

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Original NASA ASRS Text

Title: An A320 Captain reported unexplained aircraft vibration believed to have been related to the flight control systems.

Narrative: While in cruise with the autopilot engaged; we experienced a continuous heavy tail vibration. Our speed at the time was .77 Mach; at an altitude of 32;000 feet. We contacted the Flight Attendant in the aft end of the aircraft to ascertain what the cabin crew was experiencing. The Flight Attendant said that there was a continuous heavy vibration and shaking in the aft galley; extending all the way up to mid-wing area. The First Officer and I discussed and evaluated the situation; and determined the safest course of action was to descend into more dense air and reduce airspeed; suspecting a flight control problem. We contacted ATC; requesting FL 300 and reducing speed to .75 Mach. The vibration continued in frequency and intensity. We continued descent to FL 260; speed 270 kts; at which point the vibration decreased to continuous light vibration from the tail section. We had dispatched with yaw damper #2 inoperative; and we suspected at this point we may be experiencing a problem with yaw damper #1.In cruise; upon realizing a flight control problem; we used the QRH for any possible assistance. The only info provided there was to fill out the [standardized company] vibration form; found in the FOM. Maintenance Control also requested that I disengage the autopilot and put in control inputs to further assess the area of concern. I denied that request in the best interest of my crew and passengers. Thus far there is no known cause for the vibration. [I am] awaiting word from the Fleet Manager concerning maintenance's findings. Company radio was very unreliable when we needed it most. Due to the seriousness of the situation; typing our problem via ACARS would have been time consuming and would take one pilot out of the loop regarding the safe management of the aircraft.Maintenance's request that I turn off the autopilot and enter 'some control inputs' at cruise altitude was discomforting; to say the least. I refused their request; in the interest of safety for my crew and passengers. Please advise Maintenance Control that we don't 'test fly' an aircraft with passengers and crew on board. We were hoping for some further direction on alleviating our problem from the QRH. We found the information provided inadequate and of no assistance. We suggest adding to the QRH suggested airspeeds; Machs; and altitudes that may possibly reduce flight control vibration.We kept ATC informed of our flight control situation and they provided assistance as required. We informed Dispatch and Maintenance Control of our situation via company radio. Dispatch asked us if we were diverting. We informed them that we would continue to our destination unless the situation deteriorated. ATC; along with Dispatch; coordinated priority handling and we proceeded with no delays. Descent; approach; and landing were normal.

Data retrieved from NASA's ASRS site as of April 2012 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.