An A319 at FL250/M 0.81 had an medium frequency flutter which developed into a extreme airframe vibration. The crew declared an emergency and while slowing the anomaly disappeared at 270 KTS.

Narrative

During steady-state high speed cruise at medium altitude; suspect some anomaly in either the elevator or the stabilizer system caused a medium frequency 'flutter' that intensified over time resulting in EXTREME airframe vibration throughout entire structure. Flight control page indicated nothing out of the ordinary as far as control displacement and hydraulics page showed no 'bumping or surging.' Aircraft was gradually slowed so as not to cause immediate and abrupt changes in aerodynamic loads or associated pitch changes. Vibration continued until reaching 270 KTS indicated; where it discontinued. We declared an emergency and elected to continue because it did not require immediate turns and our glide distance would take us to our destination at the reduced speed and gradual descent we were utilizing. Situation was approached as a very likely mechanical failure in pitch control and all configuration changes were performed slowly and with caution. Passengers were mortified but First Officer's PA's comforted them greatly by acknowledging there was; in fact; a problem and that we were dealing with it. I communicated with the flight attendants several times explaining our course of action and concerns; pointing out how our approach would be different than normal; etc. We descended gradually while keeping air loads back and control inputs to a minimum. Approximately 20 miles out with conservative altitude; a slow methodical configuration change was initiated until aircraft reached desired landing configuration. We had plenty of fuel on board (approximately 2 hours) so 'dragging-in' the aircraft seemed sensible for several reasons (fuel reduction; weight reduction and requiring no further control/configuration changes at lower altitude. Final approach and landing was successfully completed with emergency equipment standing by. The only noticeable trait I noticed during this extended approach was minor 'buffeting' that I suspect emanated from same control failure. Taxied to gate; notified Dispatch; Maintenance Control; advised emergency personnel we no longer needed their services and advised station personnel. Deplaning passengers and flight attendants sitting in the rearmost part of the airplane confirmed our suspicions that problem originated from the aft-most point. Aircraft was turned over to Maintenance and crew operated outbound flight with a different aircraft. I would like to know the cause! The only thing I could see on a visual inspection after we were on the ground was a suspicious area on the inboard trailing edge of the right elevator that; from the ground; appeared to have a possible crack or delamination. I just don't know. Problem had to be caused by some sort of 'play' or looseness or failure allowing for undesired movement of either the stabilizer or the elevator. Improve emphasis on maintenance/inspection of flight controls as they are the single-most important area for safety concern. Guidance in QRH in handling potential flight control failures (including mechanical breakage) or losses based upon engineering and flight test input (and not 'hypothetical' simulator based procedures created by instructor pilots without test pilot knowledge) would be helpful. It is far more difficult to ascertain exactly what a pilot is faced with while flying a fly-by-wire aircraft since no 'feedback' through the control stick is available.