Narrative:

Flight planning; pre-flight; taxi and takeoff were normal. On climbout; passing about FL274 and at 310 KIAS indicated; we began feeling a large amplitude vertical vibration/shaking. I leveled the jet at FL280 and quickly performed a vibration check (autopilot off; input one axis of control at a time to isolate the vibration). (At this point; the flight attendants called from the back with more than a touch of worry in their voices.) nothing seemed to work. Granted; the control inputs I made were very small; but I did move them all. Engine parameters were normal. I decided to slow. Below 300 KTS; the vibration completely went away. After discussing the rapid onset and level of airframe shaking with my first officer (neither of us had felt that strong of a vibration before in anything we'd flown) we elected to see if it was just an anomaly or some sort of atmospheric displacement. I once again accelerated to 310 KTS at FL280. Right at 310 KTS; it started again. The amplitude rapidly built to what it was before; so I again slowed to 290 knots. The vibration continued until below 300. We discussed our options; hold and talk to maintenance; continue on to destination; or return to ZZZ. I was ok with an airplane vibrating; but the way it was doing it was so much larger than what I'd felt before that I couldn't see continuing on. We thought that since we were steady-state for a few minutes and before we committed to returning to ZZZ; we'd better try it once more. I decided to only go to 300 KIAS. At that speed; it was a slow onset. The frequency wasn't slow; but the buildup to full shaking took maybe 10 seconds instead of almost instantaneous at the higher speed. It felt very much like there was a resonant frequency at work here on some flight control at the rear of the jet. I took a quick look at the flight control page (again) looking for any movement and could find none. I could feel it very strongly in the rudder pedals; but the oscillations felt vertical to me. (I asked the flight attendants about it on the ground and they were convinced it was up-and-down; with no side-to-side movement.) that was enough for us. I had the first officer work the issues with center; he took the radios and the airplane while I did the coordination with the flight attendants; dispatch and the passengers. I sent dispatch a very quick note explaining the situation and that we were returning to ZZZ. I had the purser come into the cockpit and briefed her on what had happened and what we were going to do. I wanted her to accomplish a cabin advisory and she responded extremely well. It was pretty obvious to me when she was in the cockpit that while she was a bit shaken; she and the rest of the crew were performing well and were exceptionally capable of doing exactly what they needed to do. And they did! I called dispatch and gave him a reader's digest version of what happened and what the plan was. He had already talked to maintenance and I gave him just a little extra information for them. I let him know that while we hadn't declared an emergency; we were going to once closer in to ZZZ. Once with approach control; I declared an emergency and took over the PF responsibilities again. The approach; landing and conclusion of the flight were normal. Further information: the vibration was felt throughout the aircraft; and the flight attendants all felt it was at its worst at the rear of the jet. It shook the entire airplane very significantly. While we were shaking and I was looking at the flight control page on the east/wd; I recognized the folly in trying to see minuscule movements of any flight control. I could only see that there were arrows there; the shaking was too much for anything else. The frequency of the vibration seemed about 5-10 cycles per second (5-10 HZ). The only variation we had once in the vibration zone of 300-310 knots at FL280 was in amplitude. In other words; at the onset; the frequency was the same as it was at its worst. Only the amplitude changed. We had no indications of problems with any systems. While the vibration almost felt 'violent' on the rudder pedals (like a bunch of people were hitting the sides of the rudder with hammers; not sledge hammers; just hammers) the oscillations had no yaw moment that I could feel or detect in any way. There was no sound associated with the vibration; as we usually get with gear door problems. I never accelerated above 310 knots; so I don't know if the shaking/vibration was worse faster than that.callback conversation with reporter revealed the following information: the reporter stated that after the vibration started; a discussion with the cabin crew confirmed the flight crew's impression that the vibration had no yaw component and was strictly longitudinal. The reporter has flown fighter jets; heavy air carrier aircraft; and other aircraft. This is the strongest vibration that he has ever experienced. The aircraft was put back in service the next day after this event with maintenance reporting the nose gear door was out of rig. However; several days later the reporter discovered that the aircraft was again removed from service with maintenance citing the nose gear door seal as the issue. The reporter discounts this because the flight crew and flight attendants felt the vibration originated in the tail area.

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Original NASA ASRS Text

Title: AN A319 CAPT REPORTS A LARGE AMPLITUDE LONGITUDINAL VIBRATION STARTING AT FL274 AND 310 KTS. THE 5-10 HZ VIBRATION STOPPED BELOW 300 KTS. AN EMERGENCY WAS DECLARED WITH A RETURN TO LAND.

Narrative: FLIGHT PLANNING; PRE-FLIGHT; TAXI AND TAKEOFF WERE NORMAL. ON CLIMBOUT; PASSING ABOUT FL274 AND AT 310 KIAS INDICATED; WE BEGAN FEELING A LARGE AMPLITUDE VERTICAL VIBRATION/SHAKING. I LEVELED THE JET AT FL280 AND QUICKLY PERFORMED A VIBRATION CHECK (AUTOPILOT OFF; INPUT ONE AXIS OF CONTROL AT A TIME TO ISOLATE THE VIBRATION). (AT THIS POINT; THE FLIGHT ATTENDANTS CALLED FROM THE BACK WITH MORE THAN A TOUCH OF WORRY IN THEIR VOICES.) NOTHING SEEMED TO WORK. GRANTED; THE CONTROL INPUTS I MADE WERE VERY SMALL; BUT I DID MOVE THEM ALL. ENGINE PARAMETERS WERE NORMAL. I DECIDED TO SLOW. BELOW 300 KTS; THE VIBRATION COMPLETELY WENT AWAY. AFTER DISCUSSING THE RAPID ONSET AND LEVEL OF AIRFRAME SHAKING WITH MY FO (NEITHER OF US HAD FELT THAT STRONG OF A VIBRATION BEFORE IN ANYTHING WE'D FLOWN) WE ELECTED TO SEE IF IT WAS JUST AN ANOMALY OR SOME SORT OF ATMOSPHERIC DISPLACEMENT. I ONCE AGAIN ACCELERATED TO 310 KTS AT FL280. RIGHT AT 310 KTS; IT STARTED AGAIN. THE AMPLITUDE RAPIDLY BUILT TO WHAT IT WAS BEFORE; SO I AGAIN SLOWED TO 290 KNOTS. THE VIBRATION CONTINUED UNTIL BELOW 300. WE DISCUSSED OUR OPTIONS; HOLD AND TALK TO MAINT; CONTINUE ON TO DEST; OR RETURN TO ZZZ. I WAS OK WITH AN AIRPLANE VIBRATING; BUT THE WAY IT WAS DOING IT WAS SO MUCH LARGER THAN WHAT I'D FELT BEFORE THAT I COULDN'T SEE CONTINUING ON. WE THOUGHT THAT SINCE WE WERE STEADY-STATE FOR A FEW MINUTES AND BEFORE WE COMMITTED TO RETURNING TO ZZZ; WE'D BETTER TRY IT ONCE MORE. I DECIDED TO ONLY GO TO 300 KIAS. AT THAT SPEED; IT WAS A SLOW ONSET. THE FREQUENCY WASN'T SLOW; BUT THE BUILDUP TO FULL SHAKING TOOK MAYBE 10 SECONDS INSTEAD OF ALMOST INSTANTANEOUS AT THE HIGHER SPEED. IT FELT VERY MUCH LIKE THERE WAS A RESONANT FREQUENCY AT WORK HERE ON SOME FLIGHT CONTROL AT THE REAR OF THE JET. I TOOK A QUICK LOOK AT THE FLIGHT CONTROL PAGE (AGAIN) LOOKING FOR ANY MOVEMENT AND COULD FIND NONE. I COULD FEEL IT VERY STRONGLY IN THE RUDDER PEDALS; BUT THE OSCILLATIONS FELT VERTICAL TO ME. (I ASKED THE FLT ATTENDANTS ABOUT IT ON THE GROUND AND THEY WERE CONVINCED IT WAS UP-AND-DOWN; WITH NO SIDE-TO-SIDE MOVEMENT.) THAT WAS ENOUGH FOR US. I HAD THE FO WORK THE ISSUES WITH CENTER; HE TOOK THE RADIOS AND THE AIRPLANE WHILE I DID THE COORDINATION WITH THE FLT ATTENDANTS; DISPATCH AND THE PASSENGERS. I SENT DISPATCH A VERY QUICK NOTE EXPLAINING THE SITUATION AND THAT WE WERE RETURNING TO ZZZ. I HAD THE PURSER COME INTO THE COCKPIT AND BRIEFED HER ON WHAT HAD HAPPENED AND WHAT WE WERE GOING TO DO. I WANTED HER TO ACCOMPLISH A CABIN ADVISORY AND SHE RESPONDED EXTREMELY WELL. IT WAS PRETTY OBVIOUS TO ME WHEN SHE WAS IN THE COCKPIT THAT WHILE SHE WAS A BIT SHAKEN; SHE AND THE REST OF THE CREW WERE PERFORMING WELL AND WERE EXCEPTIONALLY CAPABLE OF DOING EXACTLY WHAT THEY NEEDED TO DO. AND THEY DID! I CALLED DISPATCH AND GAVE HIM A READER'S DIGEST VERSION OF WHAT HAPPENED AND WHAT THE PLAN WAS. HE HAD ALREADY TALKED TO MAINT AND I GAVE HIM JUST A LITTLE EXTRA INFORMATION FOR THEM. I LET HIM KNOW THAT WHILE WE HADN'T DECLARED AN EMERGENCY; WE WERE GOING TO ONCE CLOSER IN TO ZZZ. ONCE WITH APPROACH CONTROL; I DECLARED AN EMERGENCY AND TOOK OVER THE PF RESPONSIBILITIES AGAIN. THE APPROACH; LANDING AND CONCLUSION OF THE FLIGHT WERE NORMAL. FURTHER INFORMATION: THE VIBRATION WAS FELT THROUGHOUT THE AIRCRAFT; AND THE FLIGHT ATTENDANTS ALL FELT IT WAS AT ITS WORST AT THE REAR OF THE JET. IT SHOOK THE ENTIRE AIRPLANE VERY SIGNIFICANTLY. WHILE WE WERE SHAKING AND I WAS LOOKING AT THE FLIGHT CONTROL PAGE ON THE E/WD; I RECOGNIZED THE FOLLY IN TRYING TO SEE MINUSCULE MOVEMENTS OF ANY FLIGHT CONTROL. I COULD ONLY SEE THAT THERE WERE ARROWS THERE; THE SHAKING WAS TOO MUCH FOR ANYTHING ELSE. THE FREQUENCY OF THE VIBRATION SEEMED ABOUT 5-10 CYCLES PER SECOND (5-10 HZ). THE ONLY VARIATION WE HAD ONCE IN THE VIBRATION ZONE OF 300-310 KNOTS AT FL280 WAS IN AMPLITUDE. IN OTHER WORDS; AT THE ONSET; THE FREQUENCY WAS THE SAME AS IT WAS AT ITS WORST. ONLY THE AMPLITUDE CHANGED. WE HAD NO INDICATIONS OF PROBLEMS WITH ANY SYSTEMS. WHILE THE VIBRATION ALMOST FELT 'VIOLENT' ON THE RUDDER PEDALS (LIKE A BUNCH OF PEOPLE WERE HITTING THE SIDES OF THE RUDDER WITH HAMMERS; NOT SLEDGE HAMMERS; JUST HAMMERS) THE OSCILLATIONS HAD NO YAW MOMENT THAT I COULD FEEL OR DETECT IN ANY WAY. THERE WAS NO SOUND ASSOCIATED WITH THE VIBRATION; AS WE USUALLY GET WITH GEAR DOOR PROBLEMS. I NEVER ACCELERATED ABOVE 310 KNOTS; SO I DON'T KNOW IF THE SHAKING/VIBRATION WAS WORSE FASTER THAN THAT.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT AFTER THE VIBRATION STARTED; A DISCUSSION WITH THE CABIN CREW CONFIRMED THE FLT CREW'S IMPRESSION THAT THE VIBRATION HAD NO YAW COMPONENT AND WAS STRICTLY LONGITUDINAL. THE REPORTER HAS FLOWN FIGHTER JETS; HEAVY ACR ACFT; AND OTHER ACFT. THIS IS THE STRONGEST VIBRATION THAT HE HAS EVER EXPERIENCED. THE ACFT WAS PUT BACK IN SERVICE THE NEXT DAY AFTER THIS EVENT WITH MAINTENANCE REPORTING THE NOSE GEAR DOOR WAS OUT OF RIG. HOWEVER; SEVERAL DAYS LATER THE REPORTER DISCOVERED THAT THE ACFT WAS AGAIN REMOVED FROM SERVICE WITH MAINTENANCE CITING THE NOSE GEAR DOOR SEAL AS THE ISSUE. THE REPORTER DISCOUNTS THIS BECAUSE THE FLT CREW AND FLT ATTENDANTS FELT THE VIBRATION ORIGINATED IN THE TAIL AREA.

Data retrieved from NASA's ASRS site as of May 2009 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.