I can recall the exact same thing happening to an A320 straight out of C check from GAMCO.
It was an empty ferry flight with just 2 pilots, 2 engineers and a storeman on board.
We had to lock out a pack due to a faulty pack valve, so we were flying at a limit of 31,000ft as per MEL restrictions. After T/O out of Abu Dhabi we were flying along the coast of the Gulf. By the time we got about as far as Bahrain, the A/C started to vibrate badly. I was standing in the rear galley at the time and was convinced that something had fallen off. Something like an elevator servo access panel. The vibration was similar in sound and feel to a pneumatic hammer drill, and all in the vertical axis. I went to the flight deck to find both pilots bouncing in their seats unable to read their instruments due to severe vibration.
A scary time to say the least. My other engineer colleague joined us in the cockpit where we discussed the best plan of action. All credit due to the captain who used all his resources to help him with his CRM. He was in favour of trying to isolate the fault by trying different configs (SECs /ELACs / FACs) etc. We advised that it probably wasn't wise and the priority was to get the A/C back on the ground as gently as possible. He agreed with us and we declared a MAYDAY. ATC gave us a gentle 180 turn and we lined up on a long approach on the opposite runway into Abu Dhabi. As we slowed down for approach, the vibration stopped and we made an uneventful landing.
After days back in the hangar we couldn't find anything wrong with the A/C. We had dug out all the SILs and TFUs from Airbus and couldn't find anything relevant. We eventually called Airbus Tolouse who told us to replace all the hinge line bearings in both the rudder and the elevators. Even though backlash checks were well within limits. We also replaced all elevator and rudder servos. The backlash checks on the stab were also well within limits so we didn't replace it.
As we hadn't found anything really conclusive we decided to take the A/C up for a flight test. I was the engineer on board. We managed to reproduce the vibration again. Exactly the same as before. Then we started playing with the EFCS. At one point the captain who was our resident test pilot was flying it in Direct Law with everything switched off! We managed to ascertain that the vibration was purely a function of IAS. You could accelerate through the vibration and fly a MACH no and the vibration went.
Back to the drawing board and the head scratching prevailed. Airbus then told us to replace the stab jack. Even though all checks were in limits. In fairness to them, it did fix the vibration on the next flight. Our questions remained why these wear limits existed in the MM if they were not suitable for safe flight. We never got an answer, but I believe that the backlash limits on the hinge line bearings and the stab have now been reduced significantly.
My own theory however, is that there was not enough aerodynamic loading on the stab and the slightest play would cause oscillation which is amplified though the airframe. I reckon you would be able to reproduce the same vibration on many A320s if you flew at low level(sub FL30), high IAS (not MACH) and empty.
Any 320 jocks fancy trying it?
Last edited by winglit; 15th July 2008 at 23:36.