Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 16000
Aircraft Equipment Problem Less Severe
Air Traffic Control Issued New Clearance; Flight Crew Diverted; Flight Crew Landed As Precaution; General Maintenance Action
Aircraft
Climbing out the aircraft began to vibrate in a steady; quick rhythm that felt similar to a PIO (Pilot Induced Oscillation) or a bouncing sensation like being on the end of a diving board and bouncing up and down (porpoising) in the Longitudinal axis of the aircraft.  The Autopilot was engaged in the Command A (Left Side) position and was in a VNAV Climb passing through FL180 at 280 Knots. I was the Pilot flying and noticed it immediately and after just a few seconds I disconnected the autopilot to see if it might have been the cause of the bouncing (thought it could be an autopilot servo malfunctioning). The bouncing sensation continued and actually gained intensity. Next I instructed the First Officer (PM) to roll the airspeed back to 260 knots to see if this flutter or vibration would stop. At the same time I informed ATC that we needed to level off at FL180 to deal with a possible flight control issue. We descended back down to and maintained FL180. As the airspeed bled down through 270 knots I could feel the intensity of the pitch/bounce decrease; and by the time we had slowed to 260 knots the sensation had disappeared. We went through the QRH looking for an applicable procedure and only found the 'Elevator Tab Vibration' checklist. After reading through this procedure we decided that all of the steps had already been complied with in our initial reaction to this event. This did not seem to really fit the situation that we were dealing with inflight. By this time ZZZ was about 80 miles in front of us; and I elected to divert there for its long runways; maintenance; and passenger capabilities. We decided continuing straight ahead in the decent; would be the most prudent/safe way to proceed to get us on the ground in the shortest amount of time while finishing the rest of our duties involved with the Emergency and Diversion. We would land about 1000 pounds overweight. I briefed the Flight Attendants and passengers; then prepared for our arrival. The vibration actually returned as we slowed through about 240 knots; and then went away again. Our landing was uneventful with a smooth touchdown at 145000 pounds. After a quick exterior check by ARFF Emergency vehicles; we were cleared to taxi to the gate.
The reporter stated that Maintenance was unable to find a definitive cause for the oscillations and that a subsequent test flight did not produce the oscillations.
B737-800 Captain experienced short period pitch oscillations climbing through FL180 at 280 knots. The autopilot was disengaged with no change then the airspeed was decreased to 260 knots and the oscillations disappeared. The crew elected to divert to the nearest suitable airport.