B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 88
Situational Awareness; Training / Qualification
Aircraft Equipment Problem Less Severe
General Maintenance Action
Aircraft; Human Factors
When I received the aircraft from another Crew; I was told of an MEL for the number two engine vibration indication. I was also told that they had called for a Mechanic to look at the engine and that the aircraft was going to be taken out of service [at destination.] This caused me concern because it seemed to be more serious than just an indication problem.The previous crew stated that the number two engine vibration indication showed out of limits during climb and cruise (four to five units). However; they did not feel a vibration and were able to maintain limits by reducing power. Therefore they had MEL'd the vibration indicator earlier in the day. The logbook also showed two previous writeups for the number two engine vibration indication and had been cleared after testing. The Mechanic then talked to me and stated that he had done a visual inspection and did not see any obvious FOD or damage to the engine. He also stated that if the indicator showed five units of vibration it would be felt in the cockpit and/or cabin.I asked my FO what he thought and he told me that he felt comfortable taking the aircraft. Based on the information I received from the previous crew and the Mechanic; I decided that it must be an indication problem and accepted the aircraft. During the initial climb; I clearly felt a high frequency buzzing vibration in the rudder pedals and floor. The number two engine vibration indication began to climb and soon exceeded limits. The FO and I decided that perhaps the number two engine vibration indication was correct and there was a problem with the engine itself. We followed the QRH and reduced thrust in the number two engine below 90 percent N1 which brought the vibration within limits. Since we could maintain limits we decided to continue to [our destination].The vibration felt was definitely noticeable but not very strong; and it was strongest during climb and disappeared during descent at idle thrust. The flight attendants stated that they did not feel any vibration in the cabin. I wrote up the vibration in the logbook when I got to [our destination]. This was a frustrating flight for me because on one hand I felt that I was correct in accepting the aircraft based on the information given to me. On the other hand; I felt like the aircraft was not in an airworthy condition and probably should have been taken out of service earlier in the day. In addition; I had never had an engine vibration problem and I don't know what four to five units of vibration should feel like. Perhaps the indicator overstated the actual vibration and what I felt was less than what the indication showed.We don't get trained on what an engine vibration should feel like if it is out of limits; compared to within limits. Perhaps this could be discussed during training.
B737 Captain believes that he may have flown an aircraft with actual high vibration that had been MELed with a faulty vibration detector.