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What’s That Vibration?

Posted 4 years 96 days ago ago by Randy Rowles

A few weeks ago, a pilot within our company was re-positioning a helicopter from the hangar to the ramp. While hovering, he experienced a rough engine, so he landed the helicopter on the taxiway. Seeing this, I walked over and joined him in the helicopter to aid in determining the issue. It was now that I learned that experience may falsely overshadow reality.

The helicopter had a piston engine and by all appearances, the issue was carbon build up on the spark plugs. I demonstrated to the pilot how to clear the carbon from plugs which took a couple minutes. The helicopter began to run smoothly, and problem solved. The pilot continued re-positioning the helicopter to the ramp with no further issues.

Over the next few days, the helicopter developed a small vibration from the engine now and then, but the pilot addressed it and with confidence continued flying the machine. That was until the Director of Maintenance, also a company pilot, took the helicopter out for a flight. He too felt the vibration, however he wasn’t as convinced my diagnosis was correct.

After attempting to clear the plugs as I had recommended with no success, he proceeded to pull the spark plugs to see which cylinder was at issue. Well this is where things got interesting!

There was a large dent protruding from the cylinder valve cover. This wasn’t visible with the cowlings in place. He then removed the valve cover only to have the rocker arm assembly fall into his hands. The entire rocker arm assembly cast had failed and was lying inside against the valve cover. To his amazement, the push rod was straight as an arrow! We assumed a stuck valve had caused this to occur, however with no bending of the push rod, this made absolutely no sense.

After working with the engine OEM, we conducted all the required inspections and the helicopter was repaired and back in service. This event could have been much worse. 

My belief this was a simple issue and validation based upon the expected result of leaning the engine to clear the plugs overshadowed further testing of the problem. Once this event occurred a second time, maintenance should have been contacted to look closer at the issue. Having the spark plugs get carbon buildup is not totally abnormal, but consistent issues should always be investigated.

In many situations, we use our experience to convince ourselves that a simple maintenance issue is so like previous issues, our knowledge is without fault and good to continue. However, this could easily lead to a link in an error chain. Acceptable deviation based purely on your own opinion can kill you, especially regarding an operations or maintenance issue. Take the extra step and get others involved. There may much more to the situation than meets the eye! 

About Randy:  Randy Rowles has been a FAA pilot examiner for 20 years for all helicopter certificates and ratings. He holds a FAA Gold Seal Flight Instructor Certificate, NAFI Master Flight Instructor designation, and was the 2013 recipient of the HAI Flight Instructor of the Year Award. Rowles is the owner/president of Helicopter Institute.