Question for Paul McBride, the General Aviation News engines expert: I’d appreciate your tips on vibration of the left engine of the Piper Navajo Chieftain that starts at 1800-1850 rpm and ceases at 2000-2100 rpm.
Thank you in advance.
Helio Mario Neves Pimentel de Oliveira, Brazil
Answer: When it comes to vibrations on a Piper Navajo Chieftain, there are several possibilities. From my experience, one of the most common causes is the engine mount rubbers have exceeded their useful life.
When you stop and think about it, the Navajo series engines are mounted further forward than what we’ve seen in past models. While this particular installation has been very successful, we have encountered a few issues that we hadn’t encountered previously — one of them being a vibration you described.
We found that because of the weight of the engine and its mounting position, it hangs out there quite far, which means the engine mount rubbers seem to be more sensitive to operational time in service.
Maybe I can give you a better example by sharing what we learned, especially on the right engine of the Chieftain. On aircraft that were constantly flown by the same pilot, they’d call and complain about the right engine cylinder head temperature increasing in respect to the left engine.
We learned that because the right engine has the air conditioner compressor installed on one side and the alternator installed on the other side in the front of the engine, this reduces the amount of air flow coming into the cowling. Once the engine mounts begin to sag, this reduces the incoming cooling air going into the cowling, further reducing the cooling air over the cylinder heads.
If the aircraft is flown by several pilots, they may not notice the condition versus someone who flies the aircraft frequently and has observed the increase in CHT over time.
Once the engine mount rubbers are replaced with new ones, the problem is solved.
Another thing that the worn-out mount rubbers may cause is interference with the engine cowling at various power settings as you mentioned.
My best suggestion at this point is for you to replace all of the engine mount rubbers and see what the results are.
