Sunday, November 30, 2014


Vertigo /ˈvɜrtɨɡoʊ/ (from the Latin vertō "a whirling or spinning movement") is a subtype of dizziness in which a patient inappropriately experiences the perception of motion (usually a spinning motion) due to dysfunction of the vestibular system It is often associated with nausea and vomiting as well as a balance disorder, causing difficulties with standing or walking.

Auto-rough /aw-tow ruhf/ referring to a humorous term used by pilots when flying over hostile terrain or water at night. Their perception or imagination is that the engine(s) sound like they're running a little rougher than when flying over more friendly terrain.

You may remember my recent incident with a rough running engine on a flight in which I was enduring less-than-optimal weather and terrain. In case you missed it, here is a video synopsis:

My first thought was that I may have been feeling the effects of a fouled spark plug. It didn't seem rough enough to be caused by the loss of an entire cylinder, but it also didn't feel like it was purely auto-rough either. In any event, it was enough to ground the airplane until I figured out what was going on.

The first step was to remove all four spark plugs from cylinders #1 and #3, which are the two that are located on the right hand side of the engine, Visual inspection showed no fouling problems at all.  In retrospect, an easier way to do this would have been to run the engine and turn off each of the two ignition systems individually. This would serve to isolate the fouled plug if, in fact, there was one.

Having ruled out an ignition problem, at least with regards to cheap, easy repairs, my thoughts turned to the carburetors. There are two of them on the Rotax 912, and each is responsible for the two cylinders on its respective side of the engine, So more specifically, I began to suspect a problem with the right hand carb. Lending some level of credence to this theory is the fact that there is a current Service Bulletin in effect on the 912 engine, and it has to do with defective carb floats absorbing fuel and becoming too heavy to effectively manage the fuel volume being fed into the engine.

The serial number of my engine is too low to have been caught by the SB, but not so low as to rule it out entirely, especially given the symptoms I was seeing. The nice thing about the SB text is that it includes directions for testing the floats for an overweight condition. You simply drop the float bowl and take the floats to a scale for weighing. If the sum weight of the two is less than 7 grams, you're good to go.

Now, when I say "simply drop the float bowl," I am ignoring the fact that the drip tray underneath the carb has to be removed. I'm not sure what that entails in airplanes other than the RV-12, but in the 12 it means removing the entire carburetor. On the right side of the engine, that is a little tricky in that it is a fairly cramped area. I managed to get it done, and the floats weighed in at a healthy 5 grams. This was good news because Rotax has yet to release a replacement float, and when they do they will be wildly in demand, It was bad news, though, because it had exhausted my troubleshooting expertise.

By now there will be readers shouting at their screens, "What about the carb balance? Have you checked that??!?!!?"

And just now, a whole bunch of other readers are saying, "Whhaaaattt???"
Many pilots are flying on the 912 series of four stroke Rotax aircraft engines. 
One of the areas that are critical for the proper performance of these engines is that the carburetors be properly set up and maintained. 
The 912 series of engines use an altitude compensating Bing carburetor. These are very reliable, and literally trouble free if properly set up and maintained.
The compensating nature of the two carbs is intended to keep them in sync with regards to the power being delivered to each side of the engine. Different power per side = rough engine. They are kept in sync by using a pair of vacuum gauges plugged in between the two carbs. We call this carb balancing.

This is a preview of a training video on the subject. Towards the end of the short video, you will see the adjustment being made. What you can't see, feel, or hear, is what it's like to do this behind the propeller of a running engine on a 27F degree morning trying to adjust those tiny nuts while they're vibrating up a storm and your eyes are watery from the frigid blast of air.

It is a distinctly onerous task.

So, why didn't I immediately consider the carbs being out of balance? Two reasons.

First, I was distracted by the Service Bulletin. It provided what seemed to be an obvious cause.

Second, the carbs had been balanced less than 50 hours ago, and carb balancing is done on a 100 hour cycle.

And, if I'm being entirely honest there was a third reason. What might that have been? Well, see above. It is a distinctly onerous task. Psychologists call this "denial" and/or "rationalization."

I called in a mechanic.

He listened to the engine for just a few seconds before determining that it was, in fact, a problem with the carbs and that the most obvious thing to do was to attempt to balance them. I was still in denial. I had always assumed that as long as I didn't diddle around with them, they would stay in tune. The mechanic squashed that idea like a flatworm under a steam roller: if that was the case, why would you need a periodic inspection?

Well, yeah, when you put it that way...

So.... more than a little embarrassment on my part, but I got the last laugh:

I made him stand behind the prop of a running engine on a 27F degree morning.

It obviously cost me more in dollars than if I had done it myself, but I figure I gained sufficient value: the peace of mind that comes from having a knowledgeable expert do the work while I watched from the sidelines comparing his professional technique to the way that I've been doing it, and the new found knowledge that if I ever get into more trouble with the engine I now know where I can get help.

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