Wednesday, July 15, 2009

Maintenance Odds-'n'-Ends

My good friends at Three Wing Flying Services (at KBDR, and I recommend them highly and without reservation) finally managed to print maintenance log stickers for the last couple of times they laid wrenches on N631S. (It seems that they were afflicted by a computer system virus that, among other effects, trashed the driver for their thermal printer.)

The first item was an oil change, accomplished at the same time that they installed the Cessna seat-stop modification. It being summer, we put in Aeroshell 100W oil, as opposed to the Aeroshell 80W that the engine runs on in cooler weather. I've now been following this routine for about 500 hours and the engine seems to be loving it.

I always have a spectrographic analysis done on a sample of the used oil and it has come back from the lab as normally unremarkable.

The second item was triggered by one of those "hmmm" moments during my run-up a couple of weeks ago before a flight from VKX up to KBDR. After taking the engine RPM up to 1,700 and getting a good magneto check and good propeller pitch cycling, I pulled out the knob for the carburetor heat and...nothing happened. Not too good.

The wonderful Continental O-470U engine that pulls N631S along does have one important idiosyncracy. It is an ice maker that would make the Frigidaire folks proud. The carburetor is mounted on the bottom of the engine and the throttle body stands well off under the crankcase, in a comparatively cool environment. As a result, the carb is prone to the accretion of ice in the venturi section. When that happens it can choke off the flow of combustible stuff into the cylinders and the engine stops running. This can be inconvenient at any altitude greater than about 2 feet AGL. The cure for carburetor icing is to provide a valve that diverts the incoming air flow past the nice hot exhaust plumbing, thus preheating it and making ice accretion unlikely. If the carb heat is not working as designed you are left without a weapon to combat carb icing.

On the morning in question, the sky was clear and (more importantly) the humidity was low (i.e., there was a good spread between temperature and dew point). So I assessed the risk of carb icing as minimal and made the flight to KBDR. On arrival I asked the nice folks at Three Wing to take a look at N631S's carb heat valve.

The valve in question is a squarish plate that rotates open or closed on a shaft turned by a bell crank attached to a cable that is pulled by the control knob in the cockpit. In this case, the fasteners that attach the valve plate to the shaft had sheared. I guess after 32 years and 3,750 hours this kind of thing can happen. Tony at Three Wing opened up the carburetor air-box assembly and re-secured the valve to its shaft.

The last item involved the electric pitch trim control. For a while I had been not liking the feel of the yoke-mounted thumb-switch that actuates the pitch trim. It had developed a "sticky" spot. So I asked Dave at Three Wing to look at it. He agreed that it wasn't right and ought to be replaced.

The reason this matters is that the switch might fail in one of its "ON" positions and cause a runaway trim actuation. If this happens at an awkward time (say, just after takeoff) things can quickly get a bit too interesting.

The bad news is that the manufacturer wants $495 for a new switch! (No, it isn't made out of gold - just priced that way.) So for the time being, I have elected to render the electric trim option INOP and make do with manual trim adjustment. I am on the lookout for a used switch and meanwhile Three Wing pulled the electric trim circuit breaker and made a nice little placard saying "Electric Trim Inop".

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