Before explaining the details, I should say that this story is not going to describe an "oh sh!t" moment, however it did raise more than a few pulses at Goodyear airport.
So I went to the airport the other day to practice some touch and go's. The weather was not great; 5,000 ft ceilings and about 10nm visibility, so I didn't want to go for a long ride. I filled the tanks and did a normal pre-flight inspection, jumped in and headed out.
During my normal run-up procedure, one of the things that I normally check is proper operation of the MT prop controller. The electrically adjustable constant speed prop has a small box with a knob to dial an RPM from 1,700 to 2,700 RPM, and an override switch and a toggle switch to manually control the blade pitch. A simple check is done during the high RPM run-up by dialing in 1,700 RPM and making sure the fine pitch limit light turns off and the RPM holds. That worked fine.
Since everything else was normal as well, I called for taxi clearance and headed out to the runway. During this point of the day, it was pretty busy. There were several airplanes in the pattern, several awaiting departure (including a Lear jet - but we will get to that in a minute).
At the hold short line, I did my final cabin check and called for takeoff clearance, to remain in close traffic for touch and go's. I eventually got my clearance and I lined up on the runway for takeoff.
Now, the MT prop is a great product, but it definitely has a character about it that you have to understand and work with. The prop controller in the cabin transfers electrical power to the prop hub through a brush and slip ring system, similar to a de-icing boot system. An electric motor in the hub adjusts the pitch of the three blades. There is a mechanical stop system to define the limits of travel. Hitting the stop causes the motor to draw more current, which signals the controller to stop moving.
Pretty basic stuff, but the detail is that the motor in the hub is pretty darn slow. From full coarse to full fine pitch requires about 18 seconds. Fortunately most adjustments are not really that time sensitive and will be completed in just a few seconds.
The other thing to remember about any airplane, is that when you do a static full power run-up on the ground, you probably won't hit any redlines. The prop will not reach max RPM unless it is flying through the air.
OK? Back to the runway.
I was cleared on runway 21 at A3 to takeoff and maintain upwind to the river bed to avoid inbound traffic. I acknowledged the clearance and applied full throttle. After the usual brief takeoff roll, I started climbing out at 90 KIAS, which should have given me about 1,500 fpm climb.
I noticed early on that the GRT EIS 4000 started giving me an alarm for RPM, which is not too unusual. Like I said, sometimes the prop is a little slow to respond. I reduced the controller to 2600 to get some more pitch and continued as normal.
A few seconds later, the engine wound up to an extremely high RPM which was way beyond the safe operational limits of both engine and prop. So now it was interesting. I immediately pulled power back and assumed I was flying fixed pitch. Unfortunately, full fine pitch is NOT a good thing. At less than 1/2 throttle, I was at 2700 prop RPM, and not climbing very much at a now greatly reduced airspeed. At this point, I am on upwind, maybe 300 ft AGL.
Since I was still climbing, albeit at a grossly anemic rate, I didn't consider this a grave enough situation to be an emergency; just a high priority situation. I didn't want to get too far away from a runway, so I turned crosswind before the river bed and waited what seemed to be a very long time before there was enough of a break in the radio traffic that I could slip a distress call in.
I told the controller I was at reduced power due to loss of prop control, and offered to go around the pattern for landing on runway 21. Instead, ATC made a flurry of radio calls went out to redirect traffic and clear me to return to runway 3.
By this time, the burning oil smell from the excessively high RPM engine run hit me, and I was more than happy to turn direct to the numbers on runway 3. Oh, remember that Lear jet? Those guys were on their takeoff roll, which is one reason I was hesitant about turning back. While I'm sure they were none too happy about aborting their run, they were clear of the runway by the time I touched down.
Aside from an adrenaline high and raised pulses, there were no other problems.
After returning to the hangar, the first order of business was to get the cowl and spinner off for a mandatory prop overspeed inspection. I found no leaks, no electrical problems, no anomalous things of any kind. However, I did notice that the slip rings were pretty dark. I have been cleaning them at the annual condition inspection every March. For whatever reason, they were pretty black, even though I have not flown much this past summer. I used some 1,000 grit sandpaper to polish them back to shiny.
After installing the Gen-3 redrive this year, I did some full power ground runs to make sure my static RPM was still in a good operating range. So I was surprised to see that repeating that test caused the prop to go past the redline RPM. I adjusted the fine pitch stop all the way to its limit, which finally brought the RPM down to 2500. I suppose it is possible that the cold air increased performance a bit, but that seemed a little excessive.
After a few more high power ground tests, I felt confident that the problem was under control. I put everything back together, signed off the logbook and did four touch and go's. Smooth as silk, no problems.
I have drawn three conclusions from this situation.
1. Don't give a friggin engineering dissertation to ATC when you have
a problem. Just say the "E" word and say you need to land.
2. More frequent and thorough cleaning of the slip rings is needed.
3. Full power static run-ups should be done periodically just to make
sure nothing changes.
My theory is that due to the buildup on the slip rings, and the excessively fine pitch of the blades, there were current draw issues that caused the prop controller to be confused about how to change the pitch.