Symptoms of Losing the Idle Mixture Screw

We had some complicated plans for getting to Oshkosh Airventure 2014. The problem was that we had a wedding to go to in Boston on the Sunday before Oshkosh, and we had to be back at Oshkosh in time to set up the cookout on Tuesday. The plan we came up with was for me to position the airplane to Oshkosh in advance by myself, then to airline to Boston to join the girls. Then after the wedding, we would all airline back to Oshkosh and enjoy the show. I’m oversimplifying the plan here, because there is obviously no airline service to Oshkosh, but if I explain the whole thing here you’ll likely get bored and quit reading. So to get back to the Bearhawk part of the story, I loaded everything up that we would want for our trip but not need in Boston, and set out on Thursday morning for what should have been about 6 hours of flying. For some reason, the first trip to Oshkosh for several Bearhawks has historically been an unlucky one. If mine had been uneventful, I wouldn’t bother writing this entry, but as you will see, it was not.

Weather for the first half of the trip was so-so; it was good enough to go, but it was less than ideal. My plan was to stop just northeast of Lexington KY at a small airport with cheap fuel. From there I could top off and fly all the way to Brodhead WI to camp for the night, then fly the short hop to Oshkosh on Friday morning to pick up a rental car. As I made my way over the mountains towards Kentucky, I was easily able to maintain VFR conditions above variable cloud covers below. At one point I climbed as high as 11,500 briefly, and the airplane was performing well in spite of a substantial load of “stuff.”

The forecast called for clearing conditions in the Lexington area when I was scheduled to get there, but as I got closer, I could see that those improvements were running a bit behind. I was on top of a flat but solid layer that I could see was breaking up to the north. I was showing a fuel quantity on board of around 8 gallons, and while that’s an hour of flying, it’s not much! One plan was to call Lexington approach and file a pop-up IFR to get through the thin layer. Once underneath, I could cancel and proceed VFR to my original destination. The only problem with this plan was that for some reason, my primary radio (the only panel-mounted radio) was not transmitting my voice. Are you starting to see how a superstitious person might be able to wonder if these glitches were related to my destination? The only way to use that plan would have been to revert to my handheld radio, which didn’t seem wise for IFR in Class C airspace. So I looked at the map, and saw that Frankfort was the next option past Lexington. It was farther than I wanted to go, but based on wing root sight gauges and Dynon fuel flow-based calculations, I had enough fuel to get there with legal reserves.

I was flying right over the top of the LEX Class C airspace as I saw for sure that the clouds were not going to be a problem. The clouds and the airspace led me to stay higher than normal, instead of making a power reduction to start descending towards FFT. Once clear of the outer airspace ring, I reduced the throttle to near idle to start what would need to be a circling descent. That’s when the engine started missing. It wasn’t entirely obvious at idle, because in that condition the relative wind is driving the prop and engine, rather than combustion. As I opened the throttle a little to see if it was indeed missing, I confirmed that the engine was running intermittently.

This was not cause for dire concern for a few reasons. One was that because of my high descent profile, I had enough energy to make it to FFT without any engine power. Another was that I had enough altitude to tinker with it while I was on the way. Keep in mind that the power-off glide in our Bearhawk is on the order of 500 feet per minute, allowing around one minute of glide per thousand feet of altitude. This meant I had around 8 minutes to sort it out. While these factors meant the power loss was not a dire concern, it was still a pretty serious concern, mostly because I didn’t know why it was happening. I assumed that the most likely cause was fuel delivery, because I had not yet ever flown the airplane with so little fuel on board. Maybe it was sloshing around somehow and not getting to the engine. Of course I realize now that I could have ruled this out if I had checked the fuel flow gauge, but at the time I didn’t think of that.

The situation became doubly undesirable when I finally had the FFT runway in sight. It was well within my gliding range and beautiful to see, except for the two big X marks. That’s right, it was closed for resurfacing. At that point, I had no intention of landing anywhere else. An off-airport landing is much less desirable than an off-runway airport landing, and while it was not exactly safe to land on a closed runway, I could see that the runway was clear of any personnel and equipment that I might have endangered by landing there, and since I was exercising my emergency authority, coloring outside of the usual lines was allowed to get the airplane on the ground safely. If the runway had not been clear, I could have made use of the taxiway, or even the surrounding grass.

I landed uneventfully and coasted off of the runway, and attempted to restart the engine to taxi onto the apron. It would run, but not well. The airport manager met me at the airplane to amicably express his discontent for me having just landed at his closed airport. I explained that it was an emergency landing, and that we’d sort it all out, and he agreed. We pushed the airplane to a tiedown spot and I started recalculating my complex travel plans. The airport manager said that he wouldn’t be able to let me depart from the closed airport, which was really going to complicate things. Fortunately, the construction project was nearing completion, and the airport might end up reopening on Saturday, or more likely Monday. The only viable option was for me to leave the airplane there and get to the wedding, then return after the wedding to fly the airplane to Oshkosh.

After an explanation to the friendly (and quite reasonable) local FSDO inspector who wanted to hear why it was that I felt like landing at the closed airport was the safest course of action, I took a taxi to the Lexington airport. My plan was to catch an airline flight to Chicago, with plans to catch a second flight to Appleton. From there I’d try and figure out how to get to OSH, then try to figure out a place to sleep there for one night, then wake up on Friday morning to pick up the rental car and resume the original plan. Notice that the details of that plan start getting sketchy at the end. That’s part of the adventure of flying a GA airplane on a long trip- one is never really sure that he’ll get there at all, much less according to the original plan.

The flight to ORD was uneventful, though a little bit delayed. Fortunately the crew that worked that flight was also the crew for the Appleton flight, so it was also delayed. While in the gate area I noticed one of my coworkers, Mark; this is one of the few perks of having to all wear the same flammable work clothes. We had not met before, but we talked for a little while and he explained that he was picking up a rental car in Appleton to drive to Oshkosh upon arrival, and offered me a ride. This turned out to be most useful. I figured if I could get to the show grounds, I could find a place to sleep, even if it was under Chris Owens’s Camper RV, which he had pre-positioned in his usual campsite. As we flew from Chicago, I looked out the window to see the beautiful green countryside, blue sky, and calm winds. I really wish I had been able to fly the Bearhawk instead! Conditions were just perfect. I had a great visit with Mark as we drove from Appleton, and he dropped me off in the campground at OSH. It was somewhat surreal to be on the grounds before the start of the show. Things were certainly ramping up, but the airplane parking areas were primarily empty and there weren’t any crowds.

After sizing up the grass under Chris’s camper, the scant clothing that I was wearing/carrying, and the forecast overnight low, the option of sleeping there was becoming less appealing. I had brought a tent for sleeping at Brodhead, but I was having to travel light, so it was still with the airplane. Then it occurred to me that it was Thursday, and while the local hotels are historically unobtainable during the show, I thought it would be worth asking what the rate would be. When I called the Super 8 the lady told me that they had plenty of room, and the rate would be $70. I didn’t need to take a second glance at the grass to make my decision! I started walking in that direction, enjoying the memories from the special place, the beautiful sunset, and the mild temperature. I was not enjoying the airshow that the mosquitos were putting on around my head, but they only served to remind me of 100+ good reasons for an indoor sleeping space.

After checking in I walked over to Friar Tuck’s for a hot crock of french onion soup and a roast beef sandwich, my first consequential food for the day. The next morning I woke up and continued with the travel plans, including procuring a used child seat for the rental car. The seat I had planned to use remained in the airplane, a casualty of our fragmented plan. The logistics of all of this trip would have been complicated without the need for carseats! It’s fun to have little kids, but they step up the travel complexity significantly. I drove the car down to ORD and parked it as planned, and took a flight to Boston. From there I took a bus to Cape Cod to catch up with the rest of my family tree, arriving in the middle of the night. We had a great time at the wedding on Sunday, and well before the sun came up on Monday, Tabitha’s parents took us to the airport so that we could get back to the Midwest. The girls flew to ORD to pick up the rental car, and I flew to ORD to catch a flight to LEX. After another expensive car ride to FFT, I was finally back in position.

We added some fuel to the airplane, though not a top-off due to the high fuel price. The airport was not open yet, but it was expected to be open that afternoon. This gave me some time to troubleshoot the voice problem with the Garmin 430. My research over the weekend suggested that removing and reseating the radio in the tray might fix the problem, and fortunately it did. I prepared the airplane so that I could hop in and go as soon as the airport opened, and after a few hours of waiting, the manager finally gave me the thumbs up. I hopped in and started the engine, and it wouldn’t run. This was most frustrating.

It was acting like it wasn’t getting fuel. If I pumped the throttle it would stumble along, and if I got the RPM above 1500 it would run reasonably well. A local airport patron named Gene was hanging around, and he helped me with some troubleshooting. He was a long-time owner, pilot, instructor, and home builder, and was immensely useful. One problem that I found was with standing water in the airbox. The FAB instructions say to drill a hole in the back to allow water to drain, and somehow I had omitted that step. Shortly after Oshkosh, I saw a mention in one of the magazines that said this is a very common problem to pop up at Oshkosh, since so many of us keep our airplanes outside for the first time while there. I drilled a 3/16″ hole and allowed a cup or so of water to run out onto the ramp, hopeful that I had fixed the problem. I hopped back in and started the engine, only to find no change.

I’ll save you a lengthy review of everything that we checked, including draining the float bowl, which required removing the airbox, which required removing the cowl. With all of that stuff out of the way, I happened to look at the back of the carb to see a hole where the idle mixture adjustment screw was supposed to be! This explained the problem. The engine would run fine at high power settings, when the idle circuit was not in use. At low power settings, the idle circuit was running very lean, since there was a 1/4″ air hole where the needle was supposed to be. The friendly local mechanic loaned me a needle off of one of his grounded airplanes, and after reassembly, everything ran great. It turned out to be harder than I expected to replace his needle. I started the process of ordering a replacement at the Aircraft Spruce booth, but it ended up taking several weeks, in part due to the complicated nature of ordering carburetor parts from ACS. This ended up creating a most-confusing situation in the subsequent weeks, where when I saw the shipment finally leave ACS, I sent the mechanic his original needle back, but at the same time he called ACS and redirected the new needle back to FFT, presumably assuming that I had run away with his needle, etc. Our carb is the ubiquitous 10-3878, but it’s not readily clear which variants of the MA4-5 use the same part number for the needle.

The good news was that I had a running airplane and another hour or two of daylight, so I finally got out of FFT and headed north. OSH closes at night during the show (and I don’t fly the Bearhawk at night anyway), so I knew I wasn’t going to be able to make it all the way, but I also knew that closer was better. I stopped in Indiana for the night, topping off on cheap fuel and sleeping like a king in my little tent. I wasn’t sure if camping was allowed at the airport, so I asked the tower controller when the first folks started coming around in the morning, and set my alarm for a few minutes prior. The next day I was cranking the engine just as the first car was arriving, and just as the dawn sky was lightening.

I flew from there to southern Wisconsin, where I topped off on fuel again. I didn’t want to have to worry about fuel while at OSH. I arrived at about 9:30 in the morning, and taxied to the Bearhawk Aircraft display booth. Mark’s other options for display airplanes fell through, so he put ours to use. My late arrival meant that we wouldn’t have time to clean out the airplane, or otherwise make it any more presentable. While at the show, I stopped by the Precision booth to ask if they’d ever heard of the idle mixture screw falling out. They said it can happen if the needle is backed out to a point where the spring is no longer applying good pressure to immobilize the needle. This was likely the case on our carb, because I had it backed out pretty far. The guidance about how to set the idle mixture says to look for a 50 RPM rise during shutdown. I incrementally backed the needle out looking for that rise, but never saw it. If you find that yours does not seem to respond to input, be wary of backing it out too far.