The Trailing Edge: A Weird FCF and Some Recreational Maths

May 2023

For your enjoyment, a non-technical story of unexpected circumstances that was my morning. Don't worry...we currently have two very technical offerings in the works which will probably be released within the next month or so.

Recently I finished the once-a-year condition inspection of the Combat Bearhawk (that is legally not called an "annual"). The final step is a Functional Check Flight (FCF) to ensure that everything on the airplane is still working the way it should. The FCF is flown solo to minimize risk to personnel if something should go wrong. I would have flown the FCF last week, but the winds were pretty high all day on that Saturday.

I have a bunch of checks that I do airborne of various systems. One of the checks that I do is to make a landing and takeoff from Fox (KWJF), which is a tower-controlled field. The primary purpose of this is the ensure the comm radios are working properly. The approach and landing are made talking on one radio, and the takeoff and departure are made talking on the other radio.

Generally, after taking off from Fox, I simply return to Rosamond (L00) and the FCF is over. However, there was a new wrinkle today. Last Tuesday I was flying gliders at Mountain Valley (L94) in Tehachapi, and after telling myself several times not to leave my water bottle in the glider, I got distracted and left my water bottle in the glider. I figured after I was done at Fox, I would fly the 0.5 up to L94, pick up my water bottle, and fly back to L00.

So this morning I woke up early, had breakfast, and headed out to the airport. Winds were light and variable, and the skies were clear, so everyone who had access to an airplane was going to be out today. I did the ground checks and was ready to go before 0700. Fuel on board was 36.1 gallons. The airplane uses around 10 to 11 gallons an hour in cruise, so I figured about an hour for the FCF and an hour to go to and from L94 would leave me a little low, around 16 gallons. Still well above the one hour minimum that I insist on, but lower than I usually run it. Also, I don't know that I have ever tried a takeoff with the fuel that low. The front outlet of the tank might unport during the acceleration of takeoff, but the rear outlet should be able to handle it. It is gravity feed to the fuel pump, so the unporting of one line should not be a problem.

Everything was going as planned as I pulled into the run-up area to do my usual checks. A Cessna 180 pulled up and called my by name on the radio, asking what I was up to today. I'm still not sure who it was. I told him I was doing the post-inspection check flight, and he wished me a safe flight as he passed and took off.

Takeoff was nominal as I climbed out to the area and started the litany of checks. Everything was working as expected. (This makes me nervous, because I feel like there always has to be a squawk, and I can't figure out what it is if everything seems to be working.) I did have another thought on my mind which is not on the usual FCF cards. In a couple of months I will be heading to AirVenture at Oshkosh. The arrival procedure has you fly at 90 KIAS at 1800 feet. Even though I've done this three times before, I wondered what throttle setting (MAP) I would need to hold 90 KIAS so I don't have to search for it under fire. Since I needed to slow down for a stall to check the AOA indicator, I simply stopped at 90 KIAS for a while and found that it took 15 inches of MAP. However, this was at 7000 feet, and I figured there would probably be an altitude effect. Still, it was a place to start.

After finishing the airborne VOR check over Fox Field, I descended to enter the pattern at Fox. This time I stopped at 3200 feet (about 800 AGL) and tried trimming at 90 KIAS again. This time it was about 15.5 inches of MAP. That's as low of an altitude as I can go without going over into one of the adjacent valleys. I continued in for a nominal landing and takeoff.

Now I headed toward L94, climbing to 6500 feet to get over the ridge, then descending back to 5000 feet for the pattern. I fly this same pattern a lot in the PA-25 Pawnee towplane, but I've come to learn that the Combat Bearhawk cannot generate the same drag and approach angle as the Pawnee. Add to that the first half of the runway is sloped downward. Also I turned base too early, which put me high on the approach. I should have slipped it to kill some of the altitude, but didn't think too. Finally, there is a strong suspicion, supported by evidence, that the "light and variable" winds were actually a tail wind. I finally braked it to a safe landing significantly longer than I usually do in the tow plane. With a bright yellow airplane, of course everyone notices. I'll get it figured out someday.

Even though it was only about 0830, things were already hopping at the gliderport, with lots of people there getting ready for what promised to be a good flying day. I picked up my water bottle, which apparently is rather distinctive, since I was told that over the past 3 days many people had correctly identified it as being mine. It's always fun to have your mistakes in public.

While getting ready to depart, I noticed the tow plane taxiing out for an East departure, as opposed to the West arrival I had just made. I looked at the wind sock, and this was my first clue that I may have landed downwind. I taxied out for an East departure. As I was getting ready to take the runway for departure, I got a radio call by name asking if I was there to go soaring today. This seemed weird to me, since I was about to depart. I would later find out it was from Aaron Wenner, who was overhead, coming from Bakersfield and on his way to Kern Valley.

I took off, climbed over the ridge, then descended to L00. The wind sock looked limp, stuck in a crosswind, but the weather reporting was calling a light wind from the East. Thus, I turned to set up a landing for runway 8. There was another airplane on the ground who had also decided that 8 was the best choice. I landed slightly long, and taxied back to the fuel pumps, since I was now down to 20 gallons remaining.

I set everything up, and everything seemed to be working normally and sounding normal, but when I pulled the lever, a trickle came out of the gas nozzle, then nothing. I tried resetting everything and got the same result. Not knowing what to do, I called Fuzzy Zellar, who I had been told long ago was in charge of the fuel system. He didn't answer his phone. I decided to call another Skypark resident, Paul Rosales, who didn't answer his phone. Wondering what to do, my phone rang about 30 seconds later as Paul was calling me back. Paul told me he was calling from the beach of St. Croix in the US Virgin Islands, which proves that long distance isn't expensive like it used to be when I was in college. I told him my tale of woe, and he said he would text me the contact info for the guy who is actually in charge of the fuel. He also suggested I look in the hangar to see if he was there. Nobody was in the hangar, and two guys sitting nearby said that guy had just left in an airplane. I tried calling him, with little expectation of success, and my expectation was warranted.

I put all of the fueling stuff back. Faced with an airplane with very little fuel, and the next expected flight in two weeks would need full tanks, I wrestled with my options and decided I had more time available now to make the 0.3 flight to Fox to get gas. I fired up the airplane, and out of habit taxied down to Runway 26. Sitting in the run-up area and looking at the wind sock, which was now indicating an east wind, I realized I was at the wrong end of the runway. Considering that I could probably make a downwind takeoff safely, I decided that it was better (in light of everything else that had happened and I was slightly flustered for now having two sorties I hadn't planned on) to bite the bullet and taxi down to the correct end of the runway.

After the usual checks, the Combat Bearhawk took off with 19.8 gallons remaining with no problems at all. I usually request a downwind entry to the Fox pattern, but decided this time to take the shorter route to a base entry that they always want to offer me. ATIS was calling a light wind out of the east, but the Tower was still using the west runway, which probably contributed to why this landing seemed longer than usual.

I pulled up to the gas pumps and pumped in 31.06 gallons with no drama. As I was finishing up, a guy with a Super Decathlon at the other pump said he was happy to see a Bearhawk in the wild, having never actually seen one. Discussions led to me telling him I worked at USAF TPS and he said he worked at Scaled Composites. He left to meet his student, and I climbed back in for the fifth takeoff of the day. This time I didn't have to tell the controller I was departing to the north--he remembered from earlier.

I flew the 0.3 leg back to L00, taking off from KWJF runway 24 but landing L00 runway 8. As I turned to downwind, another radio call by name (who are all of these people who recognize my airplane?) telling me he was on the crosswind and would be behind us in the pattern. Another acceptable landing and I taxied back to the hangar. Before I put the airplane away, I texted the guy in charge of the fuel and told him of my problem. Later, he would text me back apologizing, saying that there was 2000 gallons of trapped fuel in the tank (basically, it was "empty") and that another fuel shipment was scheduled for Monday.

And later I would realize that I left my water bottle in another airplane, this time the Combat Bearhawk. It finally made it home later in the day.

Here comes the recreational maths...

I thought 2000 gallons seemed like a lot for unusable fuel (my airplane only holds 50 gallons), but I knew that volume is a tricky subject to estimate. I wondered just how deep that would be in the tank. Rather than drive out to the airport to measure the tank, I used the Google maps satellite view to measure the size of the tank. I knew there would be errors, but it would be close enough. The tank was estimated to be 9 feet in diameter and 34 feet long. I was going to walk you through all of the equations, but then thought better of myself. Based on those dimensions, I estimated the capacity of the tank to be 16,180 gallons. Thus, that would be 12.3% of the capacity that was unusable. Looking up some formulae for area of a segment of a circle, I estimated that the remaining 2000 gallons would be about 19 inches deep at the center of the tank, with a free surface 7 feet wide.

That still seems like a lot of unusable fuel. I wonder if it is that much to keep sediment and other krap from being delivered out the hose?

Anyway, after five sorties and temperatures already in the 80s (F) and on their way to the 90s, I was already pretty worn out, and it was only lunchtime.

Thus went my day of unexpected events. Fortunately, nothing was broken and no one was hurt.

Confession is good for the soul.

- Russ Erb