October 1, 1991
It was a beautiful fall afternoon in Texas. My wife and I had flown our much-modified Cessna 170 to Austin to see the University of Texas Longhorns play football. When the game was over, we headed home.
I was relaxed during my preflight inspection. After all, it was my airplane. It was always in my own locked hangar when it wasn't flying, and because I am an A&P, I maintained the aircraft myself. I knew there was nothing wrong with it. I had supervised the refueling in Austin and knew that the correct quantity of 100LL to fill both tanks had been put on board. On my walk-around, I did the same thing many high-wing aircraft owners do. I just looked at the fuel tank caps from the ground. I knew the tanks were full, and the caps appeared to be in place, so I didn't go look for a ladder. I was satisfied that we were ready to fly.
Our home airport in Duncan, Oklahoma, would be easy for me to find. Go north out of Austin, pick up V17, and follow it for 252 nm to Duncan. I had made this flight every fall to every home ball game for so many years that, in the very good VFR weather we had, I didn't even need to look at my charts. I knew every landmark along the route. As soon as we cleared the Austin ARSA, I turned the volume down on both com radios and relaxed. The weather was so good that I didn't even look at the nav receivers. I certainly did not expect any problems flying the same route I had flown close to a hundred times before. Much later, I realized that, by being so complacent, I set myself up for what happened.
About an hour out of Austin, we were level at 6,500 feet about 27 nm northwest of Waco on V17 when I saw the first indication that I had trouble. Both of my electric fuel gauges were showing one-half full when they should have been showing about three-fourths each. Were those gas caps really put on right, or was fuel being siphoned out of the tanks? I didn't know. At this point, I easily could have turned back to Madison Cooper Airport at Waco. It was only about 15 minutes away. A precautionary landing to check out the problem would have made sense, but I knew there were a number of airports along the route. I figured I could land later if necessary. I switched the fuel from "Both" to "Left" to see if this would make any difference. Thirty minutes later, both fuel gauges were down to one fourth. I still was not sure what was happening, but I was beginning to consider my options. We were about 45 minutes from home. If fuel really was being siphoned out of the tanks, we wouldn't make it at the rate the gauges were going down. I elected to continue toward home for a time.
When both fuel gauges reached the empty mark, we were still about 15 minutes from the Duncan airport. I decided to call Fort Sill Approach, which controls our local airspace. I figured that I was committed to try to make the Duncan airport but that if I didn't make it, I wanted someone to know where we were. A call to Fort Sill brought no response. I then tried the other radio and another mike. Still no answer. About that time, l realized that I couldn't hear Fort Sill talking to anyone else. This was unusual. Then at long last I looked at the ammeter. When I saw the ammeter showing a high rate of discharge, "the light came on" as they say. I knew at once that I had a failure in the charging system. I knew the battery was almost discharged, and that explained why the radios wouldn't work. It probably also explained why the electric fuel gauges were now both pointing at empty. Probably. I had not actually put my hands on the fuel tank caps. I didn't know they were installed properly. There was still a possibility that fuel was being siphoned out of the tanks and that the gauges were reading empty because the tanks really were empty or nearly so. I maintained 6,500 feet until within gliding distance of the Duncan airport. The approach and landing were uneventful, and the fuel tanks only required the amount to fill that they should after a 2.3-hour flight.
I did some serious thinking after this flight and reached the following conclusions:
The ammeter must be included in the instrument scan on VFR flights just as it is on IFR flights. The most important conclusion of all is that, at least on Cessna airplanes, if there is a failure in the charging system, the electric fuel gauges will go to empty as battery power is lost. The gauges do not change instantly, but they do move much faster than they should for the amount of fuel being used. A pilot who does not understand this easily could decide to make a precautionary landing in an open field when the fuel gauges read empty.
Jack Bradley, AOPA 792121, holds a commercial pilot certificate with a multiengine rating. A retired petroleum engineer, Bradley owns a Cessna Turbo Centurion II. He has accumulated 5,600 hours in 31 years of flying.
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