Never Assume The Solution

By Bobby Day 

On a hot summer afternoon, I took my cousin’s wife and their foreign exchange high school student guest for a short hop from my home field to a neighboring airport about 20 miles away.  It was the exchange student’s first ride in a small plane and, since my plane had an auto fuel STC, I was going to fill up on mogas (auto fuel) that was available at our destination.  After the short refueling stop we piled back into the plane, fired it up and prepared to depart.  Run up was normal - except that the RPM drop on the carb heat check seemed a little greater than normal.  I made the usual radio call, back taxi to the end of the runway, position for take off, radio call for departure, full throttle, acceleration, and suddenly total silence!  With no coughing, spluttering or anything else, the engine decided that it was time for a break. 

We had enough momentum to coast off of the runway back into the ramp area.  It was time to figure out what was causing the problem?  I noticed, very quickly, that the electric fuel pump would run continuously and not build up pressure so obviously we had a vapor lock in the fuel system.  This had never happened to me before, but logic supported it.  Auto fuel has a lower vapor pressure than avgas, Normally, I fly to my destination, park the plane, go somewhere, come back quite a bit later after the engine has had time to cool down, and then depart.  This time we had only been on the ground with the engine shut down for 10 to 15 minutes before starting up again.  This didn’t allow the engine to cool.  On a hot summer day shortly after shut down, conditions are going to be as hot under the cowling as they would normally ever get.  Combine that with the lower vapor pressure of mogas and the conditions are favorable for vapor lock to occur. 

We opened the cowling up to provide some ventilation and waited for things to cool off.  Meanwhile, I did some cogitating about how to prevent this from happening in the future.  I decided that I would only put mogas in one tank and keep avgas in the other.  Then, if I was going to make a short stop I just had to shift to the avgas tank several minutes before landing to purge the mogas from the fuel system in the cowling area and I should not have a repeat of the problem.  After waiting long enough, the electric fuel pump confirmed that it was now pumping fluid again instead of vapor by slowing down to a slow ticking and by building up fuel pressure. 

We climbed back in, did another run up with everything normal, took off and had a nice flight back to home base. --- End of story.  Not exactly!!! 

Several hours later, I went back out to the airport with my cousin to take him flying.  I started the plane, taxied to the run up area near the end of the runway; experienced a good run up, position for take off, radio call for departure, full throttle, acceleration, suddenly total silence!  Sound familiar???  Needless to say, this time there was definitely no vapor lock. 

While there had definitely been vapor lock present at the previous occurrence, it does not seem to have been the cause of this engine failure.  Some experimenting showed that the problem could be repeated although not every time and when it did occur, the engine could be kept running by leaning the mixture.  The next thought of probable cause was the potential for having taken on some bad fuel earlier in the day.  I alerted the airport where I had fueled of the potential problem.  The next day I took advantage of some of the laboratory capabilities of my employer and got a sample of fuel analyzed.  The result was that no problem was found with the fuel.  Next stop was the aircraft maintenance shop where the carburetor was disassembled and inspected – no problem found.  Now what??   

The mechanic said that he knew of a few cases aircraft had experienced power reductions but not total loss of power due to muffler internals coming apart and partially blocking the muffler outlet.  A look up the exhaust pipe with a flashlight revealed a bright shiny piece of metal reflecting light back at me and partially covering the muffler outlet to the exhaust pipe.  This was in spite of two wire bales installed in the top of the exhaust pipe specifically for the purpose of preventing this problem.  It had apparently been moving around in the bottom of the muffler and sometimes blocking the outlet and sometimes not blocking the outlet.  A new muffler fixed the problem.  The internals of the old muffler were actually so brittle that they would shatter almost like glass when struck with a hammer. 

So - for every problem there is quick, simple, readily apparent wrong answer.  If the answer comes too easily, be very suspicious. 

Editor’s Note:  Bobby Day is an instrument pilot that flies out of Clarksville (AR).