Identify misfueled aircraft


fc-cover-photo-smJet fuel is kerosene, while most light aircraft piston (reciprocating) engines burn gasoline. (Diesel aircraft engines are the exception – they run on jet fuel.) Kerosene burns hotter than gasoline and has a different flash point, so it doesn’t explode at the “normal” time in a gasoline engine’s power stroke. The effect of this is that when a piston aircraft is misfueled with jet fuel, the engine will run okay at low power settings at least for a while, but at high power settings the engine will overheat and seize.

The bad news is that high power settings are first used on takeoff, so in the classic misfueling incident a piston aircraft takes off successfully and then suffers engine failure at the worst possible time, a few hundred feet above the ground.

While I’ve never personally experienced a misfueling, one occured at an FBO where I once instructed. The airplane was a Piper Navajo with two brand new piston engines, and shortly after takeoff both engines quit. Luckily the pilots had enough altitude to return to the field and land safely. Afterwards we instructors learned valuable lessons from the mishap. At our next staff meeting the chief mechanic distributed two unmarked fuel samples: one pure avgas, and the other the mixture removed from the afflicted Navajo. He challenged us to identify which was which. Despite lots of sniffing and color-checking, no one could tell the difference!

It turned out that the Navajo’s misfueling had resulted in a mixture containing about 10% jet fuel, which was enough to kill the engines, but not enough to cause detectable changes in the fuel color or odor. The mechanic then taught us the following tricks for ruling out the presence of jet fuel in avgas.

1) When sampling fuel from your piston aircraft, dribble a little on your fingertips. Gasoline should rapidly dry completely away. But kerosene is oilier than gas, and if there’s some in the fuel you’ll likely be left with an oily feeling on your fingers. (There’s evidence that gasoline absorbed through the skin may cause long-term health effects; when possible proceed directly to the next step.)

2) You can further test the fuel using “paper chromatography,” which you probably learned in high school chemistry. Drip a drop of fuel on a clean paper towel. Straight avgas will evaporate leaving little or no visible residue. But in a mixture of gasoline and jet fuel, the kerosene migrates outward through the paper to the edge of the absorbed drop, and after the gas evaporates leaves a tell-tale ring. Knowing that could one day save your life.

©2009, 2013 Gregory N. Brown

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