Accident Analysis Compels Pilots to Check Fuel

How can you tell if jet-A mixes in with avgas?

Airport fuel station
Fuel farms at most airports clearly distinguish jet-A from 100LL, but a prudent pilot verifies the type of fuel.Julie Boatman

With a growing number of light aircraft taking on jet-A instead of avgas—from a diesel-powered Cessna 172 to the single-engine Cirrus VisionJet—a prudent pilot stays on guard to supervise fueling if at all possible. But anecdotally speaking, many of us have operated under the impression that misfuelling a piston airplane burning 100LL with jet fuel is tough to do, given the difference in filler-neck sizes and pump-nozzle diameters.

However, just a little more than five years ago, a Cessna 421C on a Part 135 medevac flight operating out of Las Cruces, New Mexico, suffered the result of such a mix-up—and all four on board perished while the pilot attempted to return to the airport, as smoke and flames trailed from the airplane, according to witnesses on the scene.

The pilot looked like he had done all of the right things: Following his request to the line technician at the FBO for 40 gallons of fuel—20 gallons per side—he stayed there to observe the fueling, even assisting with the replacement of both fuel caps, according to the NTSB report.

Placards to announce the requirement for avgas were in place near the 421’s fuel filler ports—yet the FBO records, an interview with the technician, and the smell of kerosene around the accident site demonstrated the truth of what happened.

One critical link in the safety net failed: “In accordance with voluntary industry standards, the FBO’s jet fuel truck should have been equipped with an oversized fuel nozzle; instead, it was equipped with a smaller diameter nozzle, which allowed the nozzle to be inserted into the smaller fuel filler ports on airplanes that used aviation gasoline. The FBO’s use of a small nozzle allowed it to be inserted in the accident airplane’s filler port and for jet fuel to be inadvertently added to the airplane.”

Though it’s easy to Monday-morning quarterback the people involved in the fateful accident, it’s also easy in hindsight to understand how mistakes can lead to misfuelling, with just a moment’s inattention, or improper assumptions. One issue remains: jet-A and 100LL mix very well, creating a nearly homogenous sample in a fuel tester that may be difficult to judge. Unless the mixture is very strongly in favor of jet-A, the whiff of kerosene may be hidden by the overwhelming vapors of the more volatile avgas. Jet-A may indeed lighten the mixture’s color, but only to a pale blue—not nearly enough to register for many pilots.

Fortunately, we have a couple of ways to identify if there is jet-A present in a fuel sample (assuming you’re looking for 100% avgas). Both rely upon that volatility of avgas, and the residue that jet-A leaves behind. First, a thimble-sized drop of the mixture on paper (such as a Post-It note) can show how quickly the avgas evaporates, and leaves a stain behind if there is jet-A present in the sample. This must be observed within a few minutes of the test—after about 30 minutes the paper becomes difficult to “read.” Second, if you use a GATS fuel straining jar, with a clean mesh screen at the top, and strain the sample back out, the avgas should evaporate quickly. Any liquid that remains after a few minutes means the sample is suspect—or you need to clean your jar!

Our thanks to Jim Logajan from the Cardinal Flyers Online type group for the analysis of the various ways that pilots can test fuel.