ASRS Directline

  Issue Number 1 : March 1991

The Tiger in Your Tank

by Perry Thomas and Charles Drew

The Tiger's Trap

Recently, an airline pilot on a pleasure flight in his light twin stopped at an airport in a south eastern state to file a flight plan and fuel before continuing on the over-water portion of his flight to the Bahamas. Requesting that the fixed base operators' fueler fill the main and auxiliary tanks, he went inside to do his paperwork and get a bite to eat with his traveling companion. He returned to the aircraft about 45 minutes later, servicing both engines with oil and draining the fuel tank sumps during his pre-flight. Start-up and taxi-out were followed by engine run-up. Everything appeared normal.

After take-off, power reduction and initial climb, the pilot was cleared to Center frequency. Good cockpit discipline was a habit for this experienced pilot as he utilized his normal instrument scan. In his own words, "...during my scan I noticed the left cylinder head temperature was above the red line. The right cylinder head temperature was slightly high. I tapped the gauge and checked all other indications--oil pressure was a little bit low but in the green band. Oil temperature had risen slightly, but was also within limits. I reduced power on the left engine and notified Center that I needed to return for landing. By now the left cylinder head temperature had come down well into the green band."

After landing, the pilot taxied to the FBO's ramp, noting that all engine indications were normal as he shut down. Post-flight inspection revealed no problems and the pilot decided that he had experienced a gauge problem. Requesting that the fuel tanks be topped off, he went back inside to re-file his flight plan.

About 30 minutes later he again pre-flighted the aircraft, and using the check list completed engine start and taxi out. A thorough engine run up ensued and "...left and right engines checked OK with all engine instru ments normal. After take off I watched the cylinder head temperatures closely. As I made the first power reduction to 25 inches manifold pressure and 2500 RPM, the left cylinder head temperature began to rise. I stayed with Tower, reduced power, came back in and landed." The third taxi-in and shut-down of the day was accomplished without incident.

By now convinced that he had a mechanical problem, the pilot once again entered the offices of the FBO to search for a mechanic, no easy task on a Sunday. Entering into discussion with an FBO employee, he was informed that there was a possibility of fuel contamination. The pilot of a high wing single-engine aircraft had spilled some fuel down his arm while draining his fuel tank sumps, and had become suspicious when he noticed the faint smell and oily feel of kerosene. The single-engine pilot conferred with several other pilots also doing pre-flights and they collectively decided that the 100 low lead aviation gasoline was contaminated with jet fuel.

New Twist on an Old Problem

Subsequent investigation revealed that the 100LL avgas was indeed contaminated, but there is a different twist to this all too common occurrence. The fuelers had not made the mistake of pumping jet fuel into reciprocating engine light aircraft; it was the trucks themselves that were contaminated. Nor had the trucks been filled from the wrong storage tank at the tank farm. Upon delivery from the refinery, 8,000 gallons of jet fuel had been accidentally added to the FBO's 100LL storage tank, creating the first level of contamination. The trucks were filled from this tank, and the percentage of jet fuel was reduced again, creating the second level in the contamination. Finally, the trucks filled the aircraft tanks and the third level of contamination occurred. By now the percentage of jet fuel was so low that normal pre-flight fuel tank sump inspection did not reveal an observable color change in the blue 100LL fuel.

A number of aircraft received the contaminated fuel, of which a few actually got airborne. In the words of the reporter, "...fortunately no one was injured or killed as a result of the contaminated fuel, and the circumstances of this incident merit review to prevent a recurrence." Examination of the reporter's aircraft the next day revealed significant damage to both engines. Engines are now being replaced on several aircraft, including the reporter's light twin.

Avoiding The Tiger

Two recommendations for avoiding the dangers of fuel contamination can be based on this incident.

  1. Fuel retailers should check for fuel compatibility before fuel transfer operations.
  2. Pilots should be advised to check the smell and perhaps the feel of fuel samples in addition to visual inspection for contaminants and color.

The problem of aviation gasoline contaminated by jet fuel is not a new one. While most turbine engines have tolerance for aviation gasoline, the reverse is not true; reciprocating engines may be damaged by very low levels of jet fuel. Recent adoption of different type nozzles on the fuel truck dispensers have reduced but not, as we can see, entirely eliminated the problem. A line service manager who was consulted for information for this article suggested the possibility of different size, or different type hose connectors for storage tanks and delivery trucks, thus making inadvertent hookup to the wrong tank difficult. FBO fuel service personnel need also to practice greater vigilance in fuel transfer operations. The final check remains with the pilot, of course. The examination of a fuel sample by smell and feel in addition to the usual color check might well be advised--perhaps he may avoid "the tiger in the tank."

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