Safety Publications/Articles

Mystery Water

Where Does It Come From?

Engine-related problems, especially those related to fuel contamination, tend to occur at the beginning of a flight. Water, being heavier than avgas, collects at the bottom of the fuel tank. If the pilot fails to drain it, the water will inconveniently arrive at the engine just about the time the aircraft gets airborne or a little after. In this case, the water did not show up until much later.

A student pilot flying a Cessna 152 made a forced landing on a solo cross-country flight. Approximately 45 minutes into the flight, the Cessna's engine rpm dropped. The student pilot pulled the throttle back to the idle position and applied carburetor heat. The engine rpm continued to drop; then the engine quit. Attempts to restore engine power failed.

The pilot selected an open field for an emergency landing, but on touchdown, the nosewheel sheared off, and the engine firewall was damaged by the impact that followed. A fuel line also ruptured, and an unknown quantity of fuel spilled. Approximately one quart of liquid was recovered from the fuel system, and it was more than 90 percent water. Water was also discovered in the carburetor bowl and the filter assembly.

The airplane had been refueled two days before the accident. After the refueling, the area experienced severe thunderstorm activity. If the fuel caps were not properly secured, the heavy rain could have pushed water into the tanks. Of course, we don't know whether or not the fuel tank caps were secured correctly after the airplane was refueled.

Even if the caps were closed correctly, worn gaskets on the caps could have allowed water to enter the tanks. In the accident report, the pilot did not mention checking and securing the fuel caps during preflight, nor does the report state whether the pilot checked for water contamination in the fuel tanks.

As a rule, any significant water contamination will show up in the preflight fuel sampling. But even after this sump check, water can hide in a fuel system and may not become evident until a few minutes after engine start. However, it is unusual for the engine to run normally for 45 minutes and then quit as a result of water contamination, especially in a fuel system like that in a Cessna 152.

Still, there are lessons to be learned from this incident: Be sure fuel caps are secured immediately after refueling; fuel cap gaskets must be well maintained; and, finally, if the aircraft has been parked outside in heavy rains, pay extra attention to the possibility that the fuel system has been contaminated with a substantial amount of water.

By Bruce Landsberg

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