Contaminated fuel on the ground can lead to serious problems in the air. Here's an example of pilots that sumped their fuel and still had a contamination-related engine failure...
The Scenario
Pulled from a 2017 NASA ASRS Report, the following events occurred when a student pilot and instructor experienced an engine failure on rotation during takeoff...
During a "long cross country" training flight to meet the experience requirements for a Private Pilot Certificate an engine failure occurred at the moment of rotation. Our takeoff was aborted and the aircraft was safely taxied off the runway using remaining energy. After examination of the aircraft, a large amount of water was removed from the gascolator. Both fuel tank sumps produced no water at all.
During the previous 4 weeks, it had been raining unusually in the area. The incident aircraft had been parked in a steeply inclined parking space (nose down) for the previous 4 days without being flown. Two weeks prior to this incident, I was conducting an initial flight lesson for a new student in a C152. While sumping the gascolator, the sample did not have the blue color of 100LL fuel, and the fuel smell was slightly less than usual. It was only after a very careful inspection which lasted more than 20 seconds (and by sumping an additional sample from a known-good fuel tank) that it was determined the sample cup initially contained only water.
According to a secondary report, "while it was initially determined that the student pilot and instructor failed to notice the water contamination and the lack of blue dye in the sample cup, hours later it was realized that the samples were taken while the aircraft was parked nose-down on a steep incline. It is highly possible that the water was located in the forward portion of the fuel tanks and did not enter the fuel system until the aircraft was moved to level ground."
What Happens When Fuel Mixes With Water?
According to Dror Artzi, a 40-year aeronautical engineer, aircraft engines will tolerate a small amount of free water (read his entire presentation published here). When water concentration is 30ppm, that's 30 grams per 1000 Kg. This is usually considered to be the maximum an engine can handle.
Your engine may not fail right away when running on contaminated fuel. The first indications will likely be sputtering and a generally rough-running engine. Once enough water is mixed with fuel, combustion is no longer possible. Water is the most common contaminant in
aviation fuel. Because water it's denser than 100LL,
you'll find water settling to the lowest part of the tank.
How Can Water Get Into Your Fuel Tanks?
There are a few common reasons you may notice water in your fuel system. Here are the top causes:
- Contaminated Fuel Source: The tanks where avgas, or 100LL, are stored are susceptible to water contamination. Poor storage in fuel farms, trucks, or self-serve tanks could lead to water appearing in your fuel.
- Condensation: Over a period of time with temperature fluctuations, condensation inside your fuel tanks will develop water droplets. With enough time and the right conditions, you could end up with substantial amounts of water in your tanks.
- Improper Fuel Tank Seals: When the filler neck of a fuel tank isn't sealed properly by a fuel cap, water can seep in quickly. After heavy rain, you could find significant amounts of water in your tanks if you have bad seals.
What You Can Do To Prevent Contamination
If you find water contamination in your fuel, keep sumping the tanks until the fuel is the correct color and water-free. Try gently rocking the wings or raising/lowering the tail to move excess water to the drain points. And when you do your engine run-up, take extra time to make sure your engine is running smoothly.
Never skip sumping your fuel tanks during preflight. If your plane sits on a steep slope, move the aircraft to level ground, wait around 10 minutes, and re-sump the fuel. If you notice excessive contamination, it might be time to get a certified mechanic involved.
Have you ever experienced fuel contamination? Tell us about it in the comments below.
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