Recently a gentleman from California who flies out of a local airport. In the last few months, his aircraft has developed several fuel leaks. When he checked around, he discovered that other pilots at the same airport are also experiencing fuel leaks.

Additionally, one of the pilots noticed a higher than normal EGT for his normal fuel flow at cruise. When his engine was inspected, it appeared that several cylinders had run hot and experienced knock or pre-ignition.

On top of all that, it just smelled wrong, according to the pilot. Rather than the regular smell of 100LL, it smelled like “wet socks,” he reported.

What is wrong with the fuel?

It all starts with the aviation fuel system at the airport.

Aviation fuel systems have several differences from autogas fuel systems.

With avgas, the tank is supposed to be installed slightly off level (less than a degree). So that any free water in the tank will run to the low end and can be sumped off. Many of the tanks also have a pocket at that end so that the water is more easily located.

Avgas tanks also are supposed to be installed with floating suction. So that the fuel pulled out of the tank when you are fueling your airplane will be drawn from the top and not pick up any of the contaminants that settle to the bottom of the tank.

Another major difference between aviation and auto fuel tanks is filtration. In an aviation fuel system, there should be a filter separator that removes any free water and any particulates from the fuel.

The problems begin gradually. Over the years, the aviation fuel tanks will settle, which means the water is not drawn to the lowest point of the tank.

Instead, it often collects in a dead space at the bottom of the tank.

Over time, the water at the bottom of the tank attracts — and retains — all kinds of things that are bad for your airplanes, such as surfactants, ethanol, microbial growth, rust, dirt, and many other harmful things.

Over the years, the junk and contaminates continue to accumulate because they are more attracted to the water than the fuel. As it becomes more contaminated, it begins to smell bad.

This becomes a big problem when new fuel is added to the tank, stirring up the whole mess. If adequate time is not allowed to let the bad stuff settle back to the bottom of the tank, it can be drawn into the fuel inlet and then into your airplane.

Another potential problem is when the floating suction on the inlet doesn’t float due to a compromise of the float system over time. This allows the float to sink and the crud on the bottom to be drawn into the fuel inlet.

Once this bad fuel is drawn into the inlet, it enters the filter separator and can disarm the water separator part of the system. Then the free water can weaken the particulate filter and allow the contaminates to enter the aircraft tanks.

Here contaminates like ethanol will attack fuel system rubber components, causing leaks. While the microbial growth can attack metal in the fuel system. Meanwhile, particulates can plug passages in your carburetor or injection system. This can lead to bad distribution and an overheated or detonating cylinder.

Where do these contaminants come from?

The water comes from leaks in seals and fuel condensation. Other contaminates come from poor housekeeping and the distribution system.

While many fuel distributors have dedicated trucks to deliver 100LL, not all of them do. Additionally, the distribution system at most distributors have common piping and control valve systems somewhere in their plants.

The bottom line is that water and other contaminants are going to get into the fuel distribution system, so they must be managed properly.

Quality Assurance

Every avgas dispensing system needs to have a comprehensive quality assurance program.

For example, FBO or airport personnel need to check the floating suction EVERY DAY to ensure it is still floating. They also must sump the water draws until clean dry fuel is noted.

In addition, the filter separator needs to be sumped and the differential pressure across it measured and recorded it while being pumped.

I have found many systems where the differential pressure was recorded as zero for a long time, only to find that the filters had been ruptured and were not doing anything.

What can you do to protect your aircraft?

So, what should a pilot do to protect their aircraft?

First, get to know your supplier really well. Review its quality practices. I have seen some EAA chapters do this for their area.

Once you know where you get your fuel is spot on, frequent these suppliers as often as possible.

I know this can be a problem on cross country flights, but use common sense and ask questions. Most FBOs are proud of their systems and will gladly show you or discuss their operations.

Lastly, never — and I do mean never — refuel right after a fuel delivery.

If you do pull up to an airport’s fuel farm and discover it just had a fuel delivery, you’ll have to play a waiting game. How long is up to a variety of factors, including the size of the tank, as well as the amount and type of contaminants.

If it is a relatively clean tank with a working floating suction, you’re looking at about a two-hour wait.

But all bets are off as to how long everything will take to settle if the tank is in any other condition.

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