What’s That Smell?

Whether your plane has rubber bladders to hold the fuel, a wet wing, or a metal tank inside the wing, fuel leaks can result in in-flight fires or a loss of engine power.

As the pilot, you have the ability to detect fuel leaks, more often than not, by using your eyes and nose. The fuel we use in aircraft is tinted blue for 100 octane, and red for 80 octane. While this approach helps us to put the right fuel in the plane, it also helps us to identify leaks with our eyes.

The best clue that your aircraft is suffering a fuel leak is the sudden appearance of stains, but diagnosing the source is not always as simple as spotting the stain…

  • Strainers: If your fuel strainer has a sump cover, watch for a stain on that cover. It’s a sure sign that the strainer quick drain is leaking. The same goes for your walkaround – if you see discoloration on the concrete, or damage to the asphalt under your strainer quick drains, the chances that you have a leak are great.The Fix: The strainer either needs to be flushed or to have its O-ring seal replaced in the near future.
  • Fuel Tanks: These leaks will leave streamers of fuel color on your wings and airframe. If you have a persistent leak, it may cause blue or red stains down the side of the aircraft. Caution: These stains will leak out at the point at which the fuel is driven by the airflow over the wing and NOT where the leak originated.The Fix: This is a job for an experienced mechanic who is familiar with your particular type of aircraft. Get in touch with a type-club or the factory to find a service center or mechanic near you. The problem is that, with just about any type of fuel tank, fuel will wick from one part of the airplane to another. This wicking action makes it difficult to identify the origin of the leak. The fact remains: If you have a stain — and it isn’t immediately adjacent to a vent — you have a fuel leak that needs to be addressed, promptly.

Another sign of a fuel leak is the smell of fuel in the cabin. The smell may be slight, but make no mistake about it; even a slight odor of fuel is the start of a leak. That initial whiff of fuel is your early warning sign — if you can smell it more than once, you have an active leak in your fuel system. The smell will typically come up after landing, when the airflow over the wing drops off as you enter ground effect. The pressure under the wing forces the air/fuel mixture into your cabin, where you can smell it. Over time, the leak (and the smell) will become stronger, until it leaks out of the plane and causes a stain.

Warning: Just because the smell goes away after landing, does not mean that the leak has gone away. An aircraft is not a computer, there is no ‘reboot’ option that will solve your problem. If you smell it, it is there and it has *not* gone away.

Never let a fuel leak persist. Whether it’s 80 or 100 octane, AvGas is not only expensive, it’s also very flammable. All it takes is a single, poorly timed backfire, or a careless smoker on the flight line, and your plane can turn into an instant Roman candle. Pay close attention to those sights and smells of fuel leaks. The life you save will be your own.