What is gum in aviation fuel?

In aviation fuel, "gum" does not refer to chewing gum. Instead, it is a technical term for the sticky, viscous, solid deposits that can form as a result of the chemical degradation of the fuel itself.

Here's a detailed breakdown of what it is, why it matters, and how it's managed.

What Exactly is Gum?

Gum is the result of a process called oxidation. Jet fuel and aviation gasoline (avgas) are hydrocarbon-based and, when exposed to oxygen, heat, and certain metals (which act as catalysts), they can undergo chemical reactions.

Formation: These reactions create peroxides, which then polymerize (link together into long chains) to form heavier, sticky, non-volatile compounds-the gums and varnishes.

Appearance: It can range from light, dissolved resins that are not visible to thick, sticky sludges or hard, varnish-like coatings.

There are two main types measured in fuel testing:

Existent Gum: This is the amount of gum already dissolved or suspended in the fuel at the time of the test. It indicates the fuel's history and how it has been stored or handled.

Potential Gum: This is a measure of the fuel's tendency to form gum in the future under accelerated aging conditions in a lab. It predicts the fuel's stability during storage.

Why is Gum a Serious Problem in Aviation?

Gum formation is a critical concern because it can lead to catastrophic failures. Its negative effects are primarily felt in the aircraft's fuel system:

Clogged Fuel Filters and Screens: This is the most common and immediate danger. Gum can quickly plug the fine-mesh filters designed to protect the fuel control unit and injectors. A clogged filter can lead to fuel starvation and engine failure.

Fouled Fuel Gauging Probes: Gum can coat probes, leading to inaccurate fuel quantity readings in the cockpit-a major safety hazard.

Sticking Valves and Controls: Gum can cause moving parts within the fuel system (like float valves in carburetors or servos in fuel controls) to stick in position.

Fouled Fuel Nozzles and Injectors: It can deposit on and block the precise orifices of fuel injectors, disrupting the critical fuel-air mixture and causing rough operation, loss of power, or engine shutdown.

Reduced Heat Transfer: Fuel is often used as a coolant for other aircraft systems (e.g., engine oil). Gum deposits on heat exchanger surfaces act as an insulator, reducing their cooling efficiency.

What Causes Gum to Form?

The process is accelerated by:

Oxygen Exposure: Fuel in partially empty tanks has more air (and thus oxygen) headspace, speeding up oxidation.

Heat: High temperatures dramatically increase the rate of chemical reactions. Fuel stored in hot climates is at greater risk.

Time: The longer fuel is stored, the more time oxidation reactions have to occur.

Metal Catalysts: Certain metals, especially copper, are powerful catalysts for oxidation. Even tiny amounts of copper dissolved from piping or fittings can significantly increase gum formation.

Contaminants: Water, microbes ("bugs" in fuel), and other impurities can promote reactions that lead to gum.

How is it Prevented and Controlled?

The aviation industry has a multi-layered approach to managing gum:

Refinery Additives (Antioxidants): All Jet-A and Jet-A1 fuel contains additives called antioxidants. These chemicals sacrificially react with oxygen before the fuel itself can, drastically slowing the gum formation process. This is the primary defense.

Stringent Specifications: Aviation fuels are produced to incredibly strict international standards (like ASTM D1655). These standards include limits for both existent gum and potential gum to ensure the fuel is stable before it even leaves the refinery.

Proper Handling and Storage: The aviation fuel supply chain (from refinery to airport to aircraft) emphasizes:

"First-In, First-Out" (FIFO) Inventory: Using the oldest fuel first to minimize storage time.

Keeping Tanks Full: Minimizing the air headspace in storage tanks to reduce oxygen exposure.

Filtration and Monitoring: Fuel is filtered and tested at every transfer point to ensure it meets all specifications, including gum content, before it is delivered to an aircraft.

Summary