A viral video lasting barely a minute was enough to clarify why mercury is entirely banned on aircraft. Although it may seem like a harmless object, a simple thermometer can put an aircraft’s integrity at risk. The reason lies in the dangerous chemical reaction between mercury and aluminium.

The invisible enemy: how mercury destroys aluminium

Anyone who has flown knows that there are a number of items you’re not allowed to bring. Naturally we think of firearms, flammable liquids and the like. However, there is one very specific object: mercury thermometers. Why would a simple thermometer be banned? It is not the instrument per se—after all, you could bring a digital one—but the chemical element inside it that is prohibited.

In commercial flights, most structural parts of an aircraft are made of aluminium because of its lightness and strength. However, this metal has one fatal weakness: mercury (Hg). When they come into contact, a reaction known as amalgam corrosion occurs, which can rapidly weaken the metal.

Under normal conditions, aluminium is protected by an oxide layer that prevents oxygen or external agents from affecting it. But even a small scratch in that layer allows mercury to penetrate and begin a destructive reaction. When combined, the aluminium loses its solid structure and becomes a brittle mixture. This process can propagate itself, destroying large areas of the fuselage with a minute amount of mercury.

That is why if a thermometer were to break inside an aircraft, the liquid metal could leak and attack vital areas of the structure. The Federal Aviation Administration (FAA) and other aeronautical authorities expressly prohibit its transport because of the extreme risk it poses.

This video explains it in just one minute:

A real risk in flight: corrosion that perforates the plane

The phenomenon of amalgam corrosion is not only chemically devastating, but also extremely fast. In the presence of humidity—as found in a pressurized cabin or in cargo compartments—the process accelerates. Mercury acts as a catalyst that re-generates the reaction again and again, extending the damage. This means that a single drop can trigger a chain of structural deterioration, compromising essential parts of the fuselage or wings. In laboratory experiments, it has been shown how mercury “devours” aluminium until it turns to dust, which makes this element a silent yet lethal threat.

For this reason, mercury thermometers, barometers and other similar instruments are strictly banned in both hand-luggage and checked baggage. Today we use safe alternatives, such as digital or alcohol-based thermometers, precisely to avoid a chemical accident at thousands of metres of altitude.

Now we know that the prohibition of mercury on aircraft isn’t just a bureaucratic rule — it’s a vital safety measure. A minimal leak would suffice to compromise the structure of the plane. Next time you see a mercury thermometer, remember: its shiny silver could be lethal when it meets aluminium.

Reference:

• Chen Europe/Mercury-aluminum amalgam. Link

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