Twenty people had to be booted off a British Airways flight to London last week for one strange reason. Extreme heat made the plane too heavy to take off from Florence’s Amerigo Vespucci Airport, forcing passengers off to compensate for the extra fuel needed. 

The BA Embraer ERJ-190, headed for London City Airport on August 11, needed more fuel because the 35C heat made it harder to reach cruising altitude. Amerigo Vespucci's short runway, just half the length of Gatwick’s main track, also presented an issue. A British mum on the half-empty flight said staff originally wanted 36 people to get off, but in the end only about 20 volunteered.

She said: "The pilot said people had to get off because of the extreme heat. It was around 35 degrees and they needed extra fuel for the engine to run effectively.

"The staff said 36 would have to be unloaded but only about 20 people had to get off in the end."

Heat makes the air less dense, which reduces the plane's lift and engine thrust, making it harder to achieve takeoff speed and requiring a longer runway.

Pilots calculate a "density altitude", an equivalent altitude based on the temperature, to determine if a plane is able to get in the air safely.

On hot days, the density altitude can become too high, exceeding the aircraft's capabilities and forcing airlines to reduce the plane's weight by offloading passengers, cargo, or fuel to allow for a safe departure.

A BA spokesman said: "Due to the unique nature of the airfield with its short runway, extreme temperatures affect air pressure, so aircraft weight must be reduced.

"We’re sorry for the inconvenience. Our teams worked hard to get passengers to their destination as quickly as possible."