I understand the general “how” aspect in that the planes use a special fuel line to get the fuel to the plane.
How can they do it safely with the engine on however? Thinking about refuelling a car you have to turn your engine off, when a plane/helicopter refuels mid flight they obviously have their engine on. Is there differences in how they get/store fuel that makes it safer or just differences in the type of engine?
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A car uses petrol. Petrol can ignite easily at 15Celsius, any object or spark, even your clothing static charge can set petrol on fire. The exhaust pipe of a piston engine, diesel or petrol, is hot enough to ignite petrol vapours. Even in winter, the petrol stored in the ground may be hot enough to make flammable vapours.
Kerosene for aircraft is not that flammable, it makes flammable vapors at 65celsius, that’s not a temperature you find in flight, not in the tanks, not in the air. The only ignition you can have is a electrostatic spark while the two aircraft touch eachother. That contact and spark happens in the beginning of the fueling, before the parts are fully connected. Aircraft engines are hot only inside, if you spray fuel on the engine inlet you don’t have a fire. If you spray the outer part, there are cowlings that prevent fuel to get into the compartment, and the inside of it is not that hot. The only part of the engine that can set it on fire is the exhaust, but if you are flying, any exhaust covered in kerosene on fire is not a risk. Air will just blow the flame away from the plane.
The only case that we can make is a helicopter refueling. But the fueling probe is mounted so low that a fuel spill will not go on the engines. Not in normal condition. While there is a video on the internet where the heli does a strange move and cuts the supply line, spraying itself in fuel. Still no ignition happened.
There is a risk. It’s very remote. This type of refueling is done only after special training, and only in military, where taking risks is normal as you signed up for it. Military is itself a dangerous job, whatever is your role.
For civil, air refueling and external tanks are completely forbidden, no matter the situation.
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