FCOM limitations for many aircraft state:
DO NOT USE HF RADIO WHILE REFUELING
Why is this? I have heard from people that it's because most of the big airliners have a 'notch' antenna for HF; it's a small notch cut at the base of the vertical stabiliser. HF transmission power is fed to this notch and the whole fuselage acts as an antenna, radiating the power.
Can someone explain in detail and in simple terms?
The safest way to refuel an aircraft is with everything turned off and the airframe grounded (or bonded) to the fueling system. This is often not practical (particularly for airliners), so a number of other rules exist to make refueling as safe as possible -- not using HF radio while refueling is one of them.
So why is the HF radio singled out? Power, combined with the antenna design factors you mentioned.
With small radios you can easily light a fluorescent lightbulb (incidentally this is why radio geeks are such great fun at parties). HF radios can put out several hundred watts of power - this makes them more likely to generate a spark that can jump the gap between say a refueling nozzle and the refueling port/tank, igniting fuel vapors and ruining everyone's day.
HF antenna designs which use the aircraft itself to radiate part of the energy can further increase the chance of a spark.
Yes, at the free end of the HF antenna are voltages of several hundred to some kilovolts. With hundred watts the HV can reach levels to start fluorescent lamps and that is also enough to spark into free air. With some fuel dust it is like an electric detonator and TNT...
That power can cause sparks when it gets grounded through the fuel opening, that spark can ignite the tank. (Ref: ratchet freak)
