Stop fueling operations immediately!!
Direct all efforts to assist in the evacuation of the helicopter. Only after everyone is out of the helicopter, should the firefighting effort commence. The small Fire Extinguisher in the helicopter should be used to spray passengers that are on fire.
Fire Extinguishers in Helicopter generally have Halon as an extinguishing agent. Halon has great adaptability to the dramatic temperature changes that a helicopter encounters (the heat generated on the flight runway for instance).