When fog prevented returning Allied aircraft from locating and seeing their runways to land, they would be diverted to emergency FIDO equipped landing areas.
RAF night bombers which were damaged were also diverted to FIDO airfields so they could be landed safely. When FIDO was needed, the fuel pumps were started to pump flammable liquid into the pipe system and a jeep with a flaming brand lashed to its rear drove fast down both sides of the runway to ignite the fuel at the outlets in the pipes. The burners were sometimes ignited by men on bicycles or by runners on foot. This produced a row of flame along the sides of the runway that could be seen for a long way from the air. The heat from the flames evaporated suspended fog droplets so that the Allied airplanes could have suitable visibility to find the airfield and land safely. Once landed, the crews would find shelter where they could, and their planes would be refuelled and, if needed, repaired before flying back to their normal bases the next day.