Fog and FIDO

Rat Catcher, Going Postal

Fog is a bit of a bugger. It reduces visibility, is patchy and is somewhat unpredictable but what is fog?

Fog is condensed water droplets that appear close to the surface of the earth. It is often said that fog is ground borne clouds which is true but fog is created in a slightly different way. Both are created by cooling air but fog is more often that not, caused by cool humid air sitting atop slightly warmer ground that is slowly releasing its heat and therefore evaporating the moisture within. This means that fog is most common in Autumn.
It is often said that warm air holds more moisture than cold air but is this true and if so, why? Well it isn’t strictly true. Air that is hot or warm means that the air and the moisture contained within has more internal energy than cooler air. This energy gives the water molecules contained within the air more kinetic energy so they move around more. This movement means that they are less likely to group together and condense which causes water droplets which in turn creates fog. Alongside fog was smog, which is essentially the same as fog except that smoke particles combine with water molecules. Again like fog, warm air gives these particles more kinetic energy which stops them condensing so both can be eased by heating the air.

The RAF had a major problem during WW2 with aircraft returning from bombing raids early in the morning when fog was at its worst. Pilots attempting to land without even being able to find the aerodrome let alone see the runway. Many aircraft and aircrew perished as they were running short on fuel, could only navigate by stars or by landmarks and not being able to see hazards such as hills, trees and mountains. By 1942, Bomber command passed on their concerns to Churchill who took a personal interest in developments. Bomber Command obviously wanted to stem the losses of crew so Dr. John David Main-Smith who was resident at the Aircraft Establishment at Farnborough was tasked with finding a solution. After investigations, Arthur Hartly devised a system that was tested at length in an area that is prone to fog, namely the King George VI reservoir near heathrow. The project was named FIDO – Fog Investigation and Dispersal Operation and then after much testing and refinement the name remained and became Fog Intense Dispersal Of.

FIDO was a system of pipelines that ran parallel to a runway through which fuel was pumped. Along the length of each pipeline were burners that sprayed the fuel vertically. The fuel was supplied from a dedicated tank though more often, directly from the airfields’ aircraft fuel dump. Ground crew would then sprint along the pipeline, lighting the burners as they went. This created a wall of flame 6 feet high, either side of the runway which heated the air so stopping the moisture in the air condensing thereby clearing the fog.
It worked extremely well but the drain on fuel was enormous. At an average airstrip, FIDO got through 100,000 gallons of fuel per hour and often double this at emergency landing grounds which had extended runways. For example, RAF Carnaby was 750ft wide (5 times the width of a conventional runway) and was 9000ft long to enable damaged aircraft plenty of room when landing and allowed aircraft to land even in the event of aircraft being abandoned along the runway. Carnaby, like other emergency airfields had 30,000 ft of piping and 180 burners. This needed 60 ‘erks’ to operate the burners and required a dedicated fuel store of 720,000 gallons which enabled a total of 6 hours of operation.

Rat Catcher, Going Postal

This drain on precious fuel explains why FIDO was only available at 15 airfields around the UK:

RAF Blackbushe/Hartford Bridge, RAF Bradwell Bay, RAF Carnaby, RAF Downham Market, RAF Fiskerton, RAF Foulsham, RAF Gravely, RAF Ludford Magna, RAF Manston, RAF Melbourn, RAF Metheringham, RAF St. Eval, RAF Sturgate, RAF Woodbridge, Epinoy.

Rat Catcher, Going Postal

It is estimated that FIDO was responsible for the safe landing of around 11,000 aircrew in 2,500 damaged or lost aircraft.
RAF Manston was the last airfield to have FIDO which was eventually removed in 1959 however this wasn’t quite the end of FIDO.
Following the end of WW2, civil aviation took off masively owing to the numbers of pilots and ex. military aircraft that were quickly converted to civilian use. To cater for this increase, London Airport (Heathrow) expanded and wanted to maintain operations even when fog bound. To this end, some FIDO pipework was installed at Heathrow but never completed. Technological developments on aircraft such as the VC10 allowed auto land systems to take over for much of the landing process which, given the expense of FIDO, meant that the system was probably now redundant. The final nail on the coffin of FIDO was made in 1963. Hansard records that Lord Chesham announced that auto landing systems on the VC10 and the forthcoming Trident aircraft mean that there would be no purpose in installing FIDO at any other airports within the UK especially given the decision not to complete the FIDO installation at London Airport in 1961.

FIDO was a big, expensive and dramatic answer to a serious problem. Like all things, there are solutions but as ever, the costs must be justified and FIDO was overtaken by technology which was cheaper.

© Rat Catcher 2017