RAF Visual Flight Rules (VFR) require the crew to have a minimum visibility of 5.5 kilometres above , or minimum visibility of one kilometre travelling below 140 knots; if VFR conditions are lost an emergency climb must be immediately flown. Nine out of ten witnesses interviewed in the inquiry reported visibility at ground level in the fog as being as low as ten to one hundred metres at the time of the crash; in-flight visibility may have been more or less than this. The tenth witness, a yachtsman who was offshore, reported it as being , though he is regarded as a less reliable witness as he changed his testimony.