Hi Gonzo,
After the 1965 Vanguard crash at LHR
ASN Aircraft accident Vickers 951 Vanguard G-APEE London-Heathrow Airport (LHR)
The AAIB findings said PROBABLE CAUSE: "The cause of the accident was attributed to pilot error due to the following combination of events: 1) low visibility; 2) tiredness; 3) anxiety; 4) disorientation; 5) lack of experience of overshooting in fog; 6) over-reliance on pressure instruments; 7) position error in pressure instruments; 8) lacunae in training; 9) unsatisfactory overshoot procedure; 10) indifferent flap selector mechanism design; 11) wrong flap selection"
The 1st item of probable cause was attempting to land with visibilities of only around 350m to 500m.
The Board recommended " The system whereby no positive approval of an operator's weather minima is required to be given by the Ministry is unsatisfactory. It gives power whilst withholding responsibility. Positive approval or disapproval should be required."
If the DH is 200 feet and the glide slope is 3 degs, then the distance from flight deck window to touch down is 3,821' or 1164m. It was decided to make the limiting RVR half that distance (582m) rounded up to 600m, later rounded down to 550m