THIS is the time of year when airports are as often as not concealed from the approaching pilot by low cloud and mist, when not enveloped in dense banks of fog. Services are interrupted, and diversions to alternative airports are common. As one hears over central London the growl or whine of an invisible aircraft, one wonders whether this one of the daily scores of machines making this approach to London airport will, through some momentary slip on the part of an overtaxed aircrew, or through some misunderstanding with Control, or from any other cause, not make touch down safely. One reflects that perhaps the danger is more acute at other less conspicuous points of arrival, where the traffic is mainly domestic or concerned with trooping. It would of course be very wrong to suggest that air transport is as risky a business as this suggests. The figures for deaths per passenger mile, or however accident rates may be expressed, suggest that one aircraft is less likely to come to grief than an individual person is to win an astronomical sum on the football pools. None the less it remains a matter of surprise that although weapon systems can be guided at several times the speed of sound to within a few feet of a target itself travelling at near sonic speed, and perhaps manoeuvring as well, and with a high degree of reliability, no system yet in airline operation can bring an aircraft safely to ground in really bad visibility, and that there are still occasional accidents aggravated if not caused by bad weather during approach using existing aids within their carefully laid down limits of use.