coal burning heating and power generation. Cities particularly affected included the main UK centers, and their airports such as London Heathrow, Gatwick, Manchester, Birmingham and Glasgow, as well as European cities such as Amsterdam, Brussels, Paris, Zurich and Milan. Visibility at these times could become as low as a few feet (hence the London fogs of movie fame) and when combined with the soot created lethal long-persistence smog. These conditions led to the passing of the UK's "Clean Air Act," which banned the burning of smoke-producing fuel. During the immediate post-war period, British European Airways (BEA) suffered a number of accidents during approach and landing in poor visibility, which caused it to focus on the problems of how pilots could land safely in such conditions. A major breakthrough came with the recognition that in such low visibility the very limited visual information available (lights and so on) was extraordinarily easy to misinterpret, especially when the requirement to assess it was combined with a requirement to simultaneously fly the aircraft on instruments. This led to the development of what is now widely understood as the "monitored approach" procedure. One pilot is assigned the task of accurate instrument flying while the other assesses the visual cues available at decision height, taking control to execute the landing once satisfied that the aircraft is in fact in the correct place and on a safe trajectory for a landing. The result was a major improvement in the safety of operations in low visibility, and as the concept clearly incorporates vast elements of what is now known as crew resource management (although predating this phrase by some three decades) it was expanded to encompass a far broader spectrum of operations than just low visibility. However, associated with this "human factors" approach was a recognition that improved autopilots could play a major part in low-visibility landings. The components of all landings are the same, involving navigation from a point at altitude en route to a point where the wheels are on the desired runway. This navigation is accomplished using information from either external, physical, visual cues, or from synthetic cues such as flight instruments. At all times, there must be sufficient total information to ensure that the aircraft's position and trajectory (vertical and horizontal) are correct. The problem with low visibility operations is that the visual cues may be reduced to effectively zero, and hence there is