Early in 1920 the UK Air Ministry decided that, with an average of 12 air movements a day, the air traffic at London’s main airport – Croydon – needed organising.
The ministry had no template for such a task, but issued a specification for a building they believed would do the job. It was to be called an aerodrome control tower, and the working part of it was to be “15ft above ground level, with large windows to be placed on all four walls”.
Radio communication was already in use, but even primitive radar would not be developed for another 20 years.
Radio direction-finding, however, provided the Civil Aviation Traffic Officers (CATOs) with the bearing from the airport of any aircraft transmitting a radio message, thus they could provide the crew with a course to fly to arrive overhead the aerodrome. They could also provide the pilots with weather information, including visibility, wind speed and direction, but also the approximate position of other traffic in the area so the crew could keep their eyes out for it.
Navigation was primitive in aviation’s early years. Clearly identifying the destination aerodrome so the crew landed at the right one was important. The pilots were helped to find the aerodrome by a bright, strobing “lighthouse” beam – green alternating with white – which was located on a high point. When control towers came in, the light was above the tower.
Positive airfield identification was provided by very large lettering spelling out the airport name, either on the ground, or on the roof of a large hangar.
Separation between aircraft, if there was more than one near the aerodrome at any time, was assured visually by pilots looking out for other aeroplanes, with advice from the tower if necessary as to the position of potentially conflicting traffic.
Protocols about which of any two aircraft has the right to hold course and which should give way are set in the rules of the air, similar to the rules which mariners follow on the sea, and a disciplined circuit pattern over an aerodrome was a system with which pilots were familiar.
Permission to land or take off could be signalled by radio, or by a CATO shining a green aldis lamp toward the aircraft cockpit. Similarly, a red lamp would refuse permission. Firing off a green or red Verey flare from the tower was an alternative.
The UK’s principal air traffic management provider NATS is somewhat more sophisticated today! But its daily traffic tally is nearly 9,000 movements across the country, so it rather has to be.
P.S. Thanks to NATS for providing the colourised old photographs and historical detail from their archives.
2 thoughts on “ATC is 100”
A nice nostalgic little piece, David. Made me long for the days of bi-planes and wind in the wires. I calculate the average compound growth rate required to go from 12 movements a day to 9,000 in 100 years as 6.8% – a meaningless statistic, perhaps, but if it continued at that rate, we would see 18,000 movements a day in 2013. I suppose this is progress.
Time flies and memory stands still. Well Posted David; and commented Keith.