The Need to Make Flying Safer
It was a difficult and complex task to establish reliable scheduled air services. 1919 saw the launch of the world’s first scheduled international air services and with London and Paris as the prime international route. Pilots primarily used visual navigation methods to make a course for the destination airfield. Visual navigation consisted of using a compass and map reading cross-referenced against ground features such as roads, railways, rivers and towns to ascertain the aircraft’s position. If a pilot lost visual contact with the ground due to poor visibility caused by cloud cover, fog or heavy rain, it was difficult to accurately and safely verify the aircraft’s position. Ultimately this could lead to the pilot becoming lost, or worse, colliding with terrain or another aircraft.
To help pilots with the difficult task of navigating to distant airfields, the Air Ministry employed a team of ground-based personnel to assist. The Air Ministry was a government department, and the regulator of commercial aviation and airports. It employed at its airfields, Civil Aviation Traffic Officers (CATOs) and Radio Officers to handle air traffic. Radio Officers were trained and licensed to operate radio and wireless telegraphy equipment, whilst the CATOs managed the aerodrome and air traffic. London Croydon Airport was owned and operated by the Air Ministry.
An important new and essential development was radio-telephony which facilitated the development of Air Traffic Control (ATC). The advancement and experience of radio-telephony during World War One readily lent itself to support air services. London Croydon Airport was a major innovator in this area. Using radio-telephony, ground-based personnel could communicate with aircraft in flight, if it was fitted with radio, and pass operationally pertinent information such as weather reports, and also record aircraft position reports. With aircraft position reports, the information could be passed to other aircraft in the area for collision avoidance purposes.
Radio was put to good use in other ways to help aerial navigation. A useful function of radio transmission is that the signal’s source can be traced by a receiving station through equipment known as a goniometer. This information can give a bearing to or from the receiving station and is known as Direction Finding. The resulting information, when passed to a pilot, can be used to help the pilot fix the aircraft’s position in flight. If another receiving station is brought into play, the additional bearing can be used by the pilot to cross fix accurately the aircraft’s position. The Air Ministry personnel based at Croydon, improved the Direction Finding function further to a system known as Wireless Position Finding (now known as Radio Position Fixing).
Wireless Position Finding used a short radio transmission from an aircraft, received by at least two ground stations, to triangulate its position. The CATO would work out the aircraft’s position, plotting it on a map, and would transmit the information to the pilot in flight. This was a great improvement on Direction Finding as the work was performed on the ground and greatly reduced the pilot’s workload and provided navigation support. The Air Ministry approved the system as operational in February 1922, after much work in proving and calibrating the system with an accuracy of two miles and the fix obtained within two minutes. Croydon worked in conjunction with the facilities at Pulham which was Britain’s airship station at the time. The system is still used by Air Traffic Control today to support aerial navigation.
The use of radio for aerial navigation was an important milestone for developing international air routes and services – an essential first step in establishing a radio based global air navigation network. Establishing the use of radio transmissions to triangulate a navigational position was a significant development that is still in use today. The equipment is now more sophisticated, utilising satellites as part of Global Positioning Systems, but the concept of radio-triangulation remains the same.
World’s First Air Traffic Control Tower
On the 25th February 1920 the Air Ministry detailed the specification and construction of the world’s first technical building to control air traffic, the “Aerodrome Control Tower”, to be installed at Croydon Aerodrome. This was also the first time that the “Control Tower” terminology was used. The Air Ministry specification stated that the “platform of the tower to be 15 feet above ground level”, “have large windows placed in all four walls”, “with a “wind-vane to be fitted to the roof of the hut with a geared- down indicator placed inside”, “enabling the control officer to read changes of wind”. The world’s first Air Traffic Control Tower was born.
Aerial Route Traffic Control By Radio-Telephony
On 3rd June 1920, the Air Ministry published Notice to Airmen, No.61, detailing the procedures to control air traffic on the international air routes. A Notice to Airmen (NOTAM) is a legal document issued by the commercial air transport regulator, that promulgates changes to aerial navigation or impacts aircraft operations. Note no. 3 and no.6 of NOTAM No.61 were key developments. Note no.3 gave Croydon the control of all air stations in the UK and, in effect, made it Britain’s first air traffic control centre. Note no.6 identified and implemented the founding principles of air traffic control:-
3. The wireless control of all the British stations, including
stations licensed on private aerodromes, will be carried out by
6. Telephony will be used primarily for giving information
as to the whereabouts of an aircraft travelling or about to
travel along a route; secondly, for reporting weather conditions along the route to the machine.
Some key developments from NOTAM No.61 1920 were the terminology concerning “Route Traffic Control” and the intent of using ground-based personnel to provide real-time assistance and “control” of aircraft in flight. These provide pertinent air traffic information about other aircraft and weather reports- the base principles of air traffic control.
First Air Traffic Controller
G.J.H “Jimmy” Jeffs, Croydon Civilian Air Traffic Officer, was one of the great innovators in developing the new discipline. Issued with Air Traffic Control Licence No.1 dated 22nd February 1922, Jeffs developed many of the systems and procedures that were approved by the Air Ministry and are still in use. Having established over twenty-five ATC Units in the UK, it was the United States who asked that Jeffs lead the establishment and organisation of the North Atlantic Airspace during World War Two. Jeffs had a distinguished career in civil and military Air Traffic Control, culminating in the award of the CVO, OBE and the US Legion of Merit.
The innovation of “Mayday”
The step from the use of radio telegraphy (Morse code) to radio telephony (speech transmissions), saw the need for a new way to use language to ensure clearly understood messages. The quality of the radio transmissions and reception was poor by today’s standards, leading to ambiguity and misunderstanding with some messages. An issue at the time was that radio telephony was based primarily on the wireless telegraphy procedures employing the Q Code. The Q code was a series of around 220 three letter codes that abbreviated a longer message. Using these codes for spoken transmissions was not always successful, and the Morse distress code of SOS was an example. F.S. Mockford, Croydon’s Senior Radio Officer, looked at the problem to find a word for use in radio-telephony that would be the equivalent of SOS. In 1923, Mockford, who was employed by the Air Ministry and thus a civil servant, conceived the distress phrase “Mayday, Mayday, Mayday” (allegedly inspired by the French “m’aidez”, meaning “help me”) that was subsequently approved by the Air Ministry as the required radio procedure to be used in an emergency. In 1924, the UK promulgated its approval and use through the publication of The Air Pilot: Great Britain. Adopted by the International Radiotelegraph Convention of Washington in 1927, “Mayday” became the international standard distress phrase. The selection of the word “Mayday”, not connected or similar to any other operational aviation message in use, was important to help prevent misunderstanding or ambguity in message transmission. The “Mayday” distress call still saves lives today.
Air Traffic Control Tower
The Control Tower is an instantly recognisable airport technical structure. It is designed specifically to efficiently support air traffic services and has a commanding oversight of airfield operations. The basic concepts of any Control Tower remain unchanged since the inception of the first tower at London Croydon Airport in 1920.
In 1928, with the construction of an all new terminal and Control Tower at Croydon, the Air Ministry embarked on a bold and impressive new design for the tower. Imposing in size and with its square geometry, unusual amongst ATC Towers that are generally circular, it is striking in its restrained neo classical design that ingeniously houses many technical and operational functions that are not obvious to the onlooker. In sheer scale it was impressive and was the world’s tallest and biggest Control Tower at the time. The Tower was 50 feet tall itself and stretched upwards to 80 feet overall, including the aerial arrays installed on the tower’s roof. It was constructed over four floors, with three floors dedicated solely to supporting Air Traffic Control functions. Many Control Towers built after this did not surpass it in design, scale or complexity for many years.
The descriptive term “Control Tower” originated to describe the timber structure at the first London Croydon Airport and its use can be traced back to 1920. The 1928 building was a step change improvement on the timber structure it replaced on the site and was designed around the operational experience gained by the first Air Traffic Controllers (originally known as Civil Aviation Traffic Officers- C.A.T.O.). It is interesting to note that Control Towers only became compulsory in 1944.
The innovation of the London Airport Control Tower was to utilise height and to give each level a clear function. The vertical nature and height of the Control Tower provided three technical advantages for the ATC functions:-
- Clear 360 degree visibility of airborne air traffic, improved visual range and commanding view of airfield movements
- Increased operational range of radio installations mounted on the Control Tower’s roof
- Reduced topographic air mass interference to the meteorological equipment installed atop the radio mast
The Air Traffic Control Tower was built around the need to incorporate technical equipment into the design of the building and to provide effective working areas for each function employed in the Tower. The Control Tower was a world first and, when built, was the world’s tallest and largest Control Tower. In addition to the operational advantages which the Control Tower brought, each level and part of the structure served a specific function designed to facilitate improved Air Traffic Control. There was continuous innovation and experimentation regarding the equipment and procedures used in Air Traffic Control.
The tall central mast on the Tower ingeniously provided a dual purpose. It incorporated a wind anemometer feeding data down through the centre of the building to a Dines anemograph machine located on the first floor Meteorological Office of the Control Tower and also provided two way communications between the Radio Officers, Air Traffic Controllers and aircraft.
Three clocks located on the airfield sides of the balcony were visible to the pilots to ensure that the aircraft’s clock was synchronised to the correct time. The Tower’s three clocks were linked into the Administration Building’s electronic master clock system and accurate to two seconds per week. Time is a key element required for navigation and safe in-flight aircraft separation.
The top floor of the Control Tower was accessed by a spiral staircase which led into a room divided into two sections- the Control Room and the Radio Room. These were for the working positions of the Civil Aviation Traffic Officers (C.A.T.O.), Traffic Assistants and the Radio Officers- each had a specific task. The Radio Officers were licensed to operate and communicate with the airliners by radio-telephony (speech) and wireless telegraphy (Morse code), with custom-built equipment built by the Marconi Wireless Telegraph Company. Marconi’s equipment was itself important in the facilitating of Air Traffic Control. The C.A.T.O.’s, supported by the Traffic Assistants, managed the departing and arriving air traffic to and from the UK. The area of coverage extended across the English Channel, to the French Coast to the South and to the Dutch coast to the North.
Part of the advanced design of the Control Tower was the facility to remotely control airfield facilities and the remotely-located radio transmitting station. The radio transmitting station located three miles from the airport (for safety reasons), was equipped by Marconi with four 3 kilowatt custom-built transmitters and featured four 103 feet tall steel radio masts. Marconi personnel had an office and a large presence at the airport.
Additionally, airfield facilities such as inset runway lighting could be remotely controlled from the Control Tower. Inset runway lighting was an important step as it helped pilots land and take-off safely in poor visibility. Specialised runway markings (the chalk line), aided pilots during take-off and landing, as it helped pilots maintain orientation and thus prevent loss of control of the aircraft, especially in conditions of poor visibility. These features have been further developed and are standard features in runway design today.
Air Traffic Control Towers are an instantly recognisable architectural feature that are synonymous with air travel. Speech radio transmissions were the new cutting edge technology of the 1920’s that were essential for the development of Air Traffic Control. Air Traffic Control is a critical part of the global air transport network and was an essential development that wholly stemmed from the necessity to manage the safety of airliners.
The format designed and constructed at London Croydon Airport is now the standard design of all Air Traffic Control Towers.