The Semi-Automatic Level Assignment Board

(Photo: CAHS collection)

The use of paper flight progress strips in civil en route and TMA ATC ended at the turn of the 21st Century with the introduction of the computerised TAAATS system, although paper strips are still used in Control Towers.

The Flight Progress Board, and the use of flight progress strips, had been introduced to Australia in 1950. A further development of the Flight Progress Board was the Altitude Assignment Board (later the Semi-Automatic Level Assignment Board, or SLAB). The idea of the board originated from a 1948 visit to La Guardia Airport (USA) by Mr E.G. Betts, Superintendent of Communications, where he saw the operation of an air traffic coordination system. From his sketches of the device Mr Gerald Unkles, Senior ATC Supervisor, and Mr Bruce Fraser, Senior Airways Engineer, designed and built the first prototype device in Melbourne.

The following is an edited version of an article from the DCA's Civil Aviation Journal, dated June 1952.

Traffic at Essendon has been controlled with more speed and certainty since the new Australian-designed altitude assignment board came into use a few months ago. Gerry Unkles describes the operation of the system.

In any air traffic control system one major problem is co-ordination between its units. It is essential for each controller to know exactly what the others are doing. Instrument weather at a busy airport means that the Area Control Centre is feeding aircraft from converging routes to one point. From this holding point the airport control unit in the tower must take each aircraft safely through the holding procedure to its approach and landing.

Obviously Area Approach must know which altitudes are vacant in the holding sequence and must receive immediate advice when the airport control unit takes over each arriving aircraft. Similarly, airport control must have a continuous picture of the volume of traffic expected.

Controllers need to devote all their attention to swift and safe control, so this co-ordination cannot be secured adequately by a telephone line between the tower and the air traffic control centre.

Some years ago [c1947] air traffic controllers realised that it should be possible to build an automatic means of conveying this information and in the United States and in Australia this equipment is already in use. The Australian equipment, built in the workshops of DCA’s Division of Airways, is less complicated than America’s 'mechanical interlock', but similar in principle. [DCA used electrical micro-switches, activated by the flight strip holder.]
The Altitude Assignment Board consists of two parts – one for the Area Control Centre, and the other for the tower. The Area Control equipment forms an integral part of the Flight Progress Board and in its use follows exactly the normal FPB procedure.

Click here to see photos of: the SLAB in Sydney AACC; the SLAB in Launceston ACC.

How it Works

The area controller responsible for the last section of each inbound route – the Arrivals Controller – has a bay provided with the same number of sockets as there are altitudes available for holding at the destination holding point. Against each altitude are three small lamps – a green one for indicating that the altitude is vacant, a yellow one which lights to show when the altitude has been assigned, and a red one warning that this altitude is occupied by an aircraft at the holding point.

As the aircraft approach the destination holding point, the Arrivals Controller arranges his flight progress strips representing arriving aircraft according to the altitudes he assigns them to.

Transfer of Control

Assume that the aircraft has been assigned to 5000 ft. on the Tower board (which will be described later). The Area controller then knows that this has occurred because his 5000 ft yellow lamp begins to flash (reminding him to pull the strip out because control has been accepted by he tower), and the steady red lamp warns that this altitude in the stack is occupied by an aircraft. As the tower controller brings his aircraft down through the stack, he releases altitudes to the Arrivals controller and the information will be shown automatically by the appearance of a green light against an altitude in place of a red one.

Progress Through the Stack

At the tower controller’s right hand is another ‘plug’ [miniature flight strip holder] inserted in the jack representing final approach in anticipation of the aircraft appearing through the overcast at any moment. When he is satisfied that this aircraft will land, he will remove the plug and clear the next aircraft for final approach, thus allowing the aircraft which are at 3000 ft and 4000 ft to be stepped down to 2000 ft and 3000 ft. respectively. Should he have no use at that time for the altitude 4000 ft, he will press the small silver ‘release’ button alongside that altitude. This will thereby extinguish the red lamp on both his panel and the arrivals controller's panel, and illuminate the green lamp on both panels, thereby allowing the arrivals controller to make use of that altitude. The reason for his procedure is that the tower controller, having cleared the altitude, may have further use for it, perhaps because an aircraft misses its approach and must, therefore, be fed back into the stack. Only when he is satisfied that this is not going to occur does the tower controller release the unoccupied altitude, and only the definite action of pressing the silver button will make the green light appear.

Safety Features

The lamps beside each altitude are in themselves an adequate safeguard; for example, the arrivals controller will assign only those altitudes represented on the board by a green light. However, as an additional safeguard against two aircraft being given the same altitude, there is a warning device built into the arrivals controller’s board. Should he try to assign an altitude which is represented on his board by a red light, a bell will ring and continue to ring until the error is rectified.

On both boards, the traffic sequence is displayed graphically and would, therefore, be of use as a memory aid even without automatic operation. But, in addition to providing this altitude picture of a traffic sequence, the board’s value is that it does away with the need for advice as to which altitudes are available, which are occupied and which are assigned. So both controllers are relieved of co-ordination difficulties and can devote their attention to the pure control of air traffic.