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Elaborate ground organizations, depending largely on radio, for ensuring the safety of air travel

Imperial Airways liner Horatius leaving Croydon Airport

BOUND FOR PARIS. The Imperial Airways liner Horatius leaving Croydon Airport. Before the take-off and after the aircraft has taxied away from the aerodrome buildings, the pilot, with his engines running at low throttle, awaits the signal from an official waiting on the parapet of the Control Tower. When the way is clear the official signals permission by directing a white light on to the machine. The Horatius is one of several Handley Page 42 biplanes operating on various routes of Imperial Airways. Another illustration of the Horatius, showing her at Le Bourget, Paris, appears in the photogravure supplement.

TWO conditions in Great Britain complicate the problem of the control of air traffic. One is the frequency of fog and low-lying cloud, the other is the congestion of the radio services. These complications are aggravated by the fact that Croydon Airport is too small for all the traffic to and from London, and the rising ground and buildings on nearly all sides hamper incoming and outgoing aeroplanes.

Paris, served by Le Bourget Aerodrome, and Berlin, served by the big Tempelhof Aerodrome, are much more favourably situated.

The airport for London on the west side is at Heston. Like Croydon, Heston Aerodrome is Air Ministry property. A projected enlargement is intended to enable Heston to take a much bigger share of the traffic than it has hitherto been able to cope with. Other aerodromes near London have been proposed: one at Fairlop, near Ilford, Essex, only ten miles from London by railway and included in a tube railway extension programme, the other at Lullingstone, near Swanley, in Kent. There is also an emergency landing ground in Kent at Littlestone, near Lympne, a few miles west of Folkestone.

All these aerodromes are necessary, for the traffic is increasing, and it is frequently necessary to direct pilots of incoming air liners to divert their course from Croydon, when that aerodrome is fogbound, to other airports. Pilots are sometimes instructed to go to Heston, or even to Portsmouth.

Meanwhile, the great expansion and acceleration of the Empire air services operated with flying boats makes no demand upon London’s land aerodromes. For these services Southampton is the terminus.

Air traffic depends more and more on efficient radio services, and in Great Britain, because of the frequency of poor visibility, this dependence is great. The demands upon radio have increased to such an extent that the range of available wavelengths has proved to be insufficient, a problem now being seriously tackled. The control of air traffic is no longer a localized problem. Croydon airport is only one of a number of stations. There is now an Inspector of Air Traffic Control. He supervises the traffic side of the Air Ministry’s staffs at all the principal airports - Croydon, Heston, Manchester and Portsmouth. Others will be added to the list in accordance with the decision that the Air Ministry is to provide the trained staffs of a comprehensive air traffic control organization. These staffs are trained at a special school, from which appointments to the various airports are made as needed.

South-east and east of London are the air routes to the Continent, over which British, French, Dutch, German, Belgian and Swiss companies operate; from London, west, north, and northeast, run internal air lines. Radio contact must be maintained between the ground and the liners in the air. Especially in conditions of poor visibility and at night, the organization must be prepared to direct pilots on their routes, instruct them as to the altitude at which to fly, and direct their landing procedure. At Croydon the number of incoming and outgoing air liners sometimes exceeds 100 in a day, and may exceed a weekly average of 700 or so.

Croydon Airport is one of London’s show places. The number of visitors that are conducted by official guides over the administration buildings exceeds 100,000 yearly, and in addition some 70,000 are admitted to the public enclosure.

The Government pays out altogether more than £40,000 yearly for the upkeep of Croydon and its organization. The receipts from all sources do not quite meet the outlay, and there is an annual loss of about £2,000. The revenue is derived from the rentals paid by air line companies which use the aerodrome, from landing fees and from many other sources.

The equipment of the airport includes departments for the maintenance in proper condition of aircraft and engines, frequent inspection, great underground stores of petrol and oil, huge petrol lorries and a multitude of details.

On the aerodrome surface are displayed the name of the airport, a big white ring, called the landing circle, and a long straight white line for the guidance of pilots taking off in foggy weather. Close to the landing circle is a smoke wand direction indicator, the course of the white smoke showing pilots the way the wind blows on the ground, a much more useful guide than the wind “stockings” at various points.

The aerodrome boundaries are marked by lights, and there is a beacon, or lighthouse, with its particular occultation distinguishing it from those of other airports.

In supreme charge of the air traffic is the Control Officer. The Control Tower rises 60 feet above the ground, and is in two sections, divided by a glass partition. These are the control room, where a Control Officer is always on duty, and a radio room, with its big array of instruments, including the directional radio installation.

The control room is about 30 feet square, glazed on all sides and surrounded by an observation platform. From this room the Control Officer on duty regulates the departures and arrivals of air liners, special charter, taxi, and occasional private aircraft. From the control room the whole of the aerodrome is visible, from the big expanse of “tarmac” on which, generally, three or four air liners are standing, across the grass to the boundary lights and the radio beacons used for “blind approach” control. Flanking the northern side of the aerodrome is a row of hangars, occupied by various flying services and, away to the left, somewhat to the rear, are the big hangars of British and foreign air lines operating to the Continent. To the right of the Control Tower is the Croydon Aerodrome Hotel.

Among his numerous responsibilities, the Control Officer regulates the dispatch and reception of aircraft. A few years ago the pilot of an air liner taxied across the field to the best starting point and, first looking round to see if all was clear, chose the taking-off moment for himself. Coming in to land, he had again to rely entirely on his own eyes, judgment and skill. With the increase of traffic, however, such a haphazard arrangement became unworkable. Delays and danger would be incurred. Nowadays the Control Officer “keeps the ring”, and the pilot is able to concentrate on his own highly responsible job. This development has synchronized with the increase in the size and value of air liners, the number of passengers they carry and the value of their freight.

CONTROL OFFICER at Tempelhof Airport, Berlin

CONTROL OFFICER at Tempelhof Airport, Berlin. The Control Officer is in charge of all the traffic at an aerodrome. His responsibilities include the dispatch and reception of aircraft. Formerly the pilot of an air liner took off and landed when it seemed advisable to him. Nowadays air traffic is so dense that the Control Officer has to “keep the ring”.

An air liner due to start, having taken on board its passengers, mails and other freight, is not allowed to proceed without permission. The signal to leave the vicinity of the aerodrome buildings and to taxi across the field is given by a panel, bearing on a green background the initial letter in white of the company concerned, British, French, Dutch, and so on. This panel is displayed from the side of the parapet of the Control Tower. Having seen the signal, the pilot taxies his machine to the lee side of the aerodrome, almost to the edge of it, turns towards the wind, and stops.

The pilot’s eyes are on the Control Tower, the engines are running at low throttle, awaiting merely the touch of the hand which shall “open them out”, to pull the machine at ever-increasing speed, so that it becomes airborne and is able to set out on its journey.

By day and by night permission to take off is signalled by a white light directed on the field from the parapet round the Control Tower, where an official awaits the moment when all is clear for the outgoing craft. This official uses a swivelling telescope attached to a small but powerful spotlight. The telescope, merely by being sighted on the aeroplane which is to take off, accurately directs the beam of the lamp on to the machine. At the right moment the light is switched on. The pilot of the aeroplane knows then that there are no incoming aircraft, nor others going out.

For incoming machines at night, an official of the Control Tower, equipped with big binoculars and a lamp, flashes a red light as a signal not to land, or, when all is clear, a green light, giving the call sign in Morse of the aircraft concerned, followed by intermittent flashes. Until the green light signal is given the pilot must fly round in an officially ordained manner, awaiting it. Sometimes a red Very light is fired from the ground to delay the landing.

The Control Officer has to deal not merely with the regular air liners. There are also various flying services operating from Croydon, and they are also subject to his orders. In this instance, he displays one of several panels from the parapet, according to the operating firm and the position of the aeroplane on the aerodrome. There are, for example, a panel with the letter X on it, a plain green panel and a red and white striped panel.

In foggy weather, when it would be impossible to see an aeroplane on the far side of the aerodrome, only one machine at a time is permitted to leave the “tarmac”. When the Control Officer has decided that it is safe, he gives a signal to depart, and then the pilot acts upon his own responsibility. He taxies out, takes up position on the white line, which enables pilots to keep straight when they cannot see the surrounding landmarks, and, when he is ready, takes off. He at any rate knows there are no other machines in the vicinity, and can therefore concentrate on the control of his machine.

The Control Officer on duty can operate a switchboard controlling all the aerodrome lights - the orange blinking boundary lights, an occulting neon beacon, the landing floodlight, and the illuminated wind-direction indicator. Or he can shut off all lights, plunging the aerodrome in darkness.

On the walls and tables of the control room are maps of all the air routes of Europe, and on these the positions of air liners are from time to time indicated.

The Control Officer is concerned not only with British air liners, but also with any others which may be within radio range. Normally, however, only those within 100 miles or so are in touch by radio. If, for example, a machine leaves Paris for London, it keeps in touch with Le Bourget until it is about halfway, and then it picks up Croydon’s signals.

FOR BLIND APPROACH AND LANDING, the Lorenz system has been installed at Heston AerodromeOccasions for communication are frequent, especially in bad weather. Sometimes an air liner is out of sight of landmarks for great distances, perhaps for the whole of the journey between Croydon and, say, Paris or Brussels. The pilot, before he started, calculated a compass course, taking into account the speed and direction of the wind and the normal cruising speed of his machine. It is not uncommon, however, for a change in direction or speed of the wind to occur, and a pilot out of sight of fixed landmarks has no means of ascertaining the deviating effect of such changes on his course-keeping. It is, therefore, necessary for him to seek guidance.

FOR BLIND APPROACH AND LANDING, the Lorenz system has been installed at Heston Aerodrome, Middlesex, and elsewhere. A radio beacon - such as that shown in the photograph - sends out radiations which provide a radio-defined approach path. A pilot equipped with the necessary receiving instruments is enabled to keep straight on his course to the aerodrome, and marker beacons tell the pilot his exact distance from the landing ground.

This is given by directional radio. At Croydon, Lympne and Pulham (Norfolk), there are instruments which, on receipt of a call from an air liner and the request for position, indicate almost exactly the direction of that aircraft from the receiving station. From any two stations widely apart the direction of the same aircraft identified by its code number, or call sign, is different. It is then a simple matter to plot on a chart the direction lines, and to see from their point of intersection the locality of the aeroplane. This is instantly communicated to the pilot by radio, the complete operation taking only a minute. Radiotelegraphy is now in general use for all these communications. It is less subject to error than telephoning, and it is available for a greater range of wavelengths.

To relieve the heavy demands on the radio organization, increasing use is being made of homing radio.

For air traffic control a radical departure from the existing European medium frequency organization will eventually be necessary, although it will entail extensive re-equipment of aircraft. This question is the subject of international discussion, and the authorities are considering the possible relegation of two-way communication to intermediate or high frequencies for navigational beacons and ground direction-finding stations. The band of high frequencies most suited to European conditions has still to be determined.

Directional radio is extremely important and valuable. In the United States too much reliance was, until recently, placed on “beam flying”, by means of which a pilot depended entirely on keeping to a radio beam by signals warning him of tendencies to stray either to the right or to the left. Occasional unreliability of the beam, however, due to crossing by other beams or to interference, caused a pilot sometimes to lose it, and pilots once lost were sometimes not able to get back into the beam, but were compelled to make forced landings in difficult country in bad weather or at night.

The worst condition with which air traffic has to contend is fog, and it has been necessary, because of the frequency of poor visibility in Great Britain and a great part of the continent of Europe, to institute a system which is calculated to prevent collisions in the air, and also to enable an air liner to keep on its course to the aerodrome.

TO GUIDE AIR LINERS DURING FOG radio apparatus is installed at Croydon Airport

TO GUIDE AIR LINERS DURING FOG radio apparatus is installed at Croydon Airport. The radio room of the control tower is shown to the right. In this room is the directional radio installation. During fog the control officer announces by code a “controlled zone” system The radio officer sends messages to incoming air liners, telling them how to proceed on arrival in the controlled zone.

Take, for example, a condition in which the bottom of the clouds is less than 1,000 feet from the ground, and the horizontal visibility less than 1,000 yards, technically known as “visibility 3 minus”. Before entering a region in which this condition prevails, a pilot must report by radio to the nearest radio station, stating his position, height, and whether he is ascending or descending.

When it is foggy the Control Officer declares what are known as Q B I conditions. These letters have no significance as initials, but are merely part of a code. In fog the letters are displayed in an illuminated sign on the control tower, and the radio officer, instructed to do so by the Control Officer, sends radio messages to incoming liners telling the pilots how to proceed on arrival in the controlled zone. The pilots have to inform the Control Officer the expected hour and minute of arrival. They are assigned different altitudes, according to their distance from the aerodrome, those farther away being given the higher altitudes, and the nearest one the lowest, for it should be the first to land.

The problem would be simplified if air liners on a given route were all of about the same speed, but fast machines overtaking slow ones constitute a danger. It is the custom, whenever possible, to send a slow machine off in time to get sufficiently far ahead to avoid the possibility of its being overtaken on the route.

When Q B I is announced, a “controlled zone” system comes into operation . It is designed to keep the airways clear for radio-equipped aircraft, to regulate their altitudes and positions, and to forbid entry into the controlled zones except to craft which have received the permissive signal from Croydon.

Radio-equipped aircraft must communicate with the main radio station serving the communication area in which they are flying until permission is given to communicate with another, on passing to the next area.

By this system, the position is known of every aeroplane equipped with radio, and risk of collision is minimized. Aircraft not equipped with radio, when in foggy weather or in cloud, take the risk, although outside the regular traffic channels this risk may not be great.

An operation for which the control tower is not responsible, except that it must see that the system is not for any reason mechanically impaired, is the approach to an aerodrome in thick fog and the landing of the machine. This is secured by “blind approach and landing” systems. Of these, the Lorenz is the best known, and is installed at Croydon, Heston and many other aerodromes.

Oral Warning

By this, as by other systems, the pilot of an aeroplane equipped with the necessary receiving instruments is enabled to keep straight on his course to the aerodrome, at the correct altitude for approach, and eventually to land.

The pilot, when many miles from the aerodrome, receives oral warning of any tendency to diverge from it. He hears a series of Morse long notes on the one side, or Morse short notes on the other. When he is flying true, the note becomes continuous. All the systems work on the ultra-short-wave system, and though they are distinct in type, they are all intended to give three sets of signals to the approaching aeroplane. The main beacon defines the approach path up to a distance of ten or fifteen miles; an outer marker beacon warns the pilot that he is within a mile and a half of the aerodrome; and an inner marker announces his arrival within 300 yards of the aerodrome boundary.

Knowing his altitude and distance, he can manage to land safely, if not always smoothly. The technique, however, is being improved, and with the aid of highly desirable modifications in the landing carriages of aeroplanes it goes far to removing all risk from beam-guided “blind” approach and landing.

Control of air traffic has begun, and is already making astounding developments. None can foretell the future, but it is certain to expand and be elaborated in forms at present scarcely suspected. Imperial Airways flying boats are in the scheme and send out messages at regular intervals by shortwave radio, reporting speed and progress. Such signals keep the Operations Department of Imperial Airways informed not only of the position of a flying boat from hour to hour, but also of the expected time of its arrival at the next port of call.

In this Operations Department can be seen at a glance the positions of a great fleet of aircraft operating over 30,000 miles of air routes. Every stage of a journey is plotted out.

IN THE CONTROL ROOM at Croydon Airport

IN THE CONTROL ROOM at Croydon Airport. The control tower, 60 feet high, is in two sections - the control room and the radio room. The control room is about 30 feet square, glazed on all sides, and surrounded by an observation platform. On the walls and tables of the control room are maps of all the air routes of Europe, and on these the positions of air liners are indicated from time to time. From this room the Control Officer on duty regulates the departures and arrivals of air liners and other aircraft.

Click here to see the photogravure supplement to this article.

You can read more on “Blind Flying”, “Homing Radio” and “Traffic Rules of the Airways” on this website.

Air Traffic Control