THE FLOODLIT LANDING FIELD at Croydon Airport. Several requirements must be fulfilled to ensure a safe landing at night. For example, the area must be large enough to ensure that the fastest aircraft can come to a stop before running beyond the light into the darkness; no deep shadows must be cast to confuse the pilot; and the lights must be so arranged as not to dazzle the pilot irrespective of the direction in which he has to land.
THE main advantage which aviation has over other methods of travel is that of speed. This advantage would often be lost unless flying by night could be made as safe as flying by day, for the disadvantages of darkness in other forms of transport have now been overcome.
Not many years have elapsed since even the largest airports closed down completely during the night. Recently, however, great advances have been made in the lighting of aerodromes, and a specialized section of the science of illumination has developed some interesting and efficient devices with the object of making night flying safe.
The requirements of a modern airport are best understood by considering the point of view of a pilot who is approaching a strange aerodrome after dark. By the ordinary principles of navigation he has arrived in its vicinity, but he may not be certain of its exact position. This he discovers by recognizing the airport beacon, which is situated near the landing area. It must give its light over 360° and at all angles from the horizontal to the zenith, so that the pilot can see it from whatever direction and from whatever angle he is approaching.
It must be powerful enough to be visible in bad weather, for the days of fine-weather flying are long past, yet its brilliance must not be so great that it dazzles the pilot as he glides in. A further requirement is that it shall in some way identify the airport as the one which the pilot is seeking, for in these days airports are separated, in some areas, by only a few miles.
These requirements are generally fulfilled by the neon beacon, examples of which are to be found at many of the world’s most busy airports. The neon beacon generally consists of a series of neon tubes grouped round a metal framework in the form of a cylinder or cone. The light given by one of these beacons is of the characteristic orange or red neon colour, which efficiently penetrates fog or low-lying cloud.
Because of the great length of tubing used, the total amount of light is large, although its intrinsic brilliance is low. Thus a beacon of this type may be visible for fifty miles without producing a noticeable effect of dazzle at close quarters. Identification is effected by making the beacon flash a Morse code character, such as the initial letter of the airport at which it is installed. Flashing mechanism is driven by an electric motor which carries a “character disk” - a combination of cams which operate a switch.
The neon beacons operate at high voltages, generated by transformers housed in weatherproof iron boxes mounted near the tubing. The low-tension current supplied to these transformers is interrupted to give the characteristic signal. By way of refinement, a safety governor is generally fitted to the electric motor, so that if the motor fails for any reason the beacon remains permanently switched on.
In America the rotating beacon is more common. Such a beacon bears a strong resemblance to a miniature lighthouse, but incorporates many refinements. If a lamp fails, another is automatically brought into action by a changeover switch, the contacts of which are held in position by the current passing through the lamp. When this current ceases to flow - because of a filament breakage - the switch brings another lamp into circuit. In some instances the switch operates an audible or visible indicator device which ensures that a second failure will not find the first broken lamp still in position.
WIND DIRECTION INDICATED BY DAY AND BY NIGHT. To ensure that a pilot about to land knows the direction of the wind this large indicator is provided at Croydon Airport. The device consists of a T-shaped framework balanced at its centre of gravity on a pivot and free to rotate. The action of the wind on the fin (see upper picture) keeps the T pointing into the wind. The upper surface of the T is painted white for visibility by day, and is illuminated by electric light bulbs for use by night. The arms of the T are about 20 feet long and 2 feet wide. A powerful floodlighting unit, formerly used, is shown in the lower picture, as well as a boundary light.
The lens arrangements of these rotating beacons are so designed that, as the lamp sweeps through the arc of rotation, its beam varies between the horizontal and an angle of 25° above horizontal. Colour screens are used, red beacons denoting the fact that it is impossible to use the landing ground.
When our imaginary pilot has successfully found and identified the airport he is seeking, the beacon’s work is finished. The pilot’s next task is to find out the area and shape of the landing ground. This is made clear to him by the boundary lights, placed at
equal distances round the periphery of the aerodrome.
In Great Britain the boundary lights generally consist of electric lamps mounted within glass globes of orange colour. Spaced some 100 yards apart, these lamps give the pilot an impression of the landing ground outlined by pinpoints of orange-coloured light. They are connected to the electricity supply by an underground cable. Each light has its own transformer and operates at a low voltage, so that there shall be no parts at high potential above ground. Thus the risks of electric shock and fire are minimized.
A boundary light is generally mounted on a stem about three feet high, the base of which is screwed into the lid of the iron box containing the transformer. Into the base of the stem is fitted a replaceable weak section, whose purpose is to ensure that the whole lamp will collapse if merely a light pressure is applied to its top. Thus the damage caused by a collision between an aeroplane and one of the boundary lights is reduced to a minimum. The boundary lights can be made to flash simultaneously by a rotary interrupter, but this is not standard practice.
When the pilot is sure of the dimensions of the landing area, he must know the direction of the wind so that he may land against it. This information is given to him by an illuminated wind direction indicator. Such a device consists of a T-shaped framework balanced at its centre of gravity on a pivot. This pivot is free to rotate on ball bearings. The upper surface of the T is painted white and carries a number of electric lamps spaced about a foot apart. Each lamp is enclosed in a strong weatherproof glass casing, and a typical wind direction indicator carries forty of them, each of 15 watts rating.
Operated by Small Windmill
The T is kept pointing into the wind by the direct action of the wind on a fin. The two arms of the T are some twenty feet long and two feet wide; the tail fin is generally fixed on the underside and is of special streamline section to prevent the indicator from being too sensitive to momentary changes of wind direction.
Specially designed brush gear and sliprings form the electrical connexion between the supply line and the wires to the lamps. The wind direction indicator is placed adjacent to the landing area, in such a position that it is free from wind eddies occurring near trees or buildings, and also in such a position that it cannot cause an obstruction to flying.
MOUNTED ON A MOBILE CHASSIS, this type of floodlight is used at German airports. The illustration shows a trailer vehicle which can be taken to any part of the landing field. Thus the landing field can be illuminated without dazzling the pilot, whatever the direction of the wind.
There is also a wind speed indicator. This shows the pilot the strength of the wind in steps of five miles an hour on a board similar to those fixed at the side of the stage in a music hall or theatre to show the number of the next turn.
The illuminated sign, which carries a number of electric lamps, is fixed flat on the ground. Its operating mechanism consists of a small windmill which drives a governor. At various wind strengths this governor operates mercury switches which bring into action the different combinations of lamps corresponding to the appropriate figures. The windmill has a tail fin which keeps it facing into the wind. By the time the pilot has identified the aerodrome, ascertained the size and shape of the landing area, and read off the wind direction and possibly the wind strength, he will be on the point of landing. At this point it is essential that he shall be warned of any obstructions in the vicinity of the airport. High buildings, factory chimneys, steeples, masts or trees are therefore equipped with “obstruction lights”. Definite regulations describe what constitutes an aerodrome obstruction; generally all possible obstacles within 1,000 yards of an airport are considered dangerous and are equipped with obstruction lights.
These lights are of the colour known as “aviation red”, and each generally contains a lamp of 60 or 100 watts rating. They are often duplicated as a safeguard against lamp failure at an inopportune moment. Some American airports have special obstruction lights using neon tubes. Such a light gives a large splash of red light, some three feet in diameter, and is therefore easily distinguished from all other lights associated with an airway or an airport. Six U-shaped neon tubes radiate from a central support and, in the event of failure of one of the tubes, that tube is automatically cut out of circuit.
Avoidance of Dazzle
The last part of the pilot’s approach to the airport is the landing, and to make this as safe by night as by day the ground is floodlit by special equipment. It is in the development of floodlighting apparatus that the greatest advances have been made in recent years. There are several particular requirements.
THE LANDING FLOODLIGHT of the type used at Croydon Airport is built in three tiers, mounted on a steel framework. Each tier contains two tubular projection lamps of 1,000 watts, with reflectors. Thus, if a lamp fails, the light is reduced by one-sixth only.
First, a sufficient area must be illuminated to ensure that the fastest aeroplanes can complete their forward run without passing out of the lighted area into a zone of darkness. Next, the floodlighting equipment must be so placed that no deep shadows appear on undulating parts of the landing ground. From the air such shadows appear as holes in the ground and are scarcely reassuring to the pilot. Again, the light from the floodlighting unit must be kept below the horizontal, to avoid dazzling the pilot while he circles the aerodrome.
The floodlight must be capable of starting up instantaneously, and there must be no possibility of its sudden failure while a pilot is landing. Finally, the system must be so arranged that, whatever the direction of the wind, the pilot need never land in such a way that he is facing directly into a floodlight and is therefore dazzled. It is the general practice in Great Britain to use a movable floodlight or to install several fixed floodlights so arranged that the most favourable unit is switched on according to the direction of the wind. Movable floodlights may be mounted, however, on mobile chassis equipped with engine-driven generators, and these chassis may be moved round the edge of the landing area. Yet another alternative is the provision of several plug points, connected to the main power supply, at various positions round the landing ground, the movable floodlight being connected to the point that is in the most favourable position.
The method of using several floodlights and switching on the ones appropriate to the direction of the wind is the most expensive method, but it is used in most of the large airports of the world. It is economical in maintenance, once it has been installed, as it saves the expense of a continually running engine, a mobile floodlight unit and the attendance of an operator.
In the most modern airports a desk in the control tower, sometimes arranged in the form of a model of the landing ground, acts as a complete key to the lighting system. The position of the wind direction indicator is automatically recorded on this control desk, and the officer in charge is thereby enabled to switch on the floodlight most favourably placed for the conditions in force. Smaller airports frequently use a different type of floodlight. One fixed light only is used, but when, because of the direction of the wind, a pilot has to land directly in the face of this light, dazzle is prevented by a “shadow bar” in front of the floodlight. This type of floodlight generally uses three powerful lamps (of some 3 kW each) placed vertically one above the other. They are enclosed in a vertical cylindrical housing, in front of which the shadow bar is fixed. This is made of sheet metal and is attached to a ring which is pivoted on ball bearings at the top of the floodlight.
An operator stands on a platform at the back of the floodlight and keeps an approaching aeroplane in shadow by “steering” the shadow bar with this ring. At a distance of 500 feet from the floodlight a shadow some 36 feet wide is produced.
AN AIR BEACON at Merle Common, near Edenbridge, Kent. Similar beacons are erected at selected points on busy air routes to enable pilots flying at night to check their position and to keep to their course. Rotating beacons generally use a single high-power filament lamp and a specially designed reflector which directs the main beam at an acute angle above the horizon.
Fixed floodlights vary considerably in design. One of the types in extensive use at British airports consists of six or nine lamps placed in three tiers one above the other. The lamps have long horizontal filaments and are arranged at the focal points of parabolic reflectors. Such an arrangement gives a wide horizontal divergence of light but an extremely small vertical spread - conditions which mean efficient illumination of the ground and absence of upward glare.
The 9 kW type of floodlight, using nine lamps of 1 kW each, gives a beam candle-power of about 1,500,000 and gives bright illumination over an area of nearly 8,000,000 square feet. Efficient focusing of these floodlights is a matter of paramount importance, and the newest types may be focused in daylight with complete accuracy. Small handscrews are used for moving the lamps into such a position that the image of the filament, seen in the mirror, coincides with that of a wire stretched across the front of the mirror on a frame. When this condition has been fulfilled the lamp is in focus.
Another kind of lighting which has helped to make night flying safer is the rotating route beacon, which is installed, not at an airport, but at selected points on busy routes.
Intensive research has been responsible for many minor refinements in such beacons. Some of them incorporate a specially designed refractor which redirects some of the main beam to higher angles. Thus, on bad nights, the beam is discernible at various angles and at various distances. A rotating beacon installed at Allahabad, India, has been picked up by aircraft ninety miles away. “On-course” lights are used at beacon stations on regular air routes to give pilots a true indication of route direction. Two of these lights are generally used at each beacon station, each light having its beam centred on the compass direction of the next beacon station. A standard type of on-course light projects a beam of high candle-power over an angle of 6° and two weaker beams over 18°.
In many instances where route beacons have been installed in places far from towns, and often far from any form of civilization, local generating plant has had to be installed. In many parts of India, for instance, crude oil engines have been set up in specially built power houses. Electricity is the only practicable means of providing the illumination, but luckily there are many means of generating it locally. Thus, even in the most inaccessible parts of the world pilots are greeted by the welcome sight of modern 9 kW floodlights, obstruction lights, wind direction indicators, and all the refinements that make night flying as safe as possible.
HIGH-VOLTAGE DISCHARGE TUBES containing neon gas act as an identification beacon for Croydon Airport. The red glow from this beacon is visible at great distances and penetrates to a considerable extent in cloudy weather and foggy conditions.