Regulations that prevent collisions and control the manoeuvring of all classes of aircraft
DIFFERENT METHODS of showing wind direction may be used. A wind stocking, such as this one at the Brooklands Flying Club, is the most general type of wind indicator. Other types of indicators are a smoke trail in the centre of the aerodrome, or a revolving wind T. The wind T may be illuminated at night.
IT might appear that the risk of collision between aircraft moving in so vast and deep an expanse as the sky was far too remote to necessitate any special regulation of air traffic. In reality, the aviator’s margin of safety from collision is, to a great extent, confined to the distance at which he can sight another machine in the air. Conditions of poor visibility will reduce this margin by restricting his range of vision and even in clear weather the distance at which it is possible to discern a machine ahead is seldom more than a mile or so.
With cruising speeds of 200 miles an hour common today, two aircraft flying on directly opposite courses would be approaching each other so fast that a matter of seconds only would elapse before the two machines were level. In practice, therefore, the risk of collision would be great were it not for the existing regulations which make each pilot take the correct action immediately and instinctively.
These air traffic rules were first laid down by international agreement in the Convention for the Regulation of Air Navigation signed in Paris in 1919. Effect was given to the rules by legislation on the part of all nations which signed the Convention. Thus aircraft throughout the greater part of the world obey the same traffic rules which, in turn, are largely based on the older International Regulations for the Prevention of Collision at Sea.
Just as steam vessels at sea, with their greater powers of manoeuvre, must give way to sailing ships, so, in the air, power-driven aircraft must make way for balloons and gliders. With aeroplanes and airships, the lighter-than-air craft has right of way over the aeroplane, though it must give way, in turn, to the less manageable balloon or glider.
To avoid risk of collision, where neither aircraft is entitled to right of way by virtue of its type, the regulations lay down the precise action to be taken by one or both machines. Thus, when two aircraft are flying courses which will lead them to meet end-on, or nearly end-on, each of them is required to alter its course to the right. When two aircraft are on courses which cross, the machine which has the other on its own right side has the duty of keeping out of the way of the other. In overtaking, the aircraft which is being over-hauled has the right of way, and the pilot of the overtaking machine must take care to give it a wide berth as he passes.
So far these rules are identical with those obtaining at sea. When it comes to specifying the manner in which course shall be altered, the compilers of the air traffic regulations have had to take into account the aircraft’s ability to move in a third dimension, inapplicable to surface craft. The air rules therefore stipulate that every aircraft which is obliged to keep out of the way of another shall avoid passing over or under it, or crossing ahead. Similarly, when aircraft are being overtaken, the overtaking machine must keep clear by altering its course to the right and is strictly forbidden to pass by diving under the other machine.
The aircraft which has the right of way is not entirely free from responsibility and must not, by suddenly altering its course or speed, make the task of the other pilot more difficult or dangerous. If it should find itself so close that collision could not be avoided by the action of the other aircraft alone, then it, too, is required to take such action as may best serve to avert the
threatened collision. As the limited range at which an aviator can see an approaching aircraft constitutes one of the greatest risks of collision, the air navigation regulations have been specially framed to ensure that pilots have the best possible opportunity of sighting other machines. Thus it is hoped to eliminate the risk of a head-on collision between machines flying opposite courses.
To facilitate the early sighting of another machine in the air, the rules insist that, when visibility is bad, every aircraft flying beneath the clouds shall be at such a distance below the cloud-base as will enable it readily to see and be seen.
Head-on collisions are guarded against by the stipulation that pilots shall normally be seated either in the centre of the cockpit or at the left-hand side, and that all aircraft following a recognized air traffic route shall invariably keep well over to the right of it. Where the route is clearly indicated on the ground by a line of landmarks such as a road or a railway, it is sufficient if the machine keeps at least 300 yards to the right of the route; but where the route is less clearly marked and the aircraft is being guided by compass, it must keep at least 1,000 yards to the right of the route.
Unusual Combination of Lights
Should a pilot wish to cross over one of these routes he must do so at right angles and as quickly as possible. If for any reason it becomes necessary for a pilot to fly for a time with a recognized route on his right-hand side, instead of on his left, the regulations
require him to make sure that he keeps sufficiently far away from the route line to avoid all risk of collision with aircraft coming in the opposite direction with the route on their left-hand side.
The same air traffic rules apply by night as well as by day, but between the hours of sunset and sunrise all aircraft are required to carry navigation lights. Navigation lights not only make aircraft conspicuous, but also, by their character and arrangement, indicate the class of aircraft carrying them and the direction in which the aircraft is flying.
Here, again, the air rules are based on the existing regulations for marine navigation. An airship, in the same way as a steamship, has to carry four lights when under way - a white light in the bows, a red sidelight on the left side, a green sidelight on the right side and a white tail-light at the stern. Aeroplanes when flying at night carry only three lights, the two coloured sidelights and the white tail-light.
As it is only by the position of these various lights that the pilot of another machine is enabled to tell the direction in which an aircraft is travelling, the location and range of visibility are matters of great importance and are rigidly specified in the regulations.
The white headlight of an airship, for example, must show over an angle of 110° on either side of the bows and be visible for a distance of at least five miles. The sidelights carried by airships and by aeroplanes must also show over an angle of 110° from right ahead and be visible for five miles; the white tail-light, showing over a 140° arc, that is, 70° on either side of the stern, must be visible for a distance of three miles.
These light s are so placed and screened that a pilot seeing one green light ahead immediately knows that he is looking at the right-hand side of an aeroplane broadside on to him and travelling from his left to his right. If the green light is accompanied by a white light he knows that the craft ahead of him is an airship and that he is seeing both its sidelight and its white bow light.
THE BOUNDARY of an aerodrome is marked at night by aviation-yellow lights. The lights may be steady or may flash at intervals. The distance between adjacent lights must be about 100 yards. Sometimes a row of green boundary lights may be used to indicate the best path of approach.
A single white light seen in the air indicates the tail-light of an aeroplane or airship; for at this angle of approach neither of the coloured sidelights would be visible. The overtaking pilot knows by the air traffic rules that it is his duty to keep clear of the machine ahead by altering course to pass it to his right. If, however, instead of the tail-light ahead, he sees a single green light out on his own left side he keeps his course and speed. The aeroplane showing the green light has him on its right-hand side, and must keep clear of him. This it will probably do by altering course to pass astern of him.
A single red light seen ahead might indicate the presence of an aeroplane showing only its left-hand sidelight. It might equally, however, be the navigation light of a glider or free balloon, both of which are required to carry a single red light visible, so far as practicable, in all directions over a distance of at least two and a half miles. Identical though they might appear at close range, no pilot would be likely to mistake either the balloon or the glider for an aeroplane, because of the much higher speed at which the aeroplane’s, light would be travelling. He might have difficulty in distinguishing between the free balloon and the glider. As, however, it is his duty as the pilot of a power-driven aircraft to keep clear of both types, only his curiosity would suffer by the confusion.
There are a number of other forms of light signals which must be carried by aircraft in various circumstances. It is possible for an aviator flying at night suddenly to encounter a combination in the form of green, white and red lights in a horizontal line, with two more red lights immediately beneath them.
This unusual combination indicates an airship under way but out of control, and carrying its normal navigation lights of two sidelights and a bow light, with the addition of two red lights suspended, 12 feet apart, on a cable beneath the gondola. If the two sidelights suddenly disappear as he watches, the pilot knows that the airship, though still out of control, is no longer making way. By day an airship which is not under control would display two black balls or shapes, each 2 feet in diameter, in place of the two red lights.
Captive balloons or kites must be marked by lights at night to avoid risk of collision. Their mooring-point on the ground is indicated by a triangle of one green and two red lights, each 80 feet apart; at 1,000-feet intervals along the mooring cable there must be placed a red light surmounted by a white light, each visible all round the horizon for a distance of two and a half miles. Similar red and white lights must be suspended 15 feet below the balloon-basket to mark the approximate highest point of the obstruction.
When moored or taxying on the water, aircraft carry lights similar to those displayed by ships. A seaplane at anchor shows a single white riding light forward, visible all round the horizon for a distance of one mile. If, however, it has a wing span greater than 150 feet, it must carry, in addition, a white light of equal power on either wing-tip; if its length is greater than 150 feet, it must carry a fourth white light on its tail. When taxying on the water an aircraft carries a white light forward, in addition to its normal flying lights of two sidelights and a tail-light.
If caught in a fog, whether by day or at night, an aircraft at anchor on the water gives the same warning signal as a ship at anchor - a rapid ringing of a bell for five seconds at one-minute intervals. If it is moving on the water it has to give warning of its approach by two blasts of five seconds’ duration on a foghorn at intervals of two minutes.
Should an aircraft be compelled to alight on the water and, because of engine trouble or some other mishap, be unable to manoeuvre, it shows, ill addition to its tail-light, two red lights one above the other and visible in all directions at a distance of at least two miles. This signal at sea indicates a vessel which is not under control; all ships would, therefore, keep well clear of it.
AN AIRPORT OFFICIAL decides which aeroplane shall leave first at some large airports where several machines may be waiting to take off at one time. To signal to the pilot that he may take off, a highly directional light is focused on him. In this picture an official at Croydon Airport is seen directing the light, which is sufficiently powerful to be seen easily in daytime.
When aircraft are in the neighbourhood of a busy airport, with air traffic arriving and departing at frequent intervals, the risk of collision is generally greatest. Special international rules have, therefore, been laid down for the manoeuvring of aircraft on or near an aerodrome. The most important of these rules is that an aircraft about to land has the right of way over - any incoming machine. If two aircraft are approaching the same aerodrome, intending to land on it, the higher machine has to give way to the lower and, if necessary, must cruise round until the other machine has landed and taxied clear of the landing area. The only exception to this rule would be if the higher machine were in trouble and needed urgently to land. Then it would be entitled, after having given the appropriate distress signal, to land forthwith, as aircraft in distress take precedence over all other neighbouring machines.
All forms of aerobatics are prohibited close to aerodromes used for international air traffic. Aerobatics are permitted at a distance of not less than 4,000 yards from the aerodrome or, over the aerodrome, at heights above 6,000 feet. With the same object of keeping the sky immediately above an airport clear of casual traffic, the regulations prohibit flying over the landing area at a lower- height then 2,000 feet, except when departing or landing. The regulations also require every aircraft passing within 2,000 yards of the landing area and at a height of less than 2,000 feet to keep the aerodrome on its left.
So far as flying over towns or cities is concerned, the rules do not state any specific height as being the minimum. But aircraft are required to fly at such a height that they would be able to glide to the outside of the city in the event of engine trouble. Thus the larger the town or city, the higher the aircraft must fly.
The rule of the left is an all-important one in the regulation of air traffic at aerodromes, as it applies to a number of conditions which have to be observed on landing or taking off. The common prelude to a landing at any aerodrome is to make a circuit of it to ascertain the wind direction from the wind-indicator and to make sure that the aerodrome is clear of obstructions. The turning must be made clear of the landing area and the circuit must be in a left-handed, or counter-clockwise, direction. The approach to the landing must then be preceded by a descent in a straight line, beginning at least 300 yards outside the boundary line of the landing area.
When Circuits are Right-Handed
While the aircraft is landing, the left-hand rule still applies, and the regulations demand that the incoming pilot shall leave clear on his left any aircraft which is landing or has already landed. Similarly, machines taking off must pass other aircraft to the left and any circuit they may make before setting off on their course must be made left-handed and well clear of the landing area. Even when manoeuvring on the ground, an aircraft must still pass other machines to its left while on the landing area and any turns it makes must be left-handed.
Occasionally, because of wind direction or local conditions, right-handed circuits are preferable at an aerodrome. This departure from the general rule is indicated to pilots by the display on the aerodrome of a large right-angled red arrow, the upper arm of which, 20 feet long, points to the right. At other aerodromes the landing area may be regarded as divided into two approximately equal zones; the zone on the right is reserved for landings and the zone on the left for departures, the rule of keeping to the left still applying to both zones.
International air traffic regulations apply also to the distinguishing lights exhibited at aerodromes at night. The regulations stipulate that all obstructions in the vicinity, such as high chimneys or telegraph poles, must be surmounted by a fixed red light, and that, if possible, the position of the aerodrome should be indicated by a luminous beacon. The lights marking the boundary of the landing area must be about 100 yards apart and must be aviation-yellow in colour, flashing or fixed. Sometimes a row of fixed green boundary lights may be inserted to indicate the most favourable sector of approach to the landing area. The subject is amplified in the chapter “Lighting the Modern Airport”.
Wind direction may be shown by various methods, the three most common daytime systems being a wind stocking or canvas cone flown from a mast, a large T set horizontally above the ground and free to pivot so that the cross-bar of the T points into wind, or a smoke trail in the centre of the aerodrome.
RED AND WHITE SIGNS, or black and white signs, of inverted V section are used to mark parts of an aerodrome which are unsuitable for landing, or where work is being carried out. The shape of these signs is chosen because it makes them conspicuous from a height and also from an aeroplane when it is nearing the ground. Several of the signs are visible in this photograph of Croydon Airport.
At night the wind direction may be indicated by illuminating the T, or by setting out one or other of various systems of flare paths. The most common of these consists of a number of white lights laid out in the form of a T, the long arm of which is at least 350 yards in length. The light at the foot of the T indicates the place where the aircraft should first touch down, and it finishes its run towards the cross-arm of the T which is pointing into wind.
An ordinary landing T may not always indicate wind direction. It may be fixed to show the direction in which -aircraft should land irrespective of whether or not the direction indicated coincides with the direction of the wind. It may also be fixed to show the direction for landing when there is no wind. If such a provision were not made, several aircraft might attempt to approach the aerodrome at the same time from different directions.
A ball mounted on a mast is used to show when the landing T indicates a compulsory direction for landing and not the wind direction. No colour is specified for this ball, but it has to be of such a colour that it contrasts well with the background. Before landing on an aerodrome at night the pilot of an aircraft has to obtain permission by radio from the control officer (see the chapter “Air Traffic Control”). If this is not possible, he attracts attention by firing a green Very light or flashing a green lamp; then he flashes the last three letters of his registration markings in the Morse code. The aerodrome officer then grants permission by the display of a green light and the repetition of the aircraft’s three-letter signal, or refuses permission by firing a red Very light or showing a red flare on the aerodrome. Should it be necessary at any time to require an aircraft to land, the order can be given in daylight by the internationally recognized signal of three black smoke puffs coming from projectiles fired at ten-seconds intervals. At night the same peremptory order is given by three projectiles discharged at ten-seconds intervals, but bursting into white stars or lights.
If a pilot sees by day three puffs of orange-coloured smoke, each appearing at ten-seconds intervals, or at night three sets of orange-coloured stars or lights, he knows that he is in the neighbourhood of a prohibited area. Prohibited areas, to be found in most countries, generally comprise fortified zones or centres of military importance over which flying is strictly forbidden. The offending pilot must immediately alter course to avoid the area.
The air traffic regulations lay down the internationally accepted distress signals which aircraft may use when in danger and in need of immediate assistance. These signals may be given by radio, by sound or by visual signalling devices. The wireless signals are the SOS transmitted by radiotelegraphy, or the word MAYDAY (from the French m’aidez, “help!”), sent by radiotelephony.
Visual distress signals may be given by flashing S O S in Morse, by firing a succession of red Very lights at short intervals, by displaying the international code flags N C, or the international distant distress signal of a square flag with a ball or similar shape above or below it. By sound, the same urgent distress call can be made by sending S O S on any sound apparatus, such as a foghorn or siren, or merely by the continuous sounding of the apparatus.
Less urgent emergencies, as for an aircraft in difficulties but not requiring immediate assistance, may be signalled by the word PAN, transmitted by radio, by the firing of a succession of white Very lights by day or, at night, by a succession of short and intermittent flashes of the aircraft’s navigation lights. A succession of green Very lights emanating from an aircraft in flight would indicate that the pilot had an urgent message to deliver concerning the safety of some person on board or within sight, as, for example, the crew of a ship in distress. By wireless, such a message would be preceded by the letter group “XXX”.
To keep track of aircraft flying abroad from Great Britain, and which are not equipped with radio, a circling procedure is laid down. The aircraft circles an aerodrome on either side of the English Channel. This enables the authorities at the aerodrome of departure to note the registration letters of the aircraft and to inform the aerodrome on the other side of the Channel to expect the arrival of this aircraft. Should the aircraft not arrive after a reasonable time, search will be made for it. Thus it is important that no aircraft that is not proceeding abroad should circle a “departure” aerodrome without landing. Otherwise it may be mistaken for an aircraft about to cross the Channel, and later be reported missing.
A comprehensive knowledge of the “rules of the road” in the air and of the signals which aircraft must carry or may use is an essential requirement of a pilot engaged in international aviation. Upon the strict observance of these rules largely depends the safety of all who use the airways of the world.
A FLYING BOAT moored on the water shows the same distinguishing lights at night as those shown by a ship. Similarly, when moored in conditions of fog, a flying boat gives the same warning signals as those of a ship at anchor. In general, seaplanes on the surface of the water follow the same rules as those governing shipping.