XVIII. THE DEVELOPMENT OF AIR TRAVEL
It was only thirty-one years ago that Orville and Wilbur Wright first succeeded in getting their home-made craft off the ground in America. This feat proved that the ‘heavier-than-air’ problem held practical possibilities. There were pioneers in other countries too, attempting to rise a few metres in what were usually rather peculiarly constructed flying machines. It was a period of tireless experimentation; speed and altitude records were set and promptly broken. It might seem somewhat amusing today to think that the whole world stood amazed when an altitude record of, say, ten metres was suddenly leaped to thirty, or when a distance flight of two hundred metres was surpassed by a ‘flight’ of no less than one kilometre. On 20 October 1909, a Farman aircraft pushed the distance record to 76 kilometres. Exactly twenty-five years later, Scott and Black departed from Mildenhall and landed in Melbourne within three days.
Many pioneers, striving to solve the problem of flight from either idealistic or commercial motives, had immense difficulties to overcome. Until shortly before the Great War, aircraft were built primarily for the pleasure of a few sporting daredevils and possessed little practical value. Subsequently, the aeroplane found its use as an aerial weapon in the bitter struggle between nations. The desire for self-preservation or victory sharpened the wits. Large factories were built; money was no object, and the world’s finest engineers and technicians worked with feverish diligence, day and night, to improve the aircraft. The experience gained during the war gave a tremendous impetus to the development of aviation.
After the war, a large number of aircraft remained, though they were of little use in peacetime. Until then, the aeroplane had been regarded chiefly as a weapon of war rather than a means of transport. Initial attempts were made to adapt old military planes for passengers, but it was soon realised that an entirely special type of aircraft had to be designed if commercial air travel was ever to become reliable and profitable. Thus, it was not until 1920 that the first ‘airliner’ appeared, with the primary goals of safety, comfort, and speed.
Today, after fifteen years of practical operation, civil aviation has developed to such an extent that it can rightly be considered an indispensable link in international society. One need only open an aviation guide and look at the dense network of air routes spanning the globe to appreciate what a significant factor it has already become in world transport. In terms of safety, comfort, and regularity, modern air travel can hold its own against any other form of transport, while it takes first place in terms of speed. It is undoubtedly to this latter advantage that air travel owes its exceptionally rapid development. In 1920, the speed of the airliners of the day averaged 110 to 130 kilometres per hour; today, a cruising speed of 250 to 300 kilometres is the standard requirement for a modern passenger aircraft.
Until a few years ago, a major disadvantage of air transport was that it could only be carried out in favourable weather conditions and during daylight hours. Today, it is often the case that flight services continue as normal while shipping in the Channel suffers major disruption.
That it is now possible to fly safely and regularly even in poor weather and at night is due to the use of multi-engined aircraft, the technical perfection of planes and engines, the invention of so-called blind-flying instruments, the improvement of radio and weather services, and everything encompassed by ‘ground organisation’.
Before discussing these components in more detail, it is advisable to consider the subject of ‘blind flying’ more deeply.
Unlike vehicles moving on the ground, an aircraft can move in free space around three axes: the longitudinal, the lateral, and the vertical—or yaw—axis. To control the aircraft in these three directions or to counteract unwanted movements, it is equipped with three independently operating control organs. These are the ailerons for rotation around the longitudinal axis, the elevator for rotation around the lateral axis, and the rudder for movement around the vertical axis. These control surfaces are operated as follows. In small aircraft, the ailerons are connected by steel cables to a control column in the cockpit; moving this lever sideways moves both ailerons in opposite directions. When the control column is central, both ailerons are in the neutral position, flush with the wing surface. If the column is moved to the left, the right aileron goes down and the left one goes up. Clearly, as the aircraft moves through the air, pressure is exerted on these ailerons such that the craft banks to the left, thus rotating around its longitudinal axis. In large aircraft, the ailerons are operated on the same principle, but by turning a steering wheel instead of moving the control column sideways.
The elevator is operated by the same control column by moving it forwards or backwards. If the pilot pulls the lever towards him, the elevator rises. It is easy to understand that the pressure exerted by the airflow on this surface causes the aircraft to move around its lateral axis and assume a ‘climbing’ attitude.
A reverse movement puts the aircraft into a ‘dive’.
The rudder works like the rudder of a ship but is operated by pedals, the so-called ‘rudder bar’. Stepping with the left foot moves the rudder to the left and causes the craft to rotate around its vertical axis.
By using these three control organs in combination, the pilot can make his aircraft assume any desired position. In straight and level flight, all controls are in the neutral position. If the craft is knocked out of its flight attitude, whether by irregular air currents or other reasons, the pilot reacts immediately, using the relevant controls to bring the aircraft back to its original position. However, aircraft are designed so that, if their balance is disturbed, they tend to return to the normal flight attitude of their own accord. This is known as the stability of the aircraft, which can also be divided into three directions.
This stability is achieved by bringing the shape and dimensions of the various parts of the aircraft into an aerodynamically correct proportion, and by ensuring that the weight distribution—in other words, the position of the centre of gravity—is favourable relative to the centre of pressure where the upward force (lift) acts. Furthermore, so-called stabilising surfaces are added. At the tail end, there is a horizontal stabiliser to which the elevator is pivotally attached, and a vertical stabiliser or fin, to which the rudder is hinged.
One might wonder how a pilot, high in the air, can constantly determine the attitude of his aircraft. Under normal circumstances, the pilot judges this purely optically; that is to say, he compares the position of his machine with the earth’s surface and the horizon. The sense of balance plays a very subordinate role here, especially in large aircraft. Indeed, due to centrifugal forces and accelerations that can occur during certain movements of an aircraft, this sense often gives the pilot a completely false impression. This clearly shows that as soon as optical judgment is made impossible by poor visibility, flying in clouds or fog, or by darkness, the pilot could find himself in a more or less precarious position—all the more so as his sense of balance can give him a completely incorrect impression of his aircraft’s attitude.
Now, however, he is aided by his instruments, which give him precise indications of his machine’s position relative to the horizontal plane. A full set of blind-flying instruments consists of six gauges, which can be divided into two groups. The first group indicates the direction and speed of the aircraft’s movements, while the second group provides indications of the craft’s attitude. We shall discuss these six instruments in a little more detail. Firstly, the airspeed indicator, which can be considered one of the most important aids for flying without visibility. When an aircraft moves around its lateral axis, it immediately affects the speed. If the aircraft enters a climbing attitude, the air resistance increases; the aircraft rises, but the horizontal speed decreases. If the nose goes down, the aircraft will lose altitude but gain speed. The airspeed indicator therefore shows the pilot what position to keep his elevator in to make his aircraft climb at the most favourable angle, to fly horizontally, or to glide at the most economical angle. The operation of the airspeed indicator is based on the change of pressure in a tube, usually mounted on the wing away from the influence of the propellers. The pressure in this ‘Pitot tube’, named after its inventor, depends on the speed of the aircraft and is communicated via lines to the airspeed indicator, which effectively acts as an ordinary pressure gauge or manometer.
The turn indicator relies on the gyroscopic effect of a high-speed spinning rotor. This rotor, whose axis is suspended horizontally in a transverse direction within a gimbal frame, is fitted with vanes on the outside and housed in an airtight casing. This casing is connected by a pipe to a vacuum pump or the intake manifold of the engine, ensuring a constant partial vacuum inside. By opening a valve, atmospheric air can flow in through a small nozzle positioned so that the air stream hits the vanes of the rotor, setting it into a rapid spinning motion. Now, if the aircraft—and with it the turn indicator—rotates around its vertical axis, the gyroscopic action of the rotor generates a force that causes the rotor’s axis, and thus the frame in which it is suspended, to rotate. This movement is indicated by a needle on the instrument. The turn indicator thus shows every rotation of the aircraft around its vertical axis, indications which the pilot must correct with his rudder. The turn indicator is combined with a so-called transverse inclinometer or side-slip indicator. This is a curved glass tube filled with liquid containing a metal ball. During a rotation of the aircraft around its longitudinal axis—causing it to bank to the left or right—the ball moves from the centre. Using his ailerons, the pilot must bring his craft into such a position that the needle of his turn indicator is at zero and the ball of the side-slip indicator is in the centre.
With these three instruments, all movements around the three axes of the aircraft are already indicated, and one could theoretically manage with these alone.
For small aircraft, especially sporting planes, the airspeed indicator, turn indicator, and side-slip indicator are often combined into a single instrument, the so-called ‘controleur-de-vol’. For larger airliners, which must be able to fly under all conditions, it is necessary to add a so-called rate-of-climb-and-descent indicator, or variometer. This instrument indicates precisely whether the craft is flying horizontally, climbing, or descending. It is based on the changes in air pressure that occur with changes in altitude. The higher one goes, the lower the air pressure becomes. A quantity of air is kept at a constant temperature inside a thermos flask. If the aircraft rises, the pressure of the outside air decreases, creating an overpressure inside the flask, so that air flows out through a small opening. If the aircraft descends, the outside air pressure increases, causing air to flow back into the flask. This inflow or outflow of air particles, as well as its speed, is accurately indicated by the needle of the variometer. The scale of this instrument is usually calibrated in metres per second of climb or descent.
The airspeed indicator, the turn-and-slip indicator, and the variometer all belong to the group of instruments that indicate the direction and speed of disturbances from equilibrium. One cannot, therefore, determine from these how much the altitude or heading has changed after correcting the aircraft’s attitude. Consequently, a second group of instruments is added to provide direct, positive indications. These are the altimeter, the gyroscopic course indicator, and an ‘artificial’ horizon, which we shall now examine.
The altimeter is effectively a standard aneroid barometer and depends on the change in atmospheric pressure with altitude. Instead of millimetres of mercury or millibars, as in a normal barometer, the scale of the altimeter is divided into the number of metres corresponding to the prevailing air pressure. The scale is adjustable, so that before departure the needle can be set to zero or, if necessary, to the altitude of the airfield above sea level. If the aircraft has covered a distance of a few hundred kilometres, the altimeter will often no longer indicate accurately. After all, it was set to the barometric pressure of the point of departure, and there will usually be some difference from the air pressure at the destination. Especially when an aircraft moves towards the centre of a depression, these differences can be very large. Therefore, altimeters are also provided with a separate, movable barometric scale. The pilot can now ask for the barometric pressure at his destination by radio and correct his altimeter accordingly. Thus, while the variometer indicates how many metres per second the aircraft is climbing or descending, the pilot can see on his altimeter how many metres he has gained or lost over time, and always read his true flight altitude.
The turn indicator shows to which side and approximately at what ‘angular velocity’—that is, how many degrees per minute—the craft is rotating around its vertical axis. The precise change in flight direction or heading caused by such a disturbance could be determined from the position of the compass needle. However, the magnetic compass is subject to many errors. If the aircraft is not perfectly level, the compass needle deviates due to inclination (the so-called northerly turning error). Furthermore, the compass needle is sluggish in its movements and subject to acceleration errors. It is therefore necessary to add an instrument that, independent of these influences, accurately indicates the number of degrees the aircraft has deviated from its course. Such an instrument is the gyroscopic course indicator (Sperry directional gyro). Like the turn indicator, this instrument consists of a high-speed spinning rotor, but it is suspended in a frame such that, through gyroscopic action and independent of any rotation of the craft, it maintains the same position for a considerable time. The frame is connected to a compass rose, which can be set to any desired heading. At intervals, this rose must be compared with the magnetic compass and adjusted if necessary.
A very important and exceptionally interesting blind-flying instrument is, finally, the artificial horizon. It also relies on gyroscopic action. When flying in favourable weather, so that the earth is not hidden from view by clouds or fog banks, the pilot will receive the impression, if his aircraft banks into a turn, that the horizon is tilted while the aircraft itself remains horizontal. On the instrument, a miniature aeroplane is fixed in the centre. Behind it is a bar, the artificial horizon, which is connected to the frame in which the gyro axis is mounted. Through gyroscopic action, this frame always remains in a horizontal plane, thereby creating the same impression as when flying by the real horizon. If the craft begins to rotate around its longitudinal axis—banking to the left or right—the artificial horizon remains horizontal and thus appears tilted relative to the wings of the miniature aeroplane in the instrument. The pilot must therefore always ensure that this little aeroplane remains parallel to the simulated horizon. If the craft enters a climb, the horizon bar moves down, so that the miniature plane appears above the horizon. Since the artificial horizon gives a positive indication of the attitude of the lateral and longitudinal axes of the aircraft relative to the horizontal plane, flying without visibility is significantly simplified by this instrument.
It will be clear to the reader that great practice is required to fly well using these instruments alone. However, from the explanation above, it can be seen that one does not always need to consult all six instruments simultaneously. Different instruments give the same indication for certain movements, meaning that if one fails, flying on the remaining instruments is still quite possible. At most, it requires a little more effort from the pilot, as he must then deduce certain data from the combined indications of the other instruments to continue controlling his aircraft in all three directions.
Of significant importance for flying without sight of the earth is automatic control. In this case, the aircraft is steered by the blind-flying instruments themselves. That is to say, the reactions of the instruments are transmitted to the control organs by means of a servo-system, which may be hydraulic, electric, or pneumatic. The pilot no longer needs to react to the instrument readings himself, but merely needs to set and monitor them, otherwise devoting himself entirely to navigation. A disadvantage of automatic control is its great complexity, which carries a high risk of malfunction. Much has already been improved in this regard, however, and it will not be long before all large airliners are equipped with automatic pilots.
Although it is now possible to control an aircraft under all conditions without optical contact with the earth, the ‘blind flying problem’ is not yet fully solved. It is not possible, when flying without sight of the ground, to determine one’s position accurately. The drift meter cannot be used without sight of the ground, and the direction and strength of the wind, especially at high altitudes and certainly in poor weather, cannot be accurately checked from the ground. Upper winds are generally measured by releasing small balloons with a known rate of ascent, which are tracked from the ground with a theodolite. In fog or with low-hanging clouds at the meteorological observation post, such a measurement is obviously impossible, leaving one in uncertainty regarding the state of the atmosphere in and above the fog or clouds. Since the wind at high altitude is usually much stronger and of a different direction than on the ground, it clearly exerts a great influence on the heading and speed of the aircraft; even an exceptionally skilled pilot with the best blind-flying instruments would find it impossible to find a particular airport in poor visibility without special aids.
This is where radio comes to the rescue. Not only can the pilot maintain contact with ground stations in this way to receive meteorological reports, and the ground service stay precisely informed of the aircraft’s position, but radio can also provide excellent service for navigation. The travelogue of the Uiver’s flight clearly shows how often, especially during night flying, radio bearings were used to find the course to an airport.
In Western Europe, there are a very large number of radio stations, all equipped with radio direction-finding equipment. Its principle is based on the directional effect of a loop antenna, which can be rotated. If we turn it so that the plane of the loop antenna is in the direction of the transmitter, the signals transmitted will be picked up with maximum strength by the amplifier connected to the loop. If we now rotate the loop antenna, the sound will decrease in volume, and exactly when the plane of the loop is perpendicular to the direction from which the radio waves are coming, no sound will be heard in the headphones. If the loop is turned another 90 degrees so that it is again facing the transmitter, maximum volume is reached once more. Thus, if we turn the loop 360 degrees, we will find two maxima and two minima in sound volume.
To determine the direction of a transmitter, we could turn our loop antenna until the maximum sound is received. However, we would soon notice that the loop can be turned through a relatively large angle without perceiving much difference in volume. If we now turn the loop 90 degrees, we find a sharp point at which nothing at all is heard in the headphones. For this reason, the direction is always determined by the minimum, and 90 degrees is then added to the bearing found. In this way, the direction of the transmitting station relative to the direction-finder can be accurately determined. However, as previously noted, there are two minima (and two maxima) in the 360 degrees, which are exactly 180 degrees apart. This creates the possibility that our bearing is 180 degrees wrong. In practice, this rarely happens, as the ground station usually knows which side the aircraft is on. It is quite possible, however, for an aircraft to fly past the station without noticing, in which case the ground station would still bear the aircraft in the same direction. Yet it is very simple to determine on which side the transmitter is located relative to the direction-finding station. The effect of the loop antenna is then combined with that of a normal open antenna. The minimum is less sharply defined as a result, but as soon as it is known which side the aircraft is on, the fixed antenna can be switched off again to find the exact minimum.
This, in short, is the principle on which radio direction finding is based. In practice, however, it is not applied in exactly this way. After all, on a loop antenna of modest dimensions, the generally weak aircraft transmitters could only be received at relatively short distances. If the loop antenna were made large enough to capture sufficient energy, such a colossal installation would be not only very expensive but also extremely unwieldy. When one considers that such an installation must be placed on the airfield to take bearings on aircraft approaching the airport, it is clear that such an obstacle would pose a danger to aircraft coming in to the landing ground in poor visibility. Although amplification systems are now available that can make the small amount of energy received on a relatively small loop antenna sufficiently audible, the so-called ‘Bellini-Tosi system’ is still widely used. Instead of one rotatable loop antenna, this system uses two large fixed loop antennas placed perpendicular to each other. If one loop is oriented North-South, for example, the plane of the other loop is East-West. The antennas are usually triangular, suspended from a single mast and insulated from each other; the wire ends of each loop are connected to a box in which two coils are likewise mounted perpendicular to each other. In the centre of these so-called field coils is a third, the search coil, which is rotatable and connected to the receiver. By rotating this coil 360 degrees within the field coils, we likewise find two minima and two maxima, achieving the same effect as if the entire antenna system were rotated. This system of coils is called a goniometer. A pointer is attached to the axis of the search coil, perpendicular to the plane of the windings. By drawing a circle on the goniometer and dividing it into 360 degrees, the bearings found can be read off immediately.
To use these direction-finding installations for air navigation, the procedure is as follows. The aircraft PH-AIZ, flying from Berlin to Amsterdam, for instance, is above a solid cloud deck, and the pilot wants to know the heading to reach Schiphol. The radio operator of the PH-AIZ calls up the Schiphol radio station and requests a bearing. Schiphol replies: ‘Please transmit,’ whereupon the onboard operator sends a series of long signals with his key, which are received by Schiphol. The operator at Schiphol turns the search coil of his goniometer until he finds the point where the strength of the received signals is at its minimum, and then reads the number of degrees from the goniometer. He then signals the aircraft: ‘Your true bearing from Schiphol is 74 degrees.’ The pilot now knows that his true course to Schiphol is 74 + 180 = 254° and can correct his compass heading if necessary. If he wishes to know his position, Schiphol immediately calls the radio stations at Rotterdam (Waalhaven) and Twente, requesting they take a bearing on aircraft PH-AIZ. He then asks the aircraft to transmit a few signals, after which all three stations determine the direction from which they receive signals from the PH-AIZ with their goniometers in the manner already described. Waalhaven and Twente then signal the bearings found to Schiphol, where the operator has a large map of the area within the receiving range of the three stations. Thin strings are attached to the locations of Schiphol, Waalhaven, and Twente, with a protractor drawn around each. The operator stretches these strings over the protractors so that the direction corresponds with the bearings found by himself and the two other stations. Where the three strings meet on the map is the position of the aircraft. He then immediately signals the found position to the PH-AIZ.
This method appears rather cumbersome, but under normal conditions, through good cooperation between the various ground stations, a requested position can be given with great accuracy in two minutes.
This system of direction finding is used throughout Europe and partially on the route to the Indies. It is, of course, also possible to equip the aircraft with a radio direction finder, so that bearings from various stations could be taken on board and plotted on the map. Naturally, however, there is insufficient space in aircraft to accommodate such bulky chart tables. This system will certainly be applied to large aircraft in the near future, just as it has been on board ships for many years.
In America, a completely different method is used, namely so-called radio beacons. The principle is similar to the system of direction finding using goniometers. However, they do not use the directional effect in reception, but transmit so-called ‘directional’ radio signals. The whole of North America is equipped with a dense network of these radio beacons, which work as follows.
Unlike an ordinary transmitter, where radio waves are dispersed in all directions through the ether, the energy from a radio beacon is radiated in only one or more specific directions.
For an aviation radio beacon, this direction is chosen so that the radiation beam coincides with an air route. If an aircraft is not exactly on this route, the pilot hears the Morse signal A (dot-dash) or N (dash-dot) in his headphones. At regular intervals, the station also transmits its call sign in a specific group of letters, so that the pilot knows exactly which beacon he is listening to and can also determine whether he has drifted left or right of the route. He thus knows which way to steer to get back on course, and as soon as he is on the so-called ‘centre line’ of the beam, he hears both the A and the N, which now merge into a long, continuous tone. This is possible because the A and N zones overlap slightly on the centre line, and the transmission of the intermittent signals is synchronised so that the dash-dot signals fall exactly into the gaps of the dot-dash signals.
It would take us too far in this book to give a full description of all the possibilities that radio offers for air navigation. Nor can we delve into discussions on the advantages and disadvantages of the various systems. Extensive trials are still being conducted to improve existing methods and to further develop entirely new inventions in this field. With the increase in air traffic and the shortage of available wavelengths, it will soon prove necessary to change the entire radio system, so that an aircraft can find its way through the air almost independently of the ground, blindly and under all conditions, using ingenious radio instruments.
Thanks to radio, it is already possible to fly in weather conditions that were once impossible without taking great risks. Low clouds and fog banks, which a few years ago often had a very unfavourable effect on the regularity of air traffic—especially in the winter months—are now, thanks to the use of multi-engined aircraft (which makes forced landings for technical reasons practically a thing of the past), the improvement of blind-flying instruments, and especially weather reports and radio services, only very rarely an obstacle to the performance of regular air services. The greatest enemies of the commercial pilot are the danger of icing and thunderstorms. The latter phenomenon is less dreaded for the danger of lightning strikes than for the very heavy vertical air currents that typically occur in thunderstorms. Especially during night flying, when individual storms cannot be clearly seen, thunderstorms can still be an obstacle to completing a flight. During the day, it is usually possible to fly around thunderstorms, which are almost always very local.
Icing can occur at air temperatures around freezing. If an aircraft flies at this temperature through a cloud that is nearly saturated with moisture, the water particles attach themselves to the parts of the aircraft exposed to the airflow. Usually, this ice formation is slow, giving the pilot time to find an altitude where the temperature is either above freezing or very low, as icing rarely occurs below -10°C. The most dangerous weather condition for ice formation on an aircraft occurs in winter during a so-called ‘warm air incursion’. Warm, moisture-saturated air comes from the ocean and encounters air of much lower temperature over the land. Since warm air is lighter, it slides over the cold layer of air and cools rapidly as it rises, forming heavy cloud masses from which hail or rain falls. On their way to the earth, the water droplets encounter a colder layer of air. If its temperature is below freezing, the water droplets are also cooled significantly without actually freezing. A falling water droplet only freezes at a temperature lower than about 8 degrees below freezing. If an aircraft now enters such a ‘supercooled’ rain shower, the droplets immediately freeze as ice upon collision with the aircraft. As a result, a very large layer of ice can form within minutes on the leading edge of the wings and on the propellers. The lifting capacity is very unfavourably affected by the change in the wing profile caused by the ice layer, while the weight of the ice often makes the aircraft several hundred kilograms heavier. It is possible that in such a case the craft will be unable to maintain altitude. If no temperature above freezing is found at a lower altitude to allow the ice to melt quickly, the aircraft will have to land. With an excellent meteorological organisation, these specific weather conditions can be foreseen so that this danger can be avoided. Sometimes, however, especially at night, the danger of icing remains an obstacle to flight.
In America, so-called ‘de-icers’ are used on the night mail services. These are thin rubber sleeves fitted to the leading edge of the wings and tail surfaces. As soon as ice begins to form on the aircraft, these sleeves are ‘inflated’ at very short intervals by means of an air pump, after which the air is allowed to escape again. This movement constantly breaks the ice layer that tries to settle on these ‘ice breakers’, ensuring the ice has no chance to form on these parts of the aircraft most vulnerable to icing.
We have thus reached the point where, with a well-equipped airliner and an experienced crew supported by a good ground organisation, flight is possible in almost all weather conditions. Even departing from an airfield where low-hanging clouds or fog have reduced visibility to the point that even the coach bringing the passengers to the airport must drive with great caution is no obstacle for the airliner, thanks to its excellent blind-flying instruments.
Landing without visibility, on the other hand, is currently a problem that cannot yet be considered fully solved. To be sure, various systems exist by which an aircraft can now safely enter an airport with a cloud base of only a few dozen metres and horizontal visibility of a few hundred metres. Without special aids, this is of course not possible. Most of these methods for landing without visibility consist of miniature radio beacons—directional transmitters following the same principle as the American aviation beacons already mentioned. At Schiphol airport—as at other airfields in our country—such an installation has been fitted, enabling airliners to land in conditions of very poor visibility. The landing of the Pelikaan on the night of 30 December 1933, after its return from the Christmas flight to the Indies, is a striking example of this, although such landings are now frequently performed.
Once the aircraft has been guided to the airfield by means of ordinary radio bearings, the aircraft’s receiver is tuned by the operator to the wavelength of the radio beacon. The pilot now also puts on a headset and hears an intermittent signal. If this consists of three short signals in succession, he knows he is to the north of the so-called centre line of the radio beacon. Otherwise, he hears groups of two dots and a long dash. He then flies away from the airfield in a south-westerly direction for a certain number of minutes, attempting to join the centre line of the radio beacon so that, instead of dots or dashes, he hears a continuous hum. After having flown away from the field for five minutes in this manner, he makes a 180-degree turn and returns to the field, flying along the centre line. He now knows fairly accurately where he is and exactly from which direction he is approaching the landing ground. It goes without saying that the direction of the radio beacon’s centre line is chosen so that the approach path is clear of high obstacles. The pilot can thus gradually descend, knowing that after about five minutes of flying back, he should be at the edge of the airfield again. He ensures that by the time he passes the beacon, he is flying at an altitude of about 50 metres. Naturally, he has previously corrected his altimeter with the barometric pressure at Schiphol, so that the instrument does not indicate incorrectly due to differences in air pressure. As he passes the beacon, the hum—which had rapidly increased in volume shortly before—suddenly falls silent. After a few seconds, the hum begins again. This interruption of the centre-line signal is caused by the fact that there is always a so-called ‘zone of silence’ near the radio beacon in which no sounds can be received. The pilot thus knows that he has passed the beacon and that he is at a distance of 400 metres from the edge of the field on a clear approach path. He can therefore safely reduce his speed and altitude until he gains sight of the ground and can land. The aircraft is thus brought over the field in the correct landing direction by means of the beacon; the actual landing must then be performed normally, with visibility. Since there is almost always a dead calm during fog, it is possible to choose the most favourable side of the airfield for the approach, considering the dimensions and surrounding obstacles.
Experiments are now being conducted to improve the existing system at Schiphol by installing so-called marker beacons. Small beacons are placed on the centre line, with their radiation beams directed vertically upwards. When passing a marker beacon, the pilot suddenly hears a series of signals that rapidly increase in strength, reach a maximum, and immediately decrease again. This allows one to know exactly how far one is from the edge of the field, as the locations where the marker beacons are installed are known.
In Germany, a similar system has been devised, but with radio beacons operating on ultra-short waves. The pilot does not need headphones to follow the centre line with this system. On his dashboard is an instrument equipped with a needle that indicates precisely which side of the centre line the aircraft is on. The instrument’s needle deflects to the side where the centre line of the beacon is located relative to the aircraft. If the craft is flying exactly in the safe approach direction towards the landing ground, the instrument’s needle remains in the centre. This method has the advantage that the pilot can tell from the position of the needle approximately how far he is from the beacon line, reducing the chance of repeatedly overshooting the beam—which is very narrow close to the beacon—and thus approaching in a zigzag fashion, which can easily happen when monitoring signals by ear. A disadvantage, however, is that the aircraft must be equipped with an additional ultra-short-wave installation.
In the United States, attempts are also being made to solve the ‘blind landing’ problem. At Newark airport near New York, a system of short-wave radio beacons has been installed, which not only guides aircraft in a specific direction to the landing ground but also down a sloping path, ensuring they pass the boundary of the field at the correct flight altitude and can land completely ‘blind’. For this, an instrument with horizontal and vertical needles is used, which forms a right-angled cross when the aircraft is exactly on the constantly descending ‘glide path’. Pilots from the American Department of Commerce have landed with this system both under a blind-flying hood and in actual conditions of poor visibility. For large aircraft, which require a longer flare-out, this method has not yet been developed to the point where one could land safely in all conditions using instruments alone. As with the systems common in Europe, however, it is now possible in America to come in under weather conditions that were previously considered impossible. It will certainly not be long before radio-technical aids are so perfected that landing without visibility can also be carried out in practice under all conditions and with great certainty.
It will be clear to the reader what important technical studies, international cooperation, and organisation form the basis for the great and rapid development of civil aviation. Besides the aids briefly discussed in this chapter, there are numerous other subjects about which very interesting volumes could be written—for example, the night lighting of airports, the equipping of night-flying routes with light beacons, aeronautical meteorology, and the general theory of commercial flying. It would, however, lead too far from the subject of this book to go into them more deeply here.
Not everywhere, however, has ground organisation yet reached such a stage of development. On the flight of the Uiver to Australia and back, some links in the chain of radio stations were found to be missing here and there. For the regular air services between the Netherlands and the Indies, however, it can be said that this air route—with the exception of Persia—is already very well equipped in terms of radio and meteorological services. Reliable weather forecasts are available everywhere, and radio direction-finding stations provide good assistance for navigation in poor visibility. In the Dutch East Indies, however, aviation must still largely make do with a few maritime radio stations, which are generally not capable of providing the data that can be of such great importance to aircraft pilots. Various flights, such as those of the Pelikaan and the Uiver, have proven how much the journey time can be shortened if flying continues day and night. Since the ground organisation is now developed along almost the entire route to the Indies so that one can also fly regularly at night, it is to be hoped that the Dutch East Indies will not lag behind in adapting its ground organisation to modern air travel.
Generally speaking, it can be said that the technical equipment of aircraft has always kept ahead of improvements in ground organisation, thanks to the experience gained in regular air traffic and on special flights. It is therefore only a matter of time before the safety and regularity of air travel surpasses that of all other means of rapid transport.
Finally, if night flying is also applied to very long routes, aircraft will still need to be equipped with sleeping quarters for the passengers and the crew. Use will then be made of very large aircraft, in which the pilots will relieve each other regularly and must therefore—just as on board ships—‘keep watch’. The Fokker F.XXXVI could already be considered the prototype for this.
The thought that, within the foreseeable future, an air traveller on board a large airliner will go quietly to bed in the evening and wake up the next morning over a place several thousand kilometres away is no ‘utopia’, but a very near vision of the future!