A Radio System for Blind Landing of Aircraft in Fog

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The Trident used a triplex system with no common elements, so that a failure in one of the three channels could be detected and that channel eliminated. The VC10 used an Elliott duplicated monitored system.

Milestones:First Blind Takeoff, Flight and Landing, 1929

By , the Trident had carried out more than 50, in-service automatic landings. The VC10 accrued 3, automatic landings before use of the system was curtailed in for economic reasons. By , Concorde had performed nearly 1, automatic landings in passenger service. BLEU knew that an ideal system would require components based on the ground and in aircraft. The former system would have to consist of a signal without the land use problems of the cable-based system or the accuracy issues of ILS. All incoming aircraft would need to be equipped with a sensor to receive the signal, a super-precise altimeter , and a reliable autopilot.

BLEU's work resulted in an eponymous system for controlling airplane landings. The required standard was that any landing system could not cause more than one accident in every ten million landings. When it was closed due to fog, they would make test landings at London Heathrow International Airport.


In his paper [44] John Charnley, then Superintendent of the BLEU, concluded a discussion of statistical results by saying that "It is fair to claim, therefore, that not only will the automatic system land the aircraft when the weather prevents the human pilot, it also performs the operation much more precisely". The system was approved for commercial use in , and on 4 November , Captain Eric Poole landed a British European Airways flight at Heathrow with visibility of 40 meters, which was the first use of the system to land a commercial flight in such severe conditions. The BLEU played a vital role in the development of autolanding, and descendants of its system are still in use around the world today.

From Wikipedia, the free encyclopedia. Oxford University Press.

Retrieved 14 March Trafford Publishing. Peden 8 February Cambridge University Press. Retrieved 15 March Science News Letter : Popular Mechanics. Hearst Magazines. October Technology and Culture : 81— Dobson ed. RAE Tech. Memo, FS 77 p. Report 70, February I magine trying to land a jumbo jet the size of a large building on a short strip of tarmac, in the middle of a city, in the depth of the night, in thick fog.

If you can't see where you're going, how can you hope to land safely?

How planes land safely in thick fog

Airplane pilots get around this difficulty using radar , a way of "seeing" that uses high-frequency radio waves. Radar was originally developed to detect enemy aircraft during World War II, but it is now widely used in everything from police speed-detector guns to weather forecasting. Let's take a closer look at how it works! Photo: This giant radar detector at Thule Air Base, Greenland is designed to detect incoming nuclear missiles.

We can see objects in the world around us because light usually from the Sun reflects off them into our eyes. If you want to walk at night, you can shine a torch in front to see where you're going.

How Pilots Land Blind

The light beam travels out from the torch, reflects off objects in front of you, and bounces back into your eyes. Your brain instantly computes what this means: it tells you how far away objects are and makes your body move so you don't trip over things. Radar works in much the same way.

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The word "radar" stands for ra dio d etection a nd r anging—and that gives a pretty big clue as to what it does and how it works. Imagine an airplane flying at night through thick fog. The pilots can't see where they're going, so they use the radar to help them. An airplane's radar is a bit like a torch that uses radio waves instead of light.

The plane transmits an intermittent radar beam so it sends a signal only part of the time and, for the rest of the time, "listens" out for any reflections of that beam from nearby objects. If reflections are detected, the plane knows something is nearby—and it can use the time taken for the reflections to arrive to figure out how far away it is. In other words, radar is a bit like the echolocation system that "blind" bats use to see and fly in the dark. Photo: This mobile radar truck can be towed to wherever it's needed.

The antenna on top rotates so it can detect enemy airplanes or missiles coming from any direction. Photo by Shane A. Cuomo courtesy of US Air Force. Whether it's mounted on a plane, a ship, or anything else, a radar set needs the same basic set of components: something to generate radio waves, something to send them out into space, something to receive them, and some means of displaying information so the radar operator can quickly understand it. The radio waves used by radar are produced by a piece of equipment called a magnetron. Radio waves are similar to light waves: they travel at the same speed—but their waves are much longer and have much lower frequencies.

Light waves have wavelengths of about nanometers billionths of a meter, which is about — times thinner than a human hair , whereas the radio waves used by radar typically range from about a few centimeters to a meter—the length of a finger to the length of your arm—or roughly a million times longer than light waves.

Both light and radio waves are part of the electromagnetic spectrum , which means they're made up of fluctuating patterns of electrical and magnetic energy zapping through the air.

The waves a magnetron produces are actually microwaves, similar to the ones generated by a microwave oven. The difference is that the magnetron in a radar has to send the waves many miles, instead of just a few inches, so it is much larger and more powerful. Once the radio waves have been generated, an antenna , working as a transmitter , hurls them into the air in front of it. The antenna is usually curved so it focuses the waves into a precise, narrow beam, but radar antennas also typically rotate so they can detect movements over a large area.

The radio waves travel outward from the antenna at the speed of light , miles or , km per second and keep going until they hit something. Then some of them bounce back toward the antenna in a beam of reflected radio waves also traveling at the speed of light. The speed of the waves is crucially important. That's no problem, because radio waves and light travel fast enough to go seven times around the world in a second!

If an enemy plane is km miles away, a radar beam can travel that distance and back in less than a thousandth of a second.

The antenna doubles up as a radar receiver as well as a transmitter. In fact, it alternates between the two jobs. Typically it transmits radio waves for a few thousandths of a second, then it listens for the reflections for anything up to several seconds before transmitting again. Any reflected radio waves picked up by the antenna are directed into a piece of electronic equipment that processes and displays them in a meaningful form on a television -like screen, watched all the time by a human operator.

What Happens Before a Plane's Delivery Flight

The receiving equipment filters out useless reflections from the ground, buildings, and so on, displaying only significant reflections on the screen itself. Using radar, an operator can see any nearby ships or planes, where they are, how quickly they're traveling, and where they're heading. Watching a radar screen is a bit like playing a video game—except that the spots on the screen represent real airplanes and ships and the slightest mistake could cost many people's lives. There's one more important piece of equipment in the radar apparatus.

It's called a duplexer and it makes the antenna swap back and forth between being a transmitter and a receiver. While the antenna is transmitting, it cannot receive—and vice-versa. Take a look at the diagram in the box below to see how all these parts of the radar system fit together. Photo: A scientist adjusts a radar dish to track weather balloons through the sky. Weather balloons, which measure atmospheric conditions, carry reflective targets underneath them to bounce radar signals back efficiently.

With an angular signal system such as ILS, as altitude decreases all tolerances must be decreased — in both the aircraft system and the input signal — to maintain the required degree of safety. This is because certain other factors — physical and physiological laws which govern for example the pilot's ability to make the aircraft respond — remain constant.

For example, at feet above the runway on a standard 3 degree approach the aircraft will be feet from the touchdown point, and at feet it will be feet out. It will be possible if initiated at feet of height, but not at feet. Consequently, only a smaller course correction can be tolerated at the lower height, and the system needs to be more accurate. This imposes a requirement for the ground-based, guidance element to conform to specific standards, as well as the airborne elements.

Thus, while an aircraft may be equipped with an autoland system, it will be totally unusable without the appropriate ground environment. Similarly, it requires a crew trained in all aspects of the operation to recognise potential failures in both airborne and ground equipment, and to react appropriately, to be able to use the system in the circumstances from which it is intended.

Consequently, the low visibility operations categories Cat I, Cat II and Cat III apply to all 3 elements in the landing — the aircraft equipment, the ground environment, and the crew. The result of all this is to create a spectrum of low visibility equipment, in which an aircraft's autoland autopilot is just one component. The development of these systems proceeded by recognising that although the ILS would be the source of the guidance, the ILS itself contains lateral and vertical elements that have rather different characteristics.

In particular, the vertical element glideslope originates from the projected touchdown point of the approach, i. The transmitted glideslope therefore becomes irrelevant soon after the aircraft has reached the runway threshold, and in fact the aircraft has of course to enter its landing mode and reduce its vertical velocity quite a long time before it passes the glideslope transmitter. The lateral guidance from the ILS localiser would however be usable right to the end of the landing roll, and hence is used to feed the rudder channel of the autopilot after touchdown.

As aircraft approached the transmitter its speed is obviously reducing and rudder effectiveness diminishes, compensating to some extent for the increased sensitivity of the transmitted signal. More significantly however it means the safety of the aircraft is still dependent on the ILS during rollout.