Takeoffs are optional, landings are mandatory

Takeoffs are Optional, Landings are Mandatory By Jim Sparks February 1998 Imagine flying to a distant destination and finding out that the pilot cannot find a runway for landing. Back in the time of Orville and Wilbur, this was not a...


Takeoffs are Optional, Landings are Mandatory

By Jim Sparks

February 1998

Imagine flying to a distant destination and finding out that the pilot cannot find a runway for landing. Back in the time of Orville and Wilbur, this was not a problem since any farmer's field would suffice. Unfortunately, landing a 747 or MD11 in a field is not quite the same.

Instrument landing systems (ILS) provide a means of getting an aircraft into a position where a decision can be made to land or go around.

Since its inception in the 1940s, about 1,000 ILS sites have been established in the United States. The original intent was to satisfy the requirements of the Category 1 approach which was to bring the aircraft to 1/2 mile from the runway at 200 feet above the ground. If at that time the pilot could clearly identify the runway, he was safe to land. With the high degree of accuracy and reliability, Category 2 approaches soon were being used. This would bring the aircraft to the end of the runway at a height of 100 feet. Today Category 3 or "auto land" is becoming more common and permits landing in almost zero visibility.

An aircraft's approach to landing is three dimensional. The pilot is concerned with the distance to the runway as well as orientation to the runway center line. Like most modes of flight, proximity to the ground is a genuine concern. In the world of ILS, these three dimensions are monitored using radio waves. The distance element is handled by a marker beacon system while orientation to the runway centerline is achieved by the localizer, and finally the vertical path is displayed with information from the glide slope. These three elements of an ILS operate independently.

Distance to the runway is provided to the pilot by using marker beacon transmitters. There can be as many as three of these transmitter on each approach path. One is located anywhere from 4 to 10 miles from the runway and provides the initial position reference to begin the approach. This device is referred to as the outer marker. About 3,500 feet out from the end of the runway is the middle marker. It is at this point where the aircraft will be about 200 feet above ground level and the decision is made on whether or not to land. An inner marker may be installed about 1,000 feet from the runway threshold and will provide the decision height (DH) for a CAT 2 approach.

These marker beacons are all fixed, ground-based transmitters producing a 75 MHz signal. On the aircraft is a marker beacon receiver (many newer systems incorporate the marker beacon function in the navigation receiver). This system will use a dedicated belly mounted antenna. There is no tuning of this unit; only the ability to turn it on or off and maybe set the sensitivity. The 75 MHz signal is transmitted straight upwards in a cone or sometimes a fishmouth shape and is modulated.

The outer marker has a 400 Hz intermittent signal installed. When this signal enters the receiver on the aircraft, a group of frequency detectors will begin to operate and the detector sensitive to the 400 Hz modulation will activate. The result is the illumination of a blue "O" (either a light on the instrument panel or a character in an attitude directional indicator {ADI}). An audible signal can also accompany the light and the signal is transmitting the Morse code for "O."

As with any cone shape, there is a narrow base and it gets progressively wider with more elevation. The sensitivity of this system is such that when the aircraft enters the cone the light will illuminate only dimly and the audio tone will be quite faint, but as the aircraft progresses into the cone, both light and audio intensities increase until the aircraft passes the center and both indications will begin to fade. Most marker systems will provide the pilot a sensitivity selector which allows him to have the fade-in, fade-out or only one tone at the center of the cone.

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