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 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.

The middle marker works on the same principle, but the 75 MHz signal is now saddled with a 1,300 Hz modulation. When the aircraft passes through the cone of the middle marker, the second frequency detector within the Nav receiver is activated and illuminates an amber "M" accompanied by an audible frequency spelling "M" in Morse code.

Airports incorporating an inner marker use the same principle as the others. The 75 MHz carrier is delivered skyward, but this time a 3,000 Hz signal. Now the third frequency detector within the receiver is active and switches on a white light labeled "A" for azimuth as well as giving audible accompaniment with a Morse code "A."

The lateral mode of ILS is called the "localizer." This is also a radio transmitter with a series of loop antennas located 1,000 feet off the nonapproach end of the runway. An operating frequency of 108 to 111.95 MHz will permit the use of a standard navigation receiver as well as the aircraft very high omni directional range (VOR) antenna. Two radio frequency (RF) transmitters will supply a five degree wide beam with 150 Hz modulation to the right of the runway centerline and a 90 Hz modulation to the left.

As long as the aircraft is on the centerline of the runway, both signals are of an equal strength. Should the aircraft start to drift to the right, the 150 Hz signal predominates and is detected by the navigation receiver which queues the pilot. VHF transmission is considered line of sight and may be impeded by such things as taxiing aircraft and snow banks. The range of localizers is limited to a maximum of about 30 miles and is affected by terrain but still has an accuracy within 25 feet at the runway threshold.

Vertical guidance is provided by the glideslope and in principle is similar to the localizer. A glideslope transmitter is located about 1,000 feet down the runway from the landing end and displaced about 600 feet from the centerline. This device includes a UHF transmitter with an operating frequency range of 328.6 to 335.4 MHz, and like the glideslope, two modulated signals are transmitted, one at 90 Hz above the glidepath and another at 150 Hz below the glidepath. As long as the aircraft descends on the 2 1/2 to 3 degree vertically inclined beam, neither the 150 Hz or the 90 Hz predominates.

The range of the glideslope is usually less than the localizer (with 15 miles being the maximum), but still maintains an accuracy of 7 feet at 100 feet. These UHF frequencies are captured by a dedicated glideslope antenna frequently mounted in the nose of the aircraft. Although UHF frequencies are used, many newer navigation receivers can sort out and utilize the glideslope frequencies.

Pilots are also not required to tune a glideslope and a localizer. The FAA has assigned specific glideslope frequencies to specific localizer frequencies. These frequency relationships are published in their entirety in the "Airman's Information Manual" and the selection of a specific localizer commands the auto tuning of the corresponding glideslope channel.

Microwave landing systems (MLS) were developed to overcome some of the weaknesses in the localizer — glideslope systems such as obstruction of the signals by terrain or manmade obstacles. As a joint venture between the Department of Transportation, Department of Defense, and NASA, the MLS was designed to provide improved performance over the present aging ILS and was to have been fully implemented by 1998. One of the key features of MLS is the ability to deviate from the standard, straight-in approach thereby providing a precision approach to airports that are surrounded by mountains or have other obstacles in the flight path.

The MLS presently in use is a time reference scanning beam (TRSB) MLS and operates on a frequency range of 3 to 30 Ghz. By using UHF frequencies, a wider choice of ground locations are available with less concern for signal reflection from buildings or hills. Like ILS, the MLS uses two transmitters; one supplying a vertical beam and the other a lateral or azimuth beam. The precise timing of the beams provides a very accurate position to the onboard MLS receiver which in turn processes the information and determines the aircraft relative position to the airport on an internal grid.

Distance information is obtained from existing distance measuring equipment (DME). This is then provided to the flight crew on conventional flight deck displays. Equipment on board the aircraft for MLS operation includes a receiver, control panel, and external antenna.

Localizer antenna
Glideslope antenna
MLS antennas installed on a Hawker

Global positioning systems (GPS) hold the key to the future of precision instrument landings. In fact, the 1994 Federal Radio Navigation Plan calls for the implementation of GPS for Category 1 approaches this year and Category 2 /3 is scheduled for 2001. This plan also includes the phase out of the ILS and MLS we currently use by the year 2010. GPS approaches can be used as overlays for present ILS or can even stand alone. This type of approach is made by following predetermined waypoints. Differential GPS (DGPS), in addition to providing a latitude, longitude position accuracy of .01 minutes, will also exceed the current height veracity.

Several components in the flight deck will enable the pilot to observe ILS data. In general aviation, a course deviation indicator will include two cross hair pointers; one reflecting the glideslope and the other displaying the localizer. When the two pointers are aligned over the aircraft symbol the aircraft is "ON the Beam."

Another common display is the attitude directional indicator (ADI). On more complex aircraft this instrument can be fitted with a glideslope pointer and a runway symbol (usually at the bottom of the display). The glideslope pointer will move above or below the fixed aircraft symbol alerting the pilot to the position of the center of the glide path. If the pointer is above the aircraft symbol, the pilot would have to pull up to get on the center of the beam. The runway icon moves left and right of the aircraft symbol advising the pilot of position relative to runway centerline. This information is provided by the navigation receiver.

Anytime an ILS frequency is dialed in the Nav receiver commands, the ILS symbols to appear. These selected frequencies are 108 to 111.95 MHz and only odd numbered ones. Frequencies other than odd numbers will cause the ILS symbols to bias out of view (BOV). In many installations, the left and right moving runway symbol may also have the ability to rise. The "rising runway" is controlled by the radio altimeter.

As the aircraft gets within about 100 feet above ground level (AGL), the runway symbol will begin moving toward the fixed aircraft symbol. At the point where the aircraft wheels touch the runway, the runway symbol contacts the fixed aircraft symbol on the cockpit display. Dual radio altimeters are among the numerous requirements for CAT 3 landings. Many aircraft today have the ability to couple the ILS components to the aircraft auto flight system and when coupled with auto throttles, auto brakes, and auto spoilers, the flight crew is just along for the ride.