Instrument Landing System (ILS)

Instrument landing system (ILS): An electronic system that provides both horizontal and vertical guidance to a specific runway, used to execute a precision instrument approach procedure. The ILS system provides both course and altitude guidance to a specific runway. The ILS system is used to execute a precision instrument approach procedure or precision approach. The system consists of the following components:

1. A localizer provides horizontal (left/right) guidance along the extended centerline of the runway.
2. A glide slope provides vertical (up/down) guidance toward the runway touchdown point, usually at a 3° slope.
3. Marker beacons provide range information along the approach path.
4. Approach lights assist in the transition from instrument to visual flight.

Source: FAA-H-8083-15B, Instrument Flying Handbook, pg. 9-35

• Localizer. The localizer (LOC) ground antenna array is located on the extended centerline of the instrument runway of an airport, remote enough from the opposite (approach) end of the runway to prevent it from being a collision hazard. This unit radiates a field pattern, which develops a course down the centerline of the runway toward the middle markers (MMs) and outer markers (OMs), and a similar course along the runway centerline in the opposite direction. These are called the front and back courses, respectively. The localizer provides course guidance, transmitted at 108.1 to 111.95 MHz (odd tenths only), throughout the descent path to the runway threshold from a distance of 18 NM from the antenna to an altitude of 4,500 feet above the elevation of the antenna site.
• The localizer course is very narrow, normally 5°. This results in high needle sensitivity. With this course width, a full-scale deflection shows when the aircraft is 2.5° to either side of the centerline. This sensitivity permits accurate orientation to the landing runway. With no more than one-quarter scale deflection maintained, the aircraft will be aligned with the runway.
• Glide Slope. Glide slope (GS) describes the systems that generate, receive, and indicate the ground facility radiation pattern. The glide path is the straight, sloped line the aircraft should fly in its descent from where the glide slope intersects the altitude used for approaching the FAF, to the runway touchdown zone.
• The glide-slope equipment is housed in a building approximately 750 to 1,250 feet down the runway from the approach end of the runway, and between 400 and 600 feet to one side of the centerline.
• The course projected by the glide-slope equipment is essentially the same as would be generated by a localizer operating on its side. The glide-slope projection angle is normally adjusted to 2.5° to 3.5° above horizontal, so it intersects the MM at about 200 feet and the OM at about 1,400 feet above the runway elevation. At locations where standard minimum obstruction clearance cannot be obtained with the normal maximum glide-slope angle, the glide-slope equipment is displaced farther from the approach end of the runway if the length of the runway permits; or, the glideslope angle may be increased up to 4°.
• Unlike the localizer, the glide-slope transmitter radiates signals only in the direction of the final approach on the front course. The system provides no vertical guidance for approaches on the back course. The glide path is normally 1.4° thick. At 10 NM from the point of touchdown, this represents a vertical distance of approximately 1,500 feet, narrowing to a few feet at touchdown.
• Marker Beacons. Two VHF marker beacons, outer and middle, are normally used in the ILS system. A third beacon, the inner, is used where Category II operations are certified. A marker beacon may also be installed to indicate the FAF on the ILS back course.
• The OM is located on the localizer front course 4 to 7 miles from the airport to indicate a position at which an aircraft, at the appropriate altitude on the localizer course, will intercept the glide path The MM is located approximately 3,500 feet from the landing threshold on the centerline of the localizer front course at a position where the glide-slope centerline is about 200 feet above the touchdown zone elevation. The inner marker (IM), where installed, is located on the front course between the MM and the landing threshold. It indicates the point at which an aircraft is at the decision height on the glide path during a Category II ILS approach. The back-course marker, where installed, indicates the back-course FAF.
• Compass Locator. Compass locators are low-powered NDBs and are received and indicated by the ADF receiver. When used in conjunction with an ILS front course, the compass locator facilities are collocated with the outer and/or MM facilities. The coding identification of the outer locator consists of the first two letters of the three-letter identifier of the associated LOC. For example, the outer locator at Dallas/Love Field (DAL) is identified as “DA.” The middle locator at DAL is identified by the last two letters “AL.”
• Approach Lighting Systems (ALS). Normal approach and letdown on the ILS is divided into two distinct stages: the instrument approach stage using only radio guidance, and the visual stage, when visual contact with the ground runway environment is necessary for accuracy and safety. The most critical period of an instrument approach, particularly during low ceiling/visibility conditions, is the point at which the pilot must decide whether to land or execute a missed approach. As the runway threshold is approached, the visual glide path will separate into individual lights. At this point, the approach should be continued by reference to the runway touchdown zone markers. The ALS provides lights that will penetrate the atmosphere far enough from touchdown to give directional, distance, and glide path information for safe visual transition.

Source: FAA-H-8083-15B, Instrument Flying Handbook, pg. 9-35

Categories of instrument approach procedures allowed at airports equipped with the following types of instrument landing systems:

• ILS Category I: Provides for approach to a height above touchdown of not less than 200 feet, and with runway visual range of not less than 1,800 feet.
• ILS Category II: Provides for approach to a height above touchdown of not less than 100 feet, and with runway visual range of not less than 1,200 feet.
• ILS Category IIIA: Provides for approach without a decision height minimum and with runway visual range of not less than 700 feet.
• ILS Category IIIB: Provides for approach without a decision height minimum and with runway visual range of not less than 150 feet.
• ILS Category IIIC: Provides for approach without a decision height minimum and without runway visual range minimum.

Source: FAA-H-8083-15B, Instrument Flying Handbook, pg. G-9

The ILS and its components are subject to certain errors, which are listed below. Localizer and glide-slope signals are subject to the same type of bounce from hard objects as space waves.

1. Reflection. Surface vehicles and even other aircraft flying below 5,000 feet above ground level (AGL) may disturb the signal for aircraft on the approach.
2. False courses. In addition to the desired course, glideslope facilities inherently produce additional courses at higher vertical angles. The angle of the lowest of these false courses will occur at approximately 9°–12°. An aircraft flying the LOC/glide-slope course at a constant altitude would observe gyrations of both the glide-slope needle and glide-slope warning flag as the aircraft passed through the various false courses. Getting established on one of these false courses will result in either confusion (reversed glide-slope needle indications), or result in the need for a very high descent rate. However, if the approach is conducted at the altitudes specified on the appropriate approach chart, these false courses will not be encountered.

Source: FAA-H-8083-15B, Instrument Flying Handbook, pg. 7-27

The most common glide slope antenna reflects an image of its signal off the surface in front of the antenna to create the mirror image of the signal. A more expensive antenna does this electronically and is therefore easier to site. The former can be thrown off by surface snow and the latter by snow in front of the antenna.

Source: ILS Suppressed Image Antennas, page 253.

The final approach dimensions are specified [in the figure]. . . . Calculate the width of the area using the following formulae:

Perpendicular Width from RCL to the Edge of the Primary = 0.10752 (D - 200) + 700

Perpendicular Width from RCL to the Edge of the Transitional Sfc = 0.15152 (D - 200) + 1000

Where D = Distance (ft) from RWT measure along RCL.

Source: TERPS, Vol. 1, Ch. 9, ¶903

The area originates 200 feet from LTP or FTP and ends at the PFAF/Glide path intercept point (GPIP). The primary area consists of the “W” and “X” OCS, and the secondary area consists of the “Y” OCS. See figure 3-1.

Source: TERPS, Vol. 3, Ch. 3, ¶3.0

The obstacle evaluation method for descent of a is the application of a descending OCS below the . The vertical distance between the and the OCS is ROC; i.e., ROC = ( height) - (OCS height). The ROC decreases with distance from the final approach fix as the OCS and converge on the approach surface baseline (ASBL) height (see figure 1-2). The OCS slope and angle values are interdependent: OCS Slope = 102 ÷ angle; or angle = 102 ÷ OCS slope. This relationship is the standard that determines the ROC value since ROC = ( height) - (OCS height)

Source: TERPS, Vol. 1, Ch. 2, ¶203.a.

Source: Table 2-2a. Maximum GPA's, from TERPS, Vol. 3, Ch. 2, ¶2.5.

5.5.5.1 The width of the ILS/MLS/GBAS final approach protection area is much narrower than those of non-precision approaches. Descent on the glide path/MLS elevation angle must never be initiated until the aircraft is within the tracking tolerance of the localizer/azimuth.

5.5.5.2 The protection area assumes that the pilot does not normally deviate from the centre line more than half-scale deflection after being established on track. Thereafter the aircraft should adhere to the on-course, on-glide path/elevation angle position since a more than half course sector deflection or a more than half course fly-up deflection combined with other allowable system tolerances could place the aircraft in the vicinity of the edge or bottom of the protected airspace where loss of protection from obstacles can occur.

Source: ICAO Doc 8168, Vol 1, §4, Chapter 5, ¶5.5.5.