The Aircraft Landing Operation
An aircraft approaching a runway in a landing operation may be visualized as a sequence of operations involving a transient body suspended in a three-dimensional grid that is approaching a fixed two-dimensional grid. While in the air, the aircraft can be considered as a point mass in a three-dimensional orthogonal coordinate system in which it may have translation along three coordinate directions and rotation about three axes. If the three coordinate axes are aligned horizontal, vertical, and parallel to the end of the runway, the directions of motion can be described as lateral, vertical, and forward. The rotations are normally called pitch, yaw, and roll, for the horizontal, vertical, and parallel axes, respectively. During a landing operation, pilots must control and coordinate all six degrees of freedom of the aircraft so as to bring the aircraft into coincidence with the desired approach or reference path to the touchdown point on the runway.
In order to do this, pilots need translation information regarding the aircraft's alignment, height, and distan regarding pitch, yaw, and roll, and information concerning the rate of descent and the rate of closure with the desired path.
Pilots must know where their aircraft is with respect to lateral displacement from the centerline of the runway. Most runways are from 75 to 200 ft wide and from 3000 to 12,000 ft long. Thus any runway is a long narrow ribbon when first seen from several thousand feet above. The predominant alignment guidance comes from longitudinal lines that constitute the centerline and edges of the runway. All techniques, such as painting, lighting, or surface treatment that develop contrast and emphasize these linear elements are helpful in providing alignment information.
The estimation of the height above ground from visual cues is one of the most difficult judgments for pilots. It is simply not possible to provide good height information from an approach lighting system.
Consequently the best source of height information is the instrumentation in the aircraft. However, use of these instruments often requires the availability of precision ground or satellite based navigation technologies. Many airports have no such technologies, and at others only provide lateral approach guidance to certain runways. Consequently two types of ground-based visual aids defining the desired
glide path have been developed. These are known as the visual approach slope indicator (VASI) and the precision approach path indicator (PAPI) which are discussed later in this chapter. Several parameters influence how much a pilot can see on the ground. One of these is the cockpit cutoff angle. This is the anglebetween the longitudinal axis of the fuselage and an inclined plane below which the view of the pilot is blocked by some part of the aircraft,
Approach lighting systems (ALS) are designed specifically to provide guidance for aircraft approaching a particular runway under nighttime or other low-visibility conditions. While under nighttime conditions it may be possible to view approach lighting systems from several miles away, under other low-visibility conditions, such as fog, even the most intense ALS systems may only be visible from as little as 2500 ft from the runway threshold.
Studies of the visibility in fog have shown that for a visual range of 2000 to 2500 ft it would be desirable to have as much as 200,000 candelas (cd) available in the outermost approach lights where the slant range is relatively long. Under these same conditions the optimum intensity of the approach lights near the threshold should be on the order of 100 to 500 cd. A transition in the intensity of the light that is directed toward the pilot is highly desirable in order to provide the best visibility at the greatest possible range and to avoid glare and the loss of contrast sensitivity and visual acuity at short range.
The configurations which have been adopted are the Calvert system shown in Fig. 8-3 which has been widely used in Europe and other parts of the world, the ICAO category II and category III system shown in Fig. 8-4, and the four system configurations which have been adopted by the FAA in the United States shown in Fig. 8-5. The FAA publishes criteria for the establishment of the approach lighting systems and other navigation facilities at airports. Approach lights are normally mounted on frangible pedestals of varying height to improve the perspective of the pilot in approaching a runway.
The first approach lighting system was known as the Calvert system. In this system, developed by E. S. Calvert in Great Britain in 1949, includes a line of single bulb lights spaced on 100-ft centers along the extended runway centerline and six transverse crossbars of lights of variable length spaced on 500-ft centers, for a total length of 3000 ft.
For operations in very poor visibility, ICAO has certified a modification of the Calvert system, known as the ICAO category II system.
The variation calls for a higher lighting intensity to the inner 300 m of the system closest to the runway threshold. The category II and category III system adopted by ICAO consists of two lines of red bars on each side of the runway centerline and a single line of white bars on the runway centerline both at 30 m intervals and both extending out 300 m from the runway threshold. In addition, there are two longer bars of white light at a distance of 150 and 300 m from the runway threshold, and a long threshold bar of green light at the runway threshold. ICAO also recommends that the longer bars of white light also be placed at distances of 450, 500, and 750 m from the runway threshold if the runway centerline lights extend out that distance.The ALSs currently certified by the FAA for installation in the United States consist of a high-intensity ALS with sequenced flashing lights (ALSF-2), which is required for category II and category III precision approaches, a high-intensity approach lighting system with sequenced flashing lights (ALSF-1), and three medium-intensity ALSs (MALSR, MALS, MALSF).
In each of these systems there is a long transverse crossbar located 1000 ft from the runway threshold to indicate the distance from the runway threshold. In these systems roll guidance is provided by crossbars of white light 14 ft in length, placed at either 100- or 200-ft centers on the extended runway centerline. The 14-ft crossbars consist of closely spaced five-bulb white lights to give the effect of a continuous bar of light.
The high-intensity ALS is 2400 ft long (some are 3000 ft long) with various patterns of light located symmetrically about the extended runway centerline and a series of sequenced high-intensity flashing lights located every 100 ft on the extended runway centerline for the outermost 1400 ft. In the high-intensity ALSs the 14-ft crossbars of five-bulb white light are placed at 100-ft intervals and in the medium intensity ALSs these crossbars of white light are placed at 200-ft intervals
both for a distance of 2400 ft from the runway threshold on the extended runway centerline. The high-intensity ALSs have a long crossbar of green lights at the edge of the runway threshold. The ALSF-2 system, shown in Fig. 8-5a, has two additional crossbars consisting of three-bulb white light crossbars which are placed symmetrically about the runway centerline at a distance of 500 ft from the runway threshold and two additional three-bulb red light crossbars are placed symmetrically about the extended runway centerline at 100-ft intervals for the inner 1000 ft to delineate the edges of the runway surface.
The ALSF-1 system, shown in Fig. 8-5b, has two additional crossbars consisting of five-bulb red light crossbars which are placed symmetrically about the runway centerline at a distance of 100 ft from the runway threshold to delineate the edge of the runway and two additional three-bulb red light crossbars placed symmetrically about the extended runway centerline at 200 ft from the runway threshold.
The MALSR system, shown in Fig. 8-5c, is a 2400-ft mediumintensity ALS with runway alignment indicator lights (RAILs). The inner 1000 ft of the MALSR is the MALS portion of the system and the outer 1400 ft is the RAIL portion of the system. The system has sequential flashing lights for the outer 1000 ft of the system. It is recommended for category I precision approaches. The simplified short approach lighting system (SSALR) has the same configuration as the MALSR system.
At smaller airports where precision approaches are not required, a medium ALS with sequential flashers (MALSF) or with sequenced flashers (MALS) is adequate. The system is only 1400 ft long compared to a length of 2400 ft for a precision approach system. It is therefore much more economical, an important factor at small airports. the runway alignment indicator lights and these are only provided in the outermost 400 ft of the 1400-ft system to improve pilot recognition of the runway approach in areas where there are distracting lights in the vicinity of the airport. The MALS system does not have the runway alignment indicator lights or the sequential flashers.
At international airports in the United States, the 2400-ft ALSs are often extended to a distance of 3000 ft to conform to international specifications.
Sequenced-flashing high-intensity lights are available for airport use and are installed as supplements to the standard approach lighting system at those airports where very low visibilities occur frequently.
These lights operate from the stored energy in a capacitor which is discharged through the lamp in approximately 5 ms and may develop as much as 30 million cd of light. They are mounted in the same pedestals as the light bars. The lights are sequence-fired, beginning with the unit farthest from the runway. The complete cycle is repeated every 2 s. This results in a brilliant ball of light continuously moving toward the runway. Since the very bright light can interfere with the eye adaptation of the pilot, condenser discharge lamps are usually omitted in the 1000 ft of the approach lighting system nearest the runway.
Visual Approach Slope Aids
Visual approach slope aids are lighting systems designed to provide a measure of vertical guidance to aircraft approaching a particular runway. The principle of these aids is to provide color-based identification to the pilot indicating their variation from a desired altitude and descent rate while on approach. The two most common visual approach slope aids are the visual approach slope indicator (VASI), and the precision approach path indicator (PAPI).
Visual Approach Slope Indicator
The visual approach slope indicator (VASI) is a system of lights which acts as an aid in defining the desired glide path in relatively good weather conditions. VASI lighting intensities are designed to be visible from 3 to 5 mi during the day and up to 20 mi at night.
There are a number of different VASI configurations depending on the desired visual range, the type of aircraft, and whether large wide bodied aircraft will be using the runway. Each group of lights transverse to the direction of the runway is referred to as a bar. The downwind bar is typically located between 125 and 800 ft from the runway threshold, each subsequent bar is located between 500 and 1000 ft from the previous bar. A bar is made up of one, two, or three light units, referred to as boxes. The basic VASI-2 system, illustrated in Fig. 8-6, is a two-bar system consisting of four boxes. The bar that is nearest to the runway threshold is referred to as the downwind bar, and the bar that is farthest from the runway threshold is referred to as the upwind bar. As illustrated in Fig. 8-6, if pilots are on the proper glide path, the downwind bar appears white and the upwind bar appears red; if pilots are too low, both bars appear red; and if they are too high both bars appear white.