CHAPTER 4 CHAPTER 4 Airport Influences on Aircraft Performance
4.6 Automatic Landing
The original purpose in developing automatic landings was to increase regularity and save the costs of diversions. There are, however, many other advantages in the form of pilot workload, control of touchdown dispersion, softer landings, and the maintenance of flight path even in difficult wind-shear conditions. Experience with the equipment is showing increased reliability and a greater harmony between pilots and autopilots. It appears that the monitoring and takeover functions of the pilot are sufficiently undemanding for fail- passive equipment to be used even for Category IIIA. Most of the benefits therefore accrue
to the airline, whereas only full use of the system can justify the expense of the ground equipment, particularly for Category III. Also, different regulations must be brought into effect; the equipped aircraft must be allowed to bypass other stacked aircraft, and it must be provided with positive control on the ground in minimum visibility.
Safe automatic landing operations depend on the same factors that have been considered for visual landings, but with much greater emphasis on the concept of decision height. This depends on the method of operation, the specification of the equipment, and the runway visual range (RVR), as shown in Table 4.5, as well as on the obstacle- clearance criteria. The latter must take account of the demonstrated height loss during a missed approach.
The required RVR is a function of the pilot’s angle-of-view cutoff, the intensity and beam spread of the lighting system, the vertical and horizontal structure of the fog, and the location of the pilot’s eyes relative to the aids and his or her intrinsic visual reference needs. It must be measured at least at three positions along the runway. The touchdown- zone reading must be passed to the pilot within 15 seconds of reading, followed by the other readings if they are lower than the first and less than 2,625 feet (800 m).
Any proposal to operate automatic landings must show the feasibility of the proposed minima for each runway, including the adequacy of the facilities and the obstacle-clearance capability. An airport wishing to declare a runway as suitable to receive automatic landings must consider
• Obstacle clearances
• Glidepath angles
• Terrain on approach [It should be essentially flat for 1,000 feet (300 m) before threshold over a 200-foot-wide (60-m-wide) strip.]
• Runway length, width, and profile
• Conformity and integrity of the instrument landing system (ILS)
• The visual aids and their integrity
• The level of air traffic control (ATC) equipment and its meteorological monitoring requirements
The most critical aspect of performance is the ability to climb after a missed approach has been declared. It must be possible to demonstrate adequate climb performance in each of the three following flight conditions:
1. Positive net gradient at 1,500 feet (457 m) above the airfield in the cruise configuration with one engine out
2. A gross gradient not less than 3.2 percent at the airfield altitude with all engines at maximum takeoff power in the final landing configuration [This is to allow a safe overshoot (balked landing).]
3. A minimum gross climb gradient at the airfield altitude with the critical engine out and all others at maximum takeoff power in the final landing configuration but with the gear up (The gradients should be not less than 2.1 percent for twin-engine aircraft, 2.4 percent for three-engine aircraft, and 2.7 percent for four-engine
aircraft.)
Landing WAT charts indicate the maximum landing weights at which these gradients
can be achieved as functions of altitude and temperature. These performance criteria will allow safe operation in the vicinity of airfields only if used in conjunction with minimum descent altitudes and taking into account the handling of each aircraft type, the ground aids available, and the local terrain conditions. In setting minimum descent altitudes and associated decision heights, it is important to realize that there is an inevitable height loss between the decision to declare a missed approach and the establishment of a positive climb gradient, even with the demonstrated performance quoted earlier. This is due to delays in responding to pitch and power inputs and to the initial downward momentum of the aircraft.
The protected-surface funnel shown in Figure 4.9 includes surfaces to protect in the missed-approach situation. These surfaces are designed to give clearance below the climb paths guaranteed by the landing WAT limits to the remote-risk level.
FIGURE 4.9 Obstacle-assessment surfaces. W and X are approach surfaces; CDE is the footprint; Y is a transitional surface; and Z is the missed-approach surface.
The approach path is governed by the need to maintain clearance over obstacles both on the expected flight path and in the general vicinity of the airfield. The former are protected by the surfaces in Figure 4.9 or by those in Annex 14, whereas the latter (and any obstacles that break the protected surfaces) are cleared by the imposition of margins over the declared obstacle-clearance altitudes (OCAs) or obstacle-clearance heights (OCHs). The derivation of these is fully explained in ICAO PAN-OPS (ICAO 2006), and there may be variations in national legislation.
In essence, reductions in noise and fuel use are obtained by avoiding until the latest time the drag and hence the thrust that come with the selection of full landing flaps; the time at which the speed is stabilized, thus, is also delayed so as to maintain a safe margin over the stall speed. Lufthansa first developed the technique to decrease noise over a sensitive area 8 miles (13 km) out on the approach to Frankfurt. A more general version of the procedure was later adopted by the International Air Transport Association (IATA), and it is now used widely in the continuous-descent approach (CDA) procedure favored for fuel and noise minimization in air traffic management (ATM) procedures.
Studies in Europe and the United States in the 1990s, using research aircraft with advanced avionics showed that precise four-dimensional (3D position and time) flight paths
were feasible using control parameters compatible with ATC processes. This led to the development of CDA procedures for many major airports. In ATM terms, the arrival rate into the terminal area (TMA) is matched to the acceptance rate of the airport, thus minimizing radar vectors and path stretching. This should allow the use of a near-idle-thrust descent from cruise altitude and minimized maneuvering time at low altitudes.
There are some disadvantages; for example, it is not possible to operate in this way without affecting decision heights. ATC also must restrict the acceptable speed range in the early stages of the approach, and either all operations must adopt the technique or separations on the approach must increase. This can limit the proportion or even the acceptance of all turboprop aircraft at busy airports. Current ATM research programs are considering the impact of these limitations and the possibility of having segregated traffic flows based on aircraft categories. This is not based on aircraft performance but influenced by performance, and the motivation for studies is the preservation of runway movement capacity.
A final consideration in approach performance is the problem of wake turbulence. It is necessary to pay attention to the order in which aircraft of different weights are allowed to approach or take off so that smaller aircraft do not follow larger aircraft when runway capacity must be maximized. At Heathrow, the arrival runway capacity was found to vary from 34.7 to 31.8 movements per hour as the percentage of heavy aircraft varied from 10 to 50 percent. This has not improved with the advent of new large aircraft, such as the Airbus A380, and the topic is addressed in Section 4.8.