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Towards the end of the flight, aircraft enter the TMA where ATC guides them from cruising altitudes to the entry points of the Approach Sector, which are typically between FL50 and FL150. Ideally, aircraft should enter the Approach Sector in a sequence properly spaced in time. The controllers of the Approach Sector are then responsible for guiding the aircraft towards the runway for landing. The tasks of ATC in the Approach Sectors include:
1) Maintain safe separation between aircraft. This is the most important requirement for safety, in any sector and during all parts of the flight. Aircraft must always maintain a minimum level of separation. A conflict between two aircraft is defined as the situation of loss of minimum safe separation between them. Safe separation is defined by a protected zone centered around each aircraft. The level of accepted minimum separation can vary with the density of the traffic and the region of the airspace. A largely accepted shape of the protected zone is defined by a vertical cylinder, centered on the aircraft with radius 5nmi and height 2000ft (i.e. aircraft which do not have 5nmi of horizontal separation must have 1000ft of vertical separation).
2) Take aircraft from entry altitude down to intercept the localizer. Once aircraft have entered the Approach Sector, ATC must guide them from the entry altitude (FL50 to FL150) to FL15. This is the altitude at which they can intercept the localizer, i.e. the radio beacons which will guide them onto the runway. The point at which the aircraft will actually start the descent towards the runway is an important variable which has to be carefully chosen since it can affect the rest of the maneuver and the coordination with other aircraft. The reason is that aircraft fly following pre-specified speed profiles which depend on the altitude; they fly faster at high altitudes and slower at low altitudes. This implies that aircraft, flying at lower altitudes, are slower in joining the landing queue.
3) Sequence aircraft towards the runway. The air traffic controllers must direct the aircraft towards the runway in a properly spaced queue. This is done by adjusting the way-points (corners) of a standard approach route (STAR) — see Figure 1. Typically the route is composed of four legs. During their descent, aircraft are first aligned, on one of the two sides of the runway, in the direction of the runway but with opposite heading. This leg is called the left/right downwind leg, since aircraft are expected to land against the wind. Aircraft then perform a turn of approximately 90�, to approach the localizer. This second segment is called the base leg. Aircraft perform an additional turn in order to intercept the plane of the localizer with an angle of incidence of approximately 30� . The reason is that 30� is a suitable angle for pilots to perform the final turn in the direction of the runway as soon as possible when the localizer has been intercepted. It is required that aircraft intercept the localizer plane at least 5 nmi from the beginning of the runway and at an altitude of 1000 − 1500 ft, so that they can follow a 3� − 5� glide path to the runway.
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Monte Carlo Optimization for Conflict Resolution in Air Traffic Control(8)
