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Navigational landing aids are available at major airports and also at more minor well-equipped locations. The main navigational aid for pilots in landing an aircraft is the Instrument Landing System (ILS), which is available on one or several runways at large airports. ILS enables pilots to make precision landings, even in conditions of low visibility. For each runway equipped with ILS, two beams are provided from stations installed on the ground to the cockpit. One beam provides the localiser which the pilot intercepts to line up and land in the precise direction. The second beam provides the glide slope which allows pilots to descend at the optimum rate. At an airport or on a runway where ILS is not available, a VOR / DME approach may be possible (see the introductory notes to Unit 2 for an explanation). In this case the pilot will use the VOR to line up in the precise direction required but will have to calculate the best rate of descent through the information provided by the DМЕ (which informs the pilot of the distance remaining to be covered).
Where no such navigational aids exist (at small aerodromes, for example) or when navigational aids are out of service, a visual approach will be required. In a visual approach, as the name suggests, pilots use their own judgment to line up correctly and approach at a suitable role of descent. This is in any case the normal procedure for VFR traffic and it is part of every pilot's initial basic training.
The basic mechanics of landing a modern jet aircraft are similar whichever of the three types of approach a pilot may be conducting. The pilot reduces airspeed sufficiently and extends the landing gear of the aircraft (also called undercarriage). The next stage is to flare (raise the nose slightly) just prior to landing. This important movement ensures that it is the wheels of the main landing gear which touch the runway first and take most of the force of the landing. Then the pilot will gradually lower the nose gear and apply reverse thrust (a surge of power backwards from the engines which significantly slows the aircraft). Afterwards braking action can be safely applied. Applying the brakes directly without using reverse thrust is possible but not always desirable as it can significantly increase the risks of skidding, light aircraft are only equipped to apply the brakes to slow down, but they are able to stop a lot easier and quicker than a large passenger airplane.
Major airports and even small aerodromes can be congested at times and aircraft may need to enter a holding pattern while they wait for authorization to land. The normal arrangement is that they circle at different altitudes (to ensure safe separation) until the controller gives them clearance to come in and land. This arrangement is known as a stack (in a diagrammatical representation the aircraft seem to be 'on top of each other). The trend nowadays is to try and avoid stacks for commercial traffic if at all possible. Often aircraft will not take off until they can be provided with a direct route to their destination. Safety concerns have been expressed by some experts about regularly having large aircraft circling over major cities, though they have not been able to prove that there is a real danger. Increasingly it is the question of additional aircraft fuel consumption (which is expensive for the airlines) and emissions (which are of concern to environmentalists - see Unit 10) which influence the decision making process.
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