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Procedures and operations
Throughout the world, air navigation services experts, pilots and engineers are working on developing new technologies and procedures to reduce aircraft noise and, thus, make flying more environmentally friendly. Ideally, aircraft take off and land against the wind. However, under certain conditions, a clearly defined, maximum tail wind component is permitted. The runway that is currently being used for take-offs and landings is called the “runway-in-use”. Runways are named after their magnetic orientation. The surface wind is, however, not the only criterion for determining the runway-in-use. Other criteria include other low level winds as well as pilot reports about conditions in the approach, amongst other factors. It is solely up to DFS staff to determine the runway-in-use. Additional factors such as local operational regulations, which address noise abatement concerns, for example, also influence the decision. Continuous Descent Approach (CDA) Besides arrival and departure routes designed to optimally deal with noise, another way to reduce the impact of noise on the ground is to use a continuous descent approach (CDA). A positive side-effect of this is that it requires less fuel. To perform a continuous descent approach, an aircraft at a specified altitude descends at idle or near-idle power setting and glides smoothly towards the runway. The optimal starting point is cruising level as a long descent offers most benefits. This variant is extremely difficult to implement in practice as any such descent commencing at higher levels can lead the aircraft through flight levels assigned to other aircraft. This makes a significant amount of coordination necessary. Ideally, the CDA procedure entails a continuous descent of approximately 300ft/NM (100m high by 900m long, corresponding to 3°). As both the airspeed – depending on the aircraft type – and the level at which the descent commences can be different, the length of the optimal arrival route varies in each case. Arrival routes are designed with noise-abatement criteria in mind and lead over sparsely populated areas if possible. In densely populated regions, this is only possible to a certain degree. Due to the differences between the criteria for an optimal CDA on the one hand, and limiting requirements on the other, the resulting descent profiles for CDA procedures often vary greatly. Thus, descents at angles much less than 3° are called CDA procedures by other ANSPs. Other ANSPs or airports also refer to approaches with deviating inclination angles as CDAs. Until now, there is no general agreement on what qualifies an approach as a CDA. It is more work for an air traffic controller to handle a CDA than a normal approach. This is because in a CDA the pilot has to fly at speeds that each aircraft type flies when gliding (i.e. aircraft descending at idle power setting), and the decreasing flight level results from the related rate of descent. A further challenge occurs when traffic flowing from different directions have to be guided on a joint final approach. Furthermore, bad weather conditions may render a CDA approach impossible. For example, extreme head winds make it impossible to calculate the “normal” length of the arrival route with idle power and require the calculations to be changed. Thunderstorm cells or areas where turbulence has been reported not to mention wind shear are to be avoided if possible. If not, the aircraft has to fly through them at a different speed. A CDA is only possible if the succeeding traffic is unlikely to be delayed given the traffic density, if there are no safety concerns (e.g. separation) and if there are no weather-related restrictions. Low drag/Low power The flight procedure low-drag/low-power means that arriving aircraft stay in the “clean configuration” as long as possible. In other words, the landing gear, flaps and slats are extended as late as possible. This aerodynamic configuration only calls for a little thrust because the more landing aids are deployed, the louder an aircraft is.
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