Our glossary contains frequently used terms and acronyms together with their explanations.

Approach procedures

Continuous descent operations (CDO)
Continuous descent operations (also called continuous descent approach - CDA) refer to an approach procedure where the aircraft descends with minimal engine power (ideally an idle power setting) and largely avoids level flight segments. This saves fuel and reduces CO2 emissions. In some areas, a reduction in noise may occur.

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The pilot uses the aircraft controls to perform the continuous descent approach. For CDO, either specially designed arrival routes with altitude instructions and headings are used or the air traffic controller gives the pilot distance information and, if necessary, rates of descent. At airports with an instrument landing system (ILS), the continuous descent approach ends once the aircraft intercepts the beam of the ILS.

Ideally, the CDO procedure involves a continuous descent rate of approximately 300ft/NM (90 m high by 1.85 km long, corresponding to 3°). As both the airspeed – depending on the aircraft type – and the flight level at which the descent commences can be different, the length of the optimal arrival route varies in each case.

Air traffic controllers have more work with a CDO procedure than a normal approach. This is because with CDO the pilot has to fly at speeds that each aircraft type flies when “gliding” (i.e. aircraft descending at idle power setting). The result is a specific rate of descent for each aircraft type.

A further challenge arises when traffic coming from different directions has to be merged on a joint final approach. Poor weather conditions may also impede continuous descent operations. 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 continuous descent is therefore only possible if the succeeding traffic is unlikely to be delayed given the low traffic load, if there are no safety concerns (e.g. separation) and if there are no weather-related restrictions.

The use of continuous descent operations may negatively impact capacity at large airports, such as Frankfurt, particularly in peak traffic periods.