Continuous descent approach (CDA) or optimized profile descent (OPD) is a method by which aircraft approach airports prior to landing. It is designed to reduce fuel consumption and noise compared to certain conventional approaches and involves maintaining a constant three degree descent angle during landing, until meeting the instrument landing system (ILS). Instead of approaching an airport in a stair-step fashion, throttling down and requesting permission to descend to each new (lower) altitude, OPD allows for a smooth, constant-angle descent to landing.
A continuous descent approach starts ideally from the top of descent, i.e. at cruise altitude, and allows the aircraft flying its individual optimal vertical profile down to runway threshold. Some airports apply constraints to this individual optimal profile such as imposing a constant descent angle (e.g. 3 degree) or starting the CDA only after having left the holding pattern (e.g. London, at about 6000 feet) due to Air Traffic Management constraints.
Read more about Continuous Descent Approach: Studies of CDA, United States, Spain
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