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Since the early 2000s, London airports have regularly reported operational compliance with the CDO definition on a monthly and annual basis. Under the current CDO definition, London airports have reached and maintained high compliance rates.

In 2017, preliminary research performed by CAA ERCD identified that the existing CDO definition was not sufficiently sensitive to provide an effective noise measure.

These insights led to the development of height-based criteria for a low noise arrival metric that would incentivise increased initial/intermediate descent angles, but not to the extent that would necessitate any changes in speed control or aircraft configuration. To better incentivise low noise arrival performance, two height boundary conditions are proposed as illustrated below, creating three height zones or low noise categories.

Testing indicates that the criteria would rate 45-50% of arrivals in the optimum category, with around 15-20% of arrivals in the second category and 35-40% in the lowest category.
The report makes recommendations for monitoring systems to be developed to implement the proposed low noise arrival metric definitions, and be appropriately validated.
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