As the demand for air traffic rebounds and airspace becomes saturated once again, we need to balance the environmental impacts of aviation with the many benefits that air travel brings. So the development of a new noise metric designed to further incentivise quieter arrivals should be music to the ears of many. This is the story of the development of the new Low Noise Arrivals Metric, and our part in it.
How quieter arrivals are accomplished today.
Before we jump in feet first, it’s important to understand the basics about noise abatement procedures. These are low noise operations that minimise the impact of aviation noise on communities living below arrival and departure flight paths. The responsibility for following these procedures falls to pilots, air traffic controllers, and airport operators, who are accountable for the way an aircraft is flown and for monitoring and reporting adherence to the procedures. At many airports compliance rates are publicly reported and airlines are held accountable if their performance isn’t up to scratch, thus incentivising good behaviours.
As early as 1978, studies have identified best practice measures to reduce arrival noise including the likes of Continuous Descent Operations (CDO), which are designed to keep aircraft higher for longer during their approach. CDO enables continuous descent to the runway in a low drag configuration with low power and low thrust, avoiding the need for extended periods of level flight. As such, aircraft are kept higher at most stages of the descent compared to a conventional step-down approach, reducing noise, fuel burn and emissions.
The Arrivals Code of Practice (published by Sustainable Aviation, an industry body) measures an arrival as a CDO if it contains, below an altitude of 6,000ft: no level flight, or one/multiple phase(s) of level flight not longer than 2.5 nautical miles (NM).
The angle of descent affects noise on the ground, and today the optimum CDO profile is defined in the Code of Practice as having a 3 degree descent angle. If an aircraft flies an extended level segment on descent (ie longer than 2.5NM) it is categorised as a non-CDO flight, requiring additional engine power to maintain level flight at a constant speed and thus creating more noise.
CDO is the primary method of reducing noise experienced on the ground beneath arriving aircraft, and today, compliance rates are very high. This means that there is no longer an incentive to further reduce noise from arriving aircraft, leading to a review of the current CDO definition.