Better landing procedures slash jet noise and pollution

24 September 2010 by Tom Marshall

Planes could cut their noise pollution and fuel use with simple adjustments to how they approach airports to land, according to a recent study.

Plane landing

Trials at East Midlands Airport have shown that using an advanced Continuous Descent Approach (CDA) can cut noise levels by between three and six decibels - around a 50 per cent drop in acoustic energy.

The trial procedures, described in Meteorological Applications, also reduced fuel use, and therefore emissions, by around 10 per cent for the final airport approaches. This is equivalent to a 1 per cent cut in total fuel use for a typical short-haul flight.

It can take decades for a new aircraft design that's quieter and less polluting to move from the drawing board to active service with airlines. Finding better ways to use existing aircraft can bring similar benefits much more quickly.

The standard procedure for approaching an airport typically involves a number of sections of level flight. Since the behaviour of the aircraft is well-defined during each of these, they simplify air traffic controllers' task of merging closely-spaced flights.

However, they also require greater use of the aircraft's engines, flaps, and speed brakes compared to a CDA procedure. Each of these operations creates more noise, which can be heard on the ground, and burns more fuel.

Radar antenna array

MST radar antenna array at Aberystwyth

"An additional reduction in noise has a far simpler cause", says Dr David Hooper, the Project Scientist for NERC's Mesosphere-Stratosphere-Troposphere (MST) Radar Facility at Aberystwyth and one of the paper's authors.

"According to basic physics, the volume of a sound decreases as you get further away from the source," he adds. "This is because the acoustic energy gets spread across an increasingly large surface area. So if you can keep planes as high as possible for as long as possible, you'll get less noise."

The design of advanced CDA procedures relies on defining a number of fixed waypoints along the ground track approaching an airport. Aircraft rely on their computerised Flight Management Systems to fly over these within predetermined altitude and speed limits.

Since CDA reduces the scope for intervention by air traffic controllers, the procedures need to be suitable for a wide range of aircraft types and weather conditions. The scientists therefore carried out extensive computer simulations before flight trials could begin.

East Midlands Airport is located close to the major urban areas of Nottingham, Leicester, Loughborough and Derby. For the trial, the CDA waypoints were set in existing approach zones, which had been deliberately chosen to stop aircraft from flying over the most heavily-populated areas.

One of the inputs to the pre-trial simulations was a statistical wind model based on data from NERC's MST Radar at Aberystwyth. This is the only instrument in the UK capable of providing wind information at suitably short time intervals. Although Aberystwyth is 200km away from East Midlands Airport, the researchers were able to show that the statistical model was broadly applicable for any location in southern Britain.

They now hope similar procedures could be put to work at other airports and could help the aviation industry reduce its environmental impact. The approach has been tested at several airports, and is now routinely used at facilities like Los Angeles International Airport, partly as a result of earlier work by some of the study's authors.

But this is the first trial to assess the feasibility of CDA for widespread implementation in the UK by systematically studying meteorological effects. The researchers hope their work will add momentum to the technique's uptake.

During an earlier trial at Louisville International Airport in Kentucky, most pilots and air traffic controllers who took part liked the CDA procedures. In part they approved of the fact that once proper spacing between aircraft had been established, there was less need for controllers to issue frequent instructions to pilots.

It can be more difficult to implement CDA methods at busier airports which must deal with larger volumes of incoming aircraft, but there are techniques that can help manage this problem, and even at the biggest airports CDA might be useful at low-demand times of day.

People living around East Midlands Airport would have benefited from a noticeable drop in noise pollution during the trial, although they won't have experienced as dramatic an improvement as the 50 per cent drop in acoustic energy might suggest. The way our ears respond to loudness means reducing acoustic energy by half does not lead to a noise that seems half as loud. But to residents, the planes would still have sounded significantly softer.

The development of the procedure described in the paper was done under the Silent Aircraft Initiative, funded by the Cambridge-MIT Institute, a collaboration between the University of Cambridge and Massachusetts Institute of Technology.

'Meteorological influences on the design of advanced aircraft approach procedures for reduced environmental impacts' - Liling Ren, Tom G Reynolds, John-Paul B Clarke, David A Hooper, Graham A Parton and Anthony J Dore; Meteorological Applications (2010). Published online; DOI: 10.1002/met.206