To combat rising concerns of aircraft noise as a result of demand for faster air travel, NASA’s Armstrong Flight Research Center in California, US, has developed and demonstrated airframe optimisation technology that could reduce aircraft noise by up to 70%.

“The number one public complaint the Federal Aviation Administration (FAA) receives is about aircraft noise. NASA’s goal here was to reduce aircraft noise substantially in order to improve the quality of life for communities near airports. We are very confident that with the tested technologies we can substantially reduce total aircraft noise, and that could really make a lot of flights much quieter,” Mehdi Khorrami, aerospace scientist at NASA’s Langley Research Center, said in a press release.

The Armstrong Flight Research Center carried out a number of acoustic research measurement (ARM) flights to analyse and combat noise generated by non-propulsive parts of aircraft, including landing gear, cavities and wing flaps.

Air passing over the landing gear can create a lot of unwanted sound, which the team solved with porous fairings, allowing air to flow through the gear, rather than being forced around it. Substantial noise is also generated when landing gear descends from the airframe due to the large cavities left in the body of the aircraft. The researchers applied a series of chevrons near the front of the cavity featuring sound-absorbing foam, as well as a net stretching across the main cavity, which altered airflow and reduced wind sound.

Wing flap noise is also a significant contributing factor, as wind travels through gaps between the flap and the main body of the wing. The NASA team came up with the solution of an experimental adaptive compliant trailing edge (ACTE) wing flap, which increases aerodynamic efficiency by implementing a flexible seamless flap, eliminating gaps.

London’s Gatwick Airport in the UK utilises an independent Noise Management Board to stay on top of aircraft noise. The airport recently announced a drop in noise footprint following an initiative to modify the Airbus A320 family of aircraft, which make up more than half of Gatwick’s flights.

“I think all UK airports realise that their right to operate and grow must be balanced with a serious responsibility to do everything possible to mitigate the impacts of its operations, particularly environmental impacts such as noise,” says Gatwick’s head of airspace strategy and engagement Andy Sinclair.

The Noise Management Board was established in 2015 after a full review of noise problems related to arriving aircraft, and the airport has since implemented changes to the way aircraft are flown to better manage air traffic noise issues. These include changing the ‘minimum joining point’ at which aircraft can start their final approach, meaning a lower concentration of aircraft at any one time, resulting in less noise.

“We have a range of other initiatives to reduce noise and have just raised the height at which arriving aircraft start their continuous descent approach (CDA),” says Sinclair. “CDA means that aircraft use less thrust and generate less noise by descending at a continuous rate, rather than a stepped approach. In August 2016, we raised the altitude for CDA conformance for all arrivals from 6,000ft to 7,000ft to reduce noise further.”

The board is made up of community groups, locally elected representatives, and organisations such as National Air Traffic Services (NATS), the Civil Aviation Authority, Gatwick Airport, the UK Department for Transport, several airlines, and leading experts in aircraft noise.

Utilising an independent body means Gatwick can stay on top of aircraft noise, proactively implementing measures rather than waiting until a complaint comes through. According to Sinclair, Gatwick’s noise footprint has shrunk by 73% in the last 20 years, impacting just 3,400 people in 2017, compared to 12,300 people in 1997.

The actual measurement of aircraft noise is itself a matter of contention for several complainants. In April, members of the US Government’s Congressional Quiet Skies Caucus announced a provision in a newly enacted omnibus appropriations bill that directs the FAA to re-evaluate how the agency measures aircraft noise in Queens, New York, including the areas surrounding LaGuardia and JFK International Airports, as well as elsewhere in the US.

The system traditionally used under FAA regulations is known as the day-night average sound level (DNL), under which aircraft noise is measured on a scale that averages out all noise in an area – such as a flightpath over a residential neighbourhood – over a 24-hour period, with extra penalties in place for noise at night or in the early morning. Since 1981, the FAA has established a DNL of 65 decibels (dB) as the point at which federal funding was released to enable measures such as soundproofing in residents’ homes, but with advances in aviation and audio engineering, the Caucus believes this system is outmoded.

“The metric of 65 DNL has long been outdated and does not adequately measure the true impact of aircraft noise,” claimed Representative Grace Meng, founding member of the Quite Skies Caucus. “That is why it’s time for the FAA to re-evaluate it. The blistering sounds of airplane noise in Queens continues to negatively impact the quality of life of borough residents, and looking at a more accurate measurement of noise effects would go a long way towards creating quieter skies over our communities.”

Several noise-based amendments to Bill S.1405 – or the ‘FAA Reauthorization Bill’ – were proposed to the US Senate in August, including the ‘The 21st Century Noise Measuring’ amendment, which requires noise measuring methods to be updated to account for actual aircraft noise in an area, rather than being computer simulated, and the ‘Cumulative Impacts’ amendment, which would require noise to be calculated cumulatively rather than on a per-flight basis.