There is no denying that aviation has come very far technology-wise. In its early infancy, commercial aviation was an expensive and rather uncomfortable endeavor, as the turboprops that powered aircraft were loud, produced a ton of vibration and were limited in power, making passengers travel onboard an aircraft for a very long time.

As jet engines gained traction commercially, passenger comfort and airline operations improved dramatically. The first turbojets were more efficient, faster and most importantly – less noisy, thus providing a much more comfortable journey on board. Yet, aviation authorities and governments worldwide now seek to reduce how much noise aircraft produce with manufacturing limits, operational restrictions and regulations.

To counter aircraft noise pollution, the International Civil Aviation Organization (ICAO) has issued the Annex 16, which includes several chapters with noise restrictions. ICAO measures sound levels in Effective Perceived Noise (EPNdB), which relies on human annoyance, rather than general loudness.

Why is reducing aircraft noise pollution important?

Noise pollution can significantly impact the experience onboard. Airlines today aim to deliver the best product possible, including top of the line in-flight entertainment (IFE), catering, cabin layout changes and even lightning inside the aircraft are crafted in a way to make passengers as comfortable as if they were in their own home.

But one aspect of comfort for airlines is hard to control – noise. It puts carriers in a difficult position, as configuring a cabin in various ways can help to reduce the noise inside, but at the same time increase the weight of the aircraft and subsequently, boost operational costs.

According to a report by the Federal Aviation Administration of the United States (FAA), exposing a person to loud noise (higher than 90dB) for a prolonged period of time (eight or more hours per day) for several years “may cause permanent hearing loss”. A 2006 study highlighted that even at 65 dB(A), “humans become irritated from noise”. The same study tested sound levels on two Airbus A321 flights and the average noise levels in-flight were between 80-85 dB(A), just below the 90dB threshold, but way above the human annoyance sound level of 65 dB(A).

Furthermore, studies like the one by The University of New South Wales (UNSW Sydney) have shown that continuous exposure to noise can reduce the recognition of memory and cause a person to be more fatigued. A recent example of how fatigue can cause issues inside the cockpit is Air Canada Flight 759. the National Trasnportation Safety Board (NTSB), which investigated the horrofing close-call, concluded that fatigue most likely contributed to the crews’ decision making.  

However, the A321 is a fairly new aircraft and is categorized under Chapter 4 in ICAO’s Annex 16, which allows for a maximum of 92.8 EPNdB for the aircraft. On average, it emits 88.3 EPNdB during a flyover. Newer aircraft are even under stricter noise restrictions and manufacturers have to comply with Chapter 14 regulations released in 2013. As a result, newer aircraft like the Airbus A321neo emits 83.7 EPNdB, while Boeing 737 MAX generates 82.6 EPNdB.

Yet people on board aren‘t the only ones affected by noise pollution – residents that live under busy flight paths experience a lot more issues related to sound pollution.

“It may be beautiful to look at, but not to live near”

Said one New York resident, as Concorde took off for the last time from New York’s John F. Kennedy Airport (JFK). While supersonic speeds, due to sonic booms, exaggerate the effect of sound pollution, the general consensus amongst residents living near airports is similar.

One study, which researched how aircraft noise affects a person’s health, concluded that continuous noise exposure may lead to “heart disease and hypertension”. Both consequences are exaggerated if the noise exposure is during the night, but “similar daytime exposure effects have also been identified”. Another study concluded that aircraft noise can also impact a child’s ability to learn, as “aircraft noise exposure at school or at home is associated with children having poorer reading and memory skills” – so much so, that even 5 dB can delay the average reading age by 2 months.

Aviation authorities are trying to mitigate the impact of aircraft noise pollution. The previously mentioned ICAO Annex 16 and its newest Chapter 14 are written with hopes to reduce the people living under “Day Night average sound Level of 55 dB” areas by 1 million between 2020 and 2036. World Health Organization (WHO) indicates that noise levels above 55 dB during the night affect the population, as it becomes “highly annoyed and sleep-disturbed” and is more prone to cardiovascular diseases.

FAA has also launched a project called Noise Quest, which includes educational material and research about the effects of aircraft noise pollution. The agency even helps airports to acquire land near their facilities if the acquisitions are done under “noise compatibility programs”, as noise regulations also restrict operations.

Restricting operations

Airlines are very strict regarding their costs, especially low-cost carriers. While one of the reasons that they do not operate the night is to conclude maintenance on their aircraft, airports also charge a lot more for flights that land or depart during the night.

For example, London-Gatwick Airport (LGW) imposes charges upon every aircraft that lands at the airport. One of the charges is related to noise. An aircraft, compliant with ICAO Annex 16 Chapter 14 base noise emissions has to pay $26.23 (£21.65) during a summer’s (April 1 – October 31) day (5 AM – 10:29 PM), but at night (10:30 PM – 4:59 AM) that sum increases to $460.52 (£378.69). Considering the fact that low-cost carriers operate under tight margins and try to avoid every cost as much as possible, airlines try to avoid landing during the night as much as possible.

To illustrate, an easyJet Airbus A19 registered G-EZIV completed six flights from and to London-Gatwick (LGW) on August 21, 2019, according to FlightRadar24 data. The aircraft started its day at 5:50 AM, avoiding the night time noise charges, but its last flight was late to depart from Bordeaux (BOD) by 11 minutes and it landed at 10:30 PM. As a result, easyJet was forced to pay nightly fees for the flight. Considering the fact that the airline has to pay for fuel, emission taxes, parking fees, service fees and many other costs imposed on aircraft, and keeping in mind that easyJet average revenue per seat is $61.65 (£50.71), those 11 minutes can be the crucial difference between a profitable flight and a loss-making flight.

Numerous airports around the world limit their operations during the night due to noise restrictions – according to ICAO, the number of international airports who restrict night time flying in some shape or form is 161, of which 107 are located in Europe. As a result, landing and departure slot times are becoming a valuable asset that can be worth almost as much as an aircraft – in early 2016, Oman Air paid $75 million for a pair of take-off and landing slots at London-Heathrow (LHR). Thus, airlines pay extra attention to schedule planning, as profitability can depend on minutes. Increasing capacity on routes can also be very difficult, if the route is congested by other airlines – if slots are limited, the initial costs of purchasing a landing slot can determine whether the airline is able to increase Available Seat Kilometers (ASK) in their network.

Nevertheless, aircraft manufacturers try to reduce the noise that aircraft emit as much as possible.

Help of the manufacturer

But noise reduction comes back to the manufacturers like Airbus or Boeing. Fuel efficiency is by far the most important aspect of a new aircraft, but noise reduction has become increasingly influential when airlines do decide to purchase an aircraft.

Lufthansa estimates that a new Airbus A350-900 produces between 40 and 50% less noise contour compared to its quad engine cousin, the A340. The aerodynamic shape of an aircraft can help reduce the noise on an aircraft as wind bypasses it, rather than hitting the aircraft. For example, a retracted landing gear during an approach or a landing can create noise, as the wind hits it. One more case is the Airbus A320 family – the aircraft emitted a “squeaky” sound during the approach. But a vortex generator under the wings can reduce the noise by up to 6 dB, which airlines began to retrofit on their A320s back in 2014.

However, engines are one of the major contributors to overall noise emitted by an aircraft and manufacturers use an array of ways to reduce the sound. Modern engines have higher bypass ratios, thus the engine itself consumes less air and produces less noise. Also, the air that bypasses the core of the engine is muffled by the air that has passed the engine fan and due to the high bypass ratio, a lot more air passes by the fan. Engine chevrons also help, as they smooth out the mixture of the hot air from the engine core and the cold air from the engine fan, creating a turbulence-free environment around the engine.

Airlines also retrofit their engines with hush kits, but these are mostly used on older, low bypass engines like on the Boeing 737-200, which still sees active service in Northern Canada.

Health concerns

All in all, the number one reason to reduce noise pollution is to improve the quality of life of the communities that live near airports or busy flight paths. As noise pollution is proven to have detrimental effects on health and the ability to work even in the cockpit, further reducing noise emissions is crucial. However, as the general public is still largely unaware of the harm that aircraft noise can cause, according to PARTNER, education is as important as advancements in technology.