Estimated reading time 9 minutes, 25 seconds.
Noise complaints are a perennial concern in the helicopter industry, but they’ve assumed new prominence over the past two years. New York City received nearly 26,000 helicopter noise complaints in 2021, more than seven times as many as in 2019. The lockdowns and changed work patterns associated with the Covid-19 pandemic are thought to have made many communities more sensitive to helicopter noise, but irritation has been growing for some time. A survey released last year by the Federal Aviation Administration (FAA), based on data collected in 2015 and 2016, found that even then more people were “highly annoyed” by aircraft noise than ever before.
Most helicopter pilots would probably tell you that there are only two ways to cut down on noise complaints: fly higher, or avoid noise-sensitive areas altogether. When doing so is practical, that’s not bad advice. According to the FAA, doubling your altitude decreases the sound level directly under the flight path by six to seven dBA (a decibel measure weighted by how humans perceive sound). That may not be enough to disappear into the background, but it can keep you from being the obnoxious distraction that interrupts conversation.
In fact, there are many other steps that helicopter pilots can take to reduce their noise signature in flight, but not all of these are obvious. Ideally, student pilots would learn these techniques early in their flight training, so that flying to minimize noise would become second nature, like power management. If instead you were taught that helicopters are simply noisy and there’s not much you can do about it, it’s not too late to catch up. Helicopter Association International (HAI) is here to help.
Turning research into action
HAI has had a “Fly Neighborly” program since the early 1980s, but over the past few years, under funding from the FAA, it has been greatly enriched with new tools and training based on the latest research. Multiple studies over the past decade-and-a-half have given researchers a better understanding of the noise produced by helicopters in general and by certain models in particular. Notably, in 2017 and 2019, NASA, the FAA, and the U.S. Army collaborated on flight tests to validate low-noise approach procedures for a number of popular helicopter models, including the Robinson R44 and R66; Bell 206, 407, and 205; Airbus AS350, EC130 B4, and AS365; Sikorsky S-76D; and Leonardo AW139.
This research has now been incorporated into free online training modules, handouts, and other resources that can be accessed from the Fly Neighborly page of the HAI website. They include an “Introduction to Fly Neighborly Training” course hosted on the FAASafety website, an HAI Online Academy course on Fly Neighborly Auditory Techniques, and recorded webinars looking at specific noise reduction techniques for five different helicopter models (with more to come). The modules use scientific flight test acoustic data to give helicopter operators nuanced recommendations on how to reduce their noise footprint, although precisely because the material is so nuanced, it can take a while to digest.
As this story from the February 2021 issue of Vertical described, helicopters generate several different types of noise, including thickness noise created by blades moving through the air, and loading noise resulting from the lift and drag forces on the blades. However, much of the Fly Neighborly training focuses on blade vortex interaction (BVI) noise, which is created when a helicopter’s rotating main rotor blades interact with the vortical wake created on their previous passes.
“So safety is number one, but when you do have options, pick the quieter one.”
BVI is the loud, impulsive noise affectionately known in the helicopter community as “blade slap.” It is created both on the advancing side of the main rotor disc, where it is directed down and forward, and on the retreating side, where it is directed down and rearward. Although it is usually the most irritating component of helicopter noise for people on the ground, it’s also the one that pilots have the most control over.
BVI typically isn’t a factor in level flight or very shallow descents, because the vortical wake remains below the rotor. Conversely, at higher rates of descent, the wake remains above the rotor and is likewise not a factor. It’s typically at moderate descents of around 200 to 400 feet per minute at 60 to 80 knots — a regime in which most helicopter pilots feel very comfortable — that BVI is most pronounced.
Each helicopter model has a specific range of airspeed/descent rate combinations that generate the most intense BVI noise. The operational noise plots established during the 2017 and 2019 acoustic testing form the basis for HAI’s model-specific guidance, which encourages pilots to fly descent profiles that avoid the most intense BVI regions when possible. However, it gets a little more complicated than that, because deceleration on approach pitches the aircraft up, retrimming the main rotor. That can move the aircraft into the BVI region at low rates of descent, or out of it at more moderate ones.
Because the complex interplay of these various factors can be a lot for pilots to keep track of in flight, HAI suggests incorporating the guidance into standard operating procedures (SOPs) for specific noise-sensitive approaches. Its Auditory Techniques course explains how pilots can work with ground observers to evaluate the effectiveness of various procedures in mitigating noise. Having someone on the ground can be essential for determining the optimal approach, since pilots don’t hear all of the noise their helicopter is generating in the cockpit.
A 2019 paper co-authored by Juliet Page, a physical scientist and chair of HAI’s Fly Neighborly Working Group, acknowledged that helicopter operations tend to be much more dynamic than fixed-wing operations, which can make developing suitable SOPs challenging. Separate low noise procedures may have to be tailored for situations when low ceilings force helicopters closer to the ground. And HAI emphasizes that noise considerations will always need to be balanced against safety first and foremost, as well as other factors including flyability, passenger comfort, and operational costs.
Rules of thumb
While optimizing a specific approach for noise can be a complex and iterative process, there are also many simple ways in which pilots can reduce their noise impacts. As Page pointed out, just knowing the airspeed and rate of descent combinations where your helicopter is likely to be noisiest can help you avoid those regimes when there’s no particular reason to fly there.
Understanding which side of your helicopter is noisiest can also help you choose the best flight path over a noise-sensitive environment. For example, with the Bell 407, which has a counter-clockwise turning main rotor system, areas to the left side of the helicopter receive significantly less noise than areas to the right, so it’s best to keep noise-sensitive areas to the left. The opposite is true for the AS350, which has a clockwise-turning main rotor system, and projects less noise to the right.
Here are some other tips from HAI’s Fly Neighborly training: Climbing turns are quieter than level or descending turns. Accelerating climbs are quieter than steady-state or decelerating climbs, and using your collective to climb is quieter than using your cyclic. Turn away from the advancing blade when possible, and strive to maintain constant airspeed during a turn.
Becoming a more noise-conscious pilot doesn’t mean radically changing the way you fly; rather, it’s about understanding how your helicopter creates and projects sound and incorporating that knowledge into your normal aeronautical decision-making.
“Safety always comes first — noise is secondary,” Page said in one of the Fly Neighborly webinars. “So safety is number one, but when you do have options, pick the quieter one.”