What makes an aircraft flying overhead noisier?
December 17, 2013 9:36 AM   Subscribe

I've been doing research on jet aircraft noise over urban areas. And, while I've uncovered some very clear explanations of the different types of aircraft noise to share with others, there are some questions that still are eluding my Google foo. I'm looking for resources that will help me to answer a variety of specific questions about aircraft noise, whether they are discussion groups or specific authors of research papers, etc. The kinds of questions I'm seeking resources for are all related to aircraft noise, but can be somewhat different. For example:

1) I'm familiar now with the 4 Stages of aircraft noise certification, but it isn't clear to me what--if any--real improvements that Stage 4 aircraft will bring, especially when you consider arrival, lateral and departure noise separately? I've been pouring over the FAA documents but a clear explanation of this is really difficult to find.

2) I know that airframe noise can significantly increase the overall noise that listeners experience on the ground. And that airframe noise is related to how the aircraft is designed AS WELL AS how a pilot flies the plane. But there is less information about how noise levels increase with different plane maneuvers. Like bringing the landing gear down early. Or making a crosswind landing.

3) Aircraft choices made by pilots because of the instructions given to them by air traffic controllers. What choices increase noise? Using landing gear to slow a plane? Requiring pilots to keep landing gear up until necessary for landing prep? Avoiding crosswind landings? Using CDA versus step down approaches and how this is implemented?

It's difficult to find sources for this specific type of information...perhaps a commercial pilot would know?

My specific context is O'Hare airport. 4 parallel runways, with up to three planes arriving at the same time in parallel (fewer parallel operations for departures.)
posted by jeanmari to Grab Bag (5 answers total) 2 users marked this as a favorite
I'm an aero engineer and a pilot, but I don't specifically work on aircraft noise problems day-to-day. The two biggest mitigating factors that I am aware of for reducing noise on the ground are distance and power settings; sound intensity follows the inverse square law so to an observer on the ground, a plane twice as far away has a reduction in sound intensity of four. Reducing power settings can also help because moving parts in the transonic or supersonic zones cause shocks that are a hell of a lot louder than below transonic.

As a pilot, efforts to reduce noise will include trying to climb as quickly as possible over the ground (best angle of climb) to put more distance between the plane and the ground as well as reducing power as much as practicable. Designers of airframes and engines will try to avoid putting parts of the airplane in the transonic/supersonic zone, as well as smoothing contours and preventing vibrations as much as possible.

Pilots don't have all that much discretion to act around a given ATC clearance. Typical departure clearances involve taking off, climbing to a couple thousand feet, and then being cleared once again to a higher altitude. Landing clearances generally follow published instrument approaches (these are available online if you're interested in seeing where they cross over the ground). Newer technologies are allowing the development of RNP (Required Navigation Performance) approaches which allow approaches to follow non-standard paths that can avoid population areas or trim distances to save time, but these are really still in their infancy. I believe Southwest has been using them at a couple of their more major destinations.

Here is a joint MIT/FAA effort to reduce noise and emissions, you may want to look through their information. The NBAA (National Business Aircraft Association) or ALPA (Airline Pilots Association) may also have some resources available.
posted by backseatpilot at 12:24 PM on December 17, 2013

I'm a pilot, working on my commercial license. I have no specific resources about noise (perhaps someone else will) but I can tell you a bit about flying.

Crosswind landings do not of themselves affect aircraft noise. When an aircraft is flying it doesn't feel the wind, it is carried along with the wind. Unless weather conditions are extreme, the same power settings and speeds are flown in the descent regardless of the wind on the runway.

Jet aircraft always lower their landing gear at the same point on the landing approach. Aircraft have spoilers to increase drag, so there's no reason to lower the gear early. Also, ATC never tells aircraft when to lower the gear, they don't care -- pilots go by their "Ops-Spec", which is the official (very detailed) airline policy about how flights are operated. The only effect ATC has is in how aircraft are routed to get to the landing runway.

In short, when landing there is very little that pilots can do to affect the noise heard on the ground. ATC can control it by the routes assigned, and of course airlines can control it by using modern quieter aircraft.

Are you concerned about takeoff noise, or landing noise? I can't say that I've ever thought landing noise was significant unless you're directly under the aircraft within about a mile of the approach end of the runway (so the airplane is just a few hundred feet above you). Takeoff noise can be significant, and many airports have noise-abatement procedures -- pilots must throttle back after climbing a bit, and get away from populated areas before resuming a normal climb. However, safety always trumps noise abatement. If the pilots feel that it would be unsafe to reduce the power setting when you're trying to get away from the ground (and it frequently can be, depending on the weather conditions), they won't.

(I suspect an ultra-busy airport like O'Hare has very little leeway to change things around. Really, the only way to reduce aircraft noise is for people to fly less, so we need fewer flights.)
posted by phliar at 12:29 PM on December 17, 2013

To clarify: We're 11 miles away from the end of the runway, under the side of the airport that was given most of the arrivals for O'Hare--day and night--in October 2013.

Re: arriving aircraft.

I've seen landing gear down on aircraft before it is directly over us, while it is passing over us, and have also watched aircraft disappear from my view (towards the airport) without gear down yet. So there is a tremendous variation in when gear is going down.

I've been collecting data on arrivals on the east side of the airport, and I have an app that tells me close to real time the altitude, Origination-Destination, Aircraft Type, and Airline. So lowering gear can happen at different points even with the same airline (but I'll look for variation in aircraft type.) I don't have specifics on weight.

After a few hours, I can go online and pull down GPS coordinates from different points on the flight path with corresponding altitude and date/time. It's pretty easy at that point to throw data into a spreadsheet and visualize it in Google Maps. I can also pull wind direction data at different times of the day in for O'Hare but not easily at other points.

The FAA was telling us that we shouldn't be hearing thrust because planes arrive at O'Hare via Continuous Descent Approach of 3 degrees. But I'm hearing thrust in many cases. Not all. But quite a few. So, I go to my data and, yep, you can see that some planes haven't started their CDA until after they are past us and are flying low (2900-3100 ft) and level over us.

Bottom line, we're told that we shouldn't really be affected by noise this far out. But the neighborhood is definitely being affected and it's not just me. It's loud, it's unceasing. We've got three jets at a time flying parallel within 1.5-2 miles of each other, so we hear it above us, to the north and to the south...we can stand on our street and see all 2-3 aircraft overhead at one time. Unlike O'Hare's historical flight patterns, there is little to no respite unless the wind switches to the East. It's loud enough to stop conversations outside, and to wake us at 2:30 am when cargo flights come in. Our local politicians were told, "Oh, it won't be a big deal for those neighborhoods outside of the noise contour" but now that the flight patterns have been changed? They are realizing how wrong that was.

So, I'm doing my research, trying to learn as much as possible about aircraft noise...from engines, to airframe (design), to airframe (how plane is being flown.)

Senators here have introduced the Silent Skies Act to hurry up transition to Stage 4 aircraft, but didn't quite understand that Stage 4 certification will have less impact than they're hoping it would. (Possibly 3 db reduction for any of these three: approach, lateral, flyover. And 3db reduction isn't very distinguishable by the human ear.)
posted by jeanmari at 1:37 PM on December 17, 2013

Atmospheric conditions can have a huge effect. In low wind conditions, the atmosphere can form layers which act as partial reflectors of noise. Noise passing through these layers can also be refracted away from a straight line, potentially increasing the sound level at a receptor. Being downwind can also have a major effect. Have a talk with an acoustic engineer; they might be able to explain additional sources of noise.

The only way the authorities will do anything about this is if you have data. Is there someone nearby who could run an ADS-B receiver to track exactly what planes are where? Similar, logging conditions from nearby weather stations could help building up a pattern. Setting up reliable long-term noise monitors is hard and expensive.

Noise nuisance can be extremely wearing. Look for coping strategies (which could include meditation, or CBT) to get you through this.
posted by scruss at 4:21 PM on December 17, 2013

You'll hear thrusty noises as the plane is landing because the pilot (or the autopilot) uses the throttle to control the altitude. If they sink too quickly, they will throttle up to get back up into the proper path. So you won't hear the full blast of a plane taking off, but that doesn't make it any less annoying.

A 3db reduction is cutting the sound power in half. It is a lot.

What I would do is go to Radio Shack and get a sound pressure meter and record the actual noise levels you are experiencing.

I would then review the various legislation that revolved around the O'Hare expansion. There may well be provisions in there to somehow compensate homeowners whose neighborhoods were previously quiet and aren't any more.

Lastly, on a particularly active day, run some video and send it to Chuck Goudie or Pam Zekman.
posted by gjc at 6:29 PM on December 17, 2013

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