Missed opportunity or necessary avoidance? FAA Noise Metrics Report to Congress

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Missed opportunity or necessary avoidance?

Cynthia Schultz, PE


While it would be easy to take the FAA to task for ignoring the congressional mandate, it is not so easy to imagine the outcome of “what ifs” if alternative metrics were explored.

comparative noise marks

Noise compatibility can be elusive.  Airports utilize the Part 150 program to establish the noise impact footprint of the airport and attempt to affect zoning to prevent encroachment of incompatible development in 65 DNL and above contour boundaries.  With at time multiple municipalities required to adopt airport protection zones into their local zoning ordinances, airports do not always enjoy the zoning protection as requested.  Sound insulation is available to insulate incompatible uses that existed prior to 1981 and can provide for the opportunity for avigation easements but not always.  However, the program is limited to the funding available and airports compete aggressively for these funds under the current DNL threshold of significance.  55.65.70 dnl lines

The introduction of performance-based navigation flight procedures through NEXT GEN has consolidated previously dispersed flight tracks over 200 foot corridors dramatically increasing the frequency of operations over a very small areaspre and post Rnav patterns.  This concentration of flight activity and noise has created high levels of annoyance at much lower decibel levels than the FAA predicted.  The current significant impact metric, DNL does not provide for adequate analysis of concentrated flight tracks as it represents an average of all flight tracks in all directions to generate a contour that does not provide any indicators of concentrated flight paths over narrow areas.



Without developing a system of compatibility tools from which to utilize alternative metrics to address properties impacted by lower threshold noise levels at a very high frequency of operations, the alternative metrics serve no purpose. 

The FAA avoids true engagement on noise impacts other than to employ the Environmental Assessment or Environmental Impact Statement Noise Study that have long been the standard to assess noise impact of proposed changes.

If the FAA were to review and endorse alternative metrics prior to understanding what can be done with them and without knowing if Congress is going to appropriate the funds to provide mitigation based on the alternative metrics then they would be opening pandora’s box.

pandora's box and metrics


The answer may lie in rethinking the metroplex approach to reintroduce dispersion below a certain altitude rather than seeking alternative metrics. 


FAA Sidesteps Congressional Mandate to Evaluate Alternative Noise Metrics

By Barbara Lichman ,PHD, JD on June 10, 2020

Barbara Lichtman


In the FAA Reauthorization Act of 2018, Pub. L. 115254, § 188, Congress required the Federal Aviation Administration (“FAA”) to “evaluate alternative noise metrics to current average day-night level standard, such as the use of actual noise sampling to address community airplane noise concerns.” In its April 14, 2020 Report to Congress (“Report”), FAA thumbed its nose at that mandate, and chose instead to enumerate the various available metrics, without any attempt at comparative analysis of their efficacy at representing real world noise impacts when compared to Day/Night Average Sound Level (“DNL”), currently required by FAA for analysis of airport noise impacts.

equivalent of operations



From FAA’s purely technical perspective, the DNL metric “captures all the acoustic energy within a 24 hour period, adding a 10 dB penalty between the hours of 10 p.m. and 7 a.m. to account for people’s increased sensitivity to noise at night. “Report, p. 11, § 4.1. What is most notable, however, is what is not said. FAA mentions in passing but fails to evaluate the comparative benefits of any of the alternative metrics, with the most obvious omission that of the Cumulative Noise Equivalency Level (“CNEL”) metric long used to evaluate aircraft noise in California. That section imposes an additional 5 dB penalty between the hours of 7 p.m. and 10 p.m., hours when families are already at home and particularly sensitive to the noise of overflights. Instead, FAA relentlessly defends DNL, with no attempt to catalogue its failures as, for example, an average of noise created by aircraft operations over a 24 hour period which gives short shrift to the impacts of individual operations, and without the mitigation of the additional 5 dB penalty.

Even more surprising is FAA’s off-hand dismissal of “noise monitoring,” the use of fixed points at which specialized machinery records the noise created by each operation, even though that information is later used to calibrate the noise models’ estimations of which FAA appears to be so fond instead. FAA seeks to justify its own reliance on the mathematical exercise of modelling, by asserting that “real-world situations,” like “monitoring” can include “various sources of error” including non-aircraft sounds. Report, § 7. Ironically, most, if not all large airports have rejected modelling as the exclusive mechanism for determining noise impacts and, instead, utilize complex systems of monitors which record the “real world” incidence of aircraft impacts.

Finally, FAA claims that “rigorous noise measurement procedures are used in the aircraft certification process.” Report, § 2.2. Although not further discussed in its letter, FAA discloses that, in the aircraft certification process, “rigorous noise measurement” (that can only be obtained from monitoring) are used to determine “the maximum noise level that an individual aircraft can emit.” Id. FAA’s logic in verifying the noise of individual operations with independent, individual, noise measurement in the case of aircraft certification, but not where community noise analysis is concerned, remains opaque.

In short, FAA’s mandated “evaluation” of alternative noise metrics has turned into nothing more than an apologia for its own flawed analytic tool. What Congress plans to do with FAA’s letter, demand a supplement or allow it to stand, remains a question. Stay tuned for the answer.

Indeed, stay tuned for the answerS. The right metric is not the only pending question; as explained in the first section of this post, there are more issues with greater consequences that must be decided first:

  • The economic and environmental values of NextGen.
  • An appropriate assessment of lower threshold noise levels at a very high frequency of operations on defined areas within NextGen’s concentrated noise.
  • How to differentiate between the generalized noise examinations of existing Part 150 policy VERSUS specific points which are under the concentrated NextGen flight paths.
  • Then to select which select which metrics best determine the concentrated vs. generalized impact.


  • Since Congress will adhere to its own budgetary disciplines, which remedies (land acquisition, sound insulation, deviating from NextGen’s preferred track, considering a higher aircraft noise certification, etc.) meet the conjoint goals of efficiency, environment, and safety.









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2 Comments on "Missed opportunity or necessary avoidance? FAA Noise Metrics Report to Congress"

  1. Cynthia- Thank you for tackling the diverse issues about alternative metrics and FAA’s recent report to Congress.

    I challenge the analysis that the problems are about DNL vs CNEL; the relative value of one metric vs another, or noise measurements and modeling.

    Citizens affected by Nextgen long ago identified that any single metric cannot serve to understand noise from Nextgen (it’s not just frequency but additional noise from lowered altitudes in areas not anywhere near a 65 dnl contour, thus the contrast in noise from an overflight compared to ambient more noticeable). It’s a step forward that FAA conceded in their 188 report that there are various metrics that can apply to different situations. Nextgen is “different” from noise problems of the past and demands new reporting methods and a menu of mitigation options such as analyzing dispersion options.

    Instead of debating alternative metrics – we need to have analysis of Nextgen noise with a set or group of appropriate metrics, some from the list in the report.

    Both you and Ms Lichman left section 173 out which was a provision to address the 65 standard. I would be interested in knowing what you think about say decoupling the NEPA disclosure standard from mitigation/insulation standards (currently both merged as one standard) so that communities could propose alternative metrics for *disclosure* of impacts in NEPA processes. That would not be a major budgetary issue compared to insulation but it would be a start to end the current concealment of noise impacts with the 65 cutoff that the public cannot respond to because working with the 65 standard simply does not reflect reality on the ground.

  2. Another failing of the NextGen PBN design process has been the complete lack of concern, or even awareness, of the communities underneath the new hyper-focused flight paths that are outside the traditional 65 DNL contours.

    In an interesting co-incidence, your first graphic at the top of this page shows the 24 noise monitor locations (green dots) in the vicinity of KSEA. Blue is the water of the Puget Sound. You’ll notice an Island West of the airport. That’s Vashon Island. It’s accessibly only by ferry (there is no bridge or tunnel) and has, or rather had, a very rural and natural character thanks to its physical isolation. The three and a half mile moat between the Island and Seattle led people who were extremely noise sensitive to settle on the Island. The ferry is a real hardship, but for many people avoiding the noise of the city is worth even an extreme hardship.

    Around 2015 the FAA implemented two NextGen downwind arrivals PBN RNAVs (one for Southflow and one for Northflow) right up/down the spine of the Island. Arrivals (up to 250/day pre-COVID) that used to be broadly dispersed in a swath miles wide, are now in a razor sharp track 100 yards wide. They are also thousands of feet lower than they used to be due to poor design, planning and execution of the HAWKZ RNP by the FAA. Our web site has details, but the Executive Summary is that OPD has backfired and average miles in level flight per-arrival *increased* 40% with NextGen instead of decreasing and, worse, the level flight that used to be above 10,000 has now been pushed down to ~4000 feet.

    All this taken together has focused tremendous noise over an Island populated with people so sensitive to noise they marooned themselves on an Island to get away from it. This was done with zero community outreach on the Island. During the EA, with a ROD years before actual implementation, the only community outreach was done in areas which would be unaffected by this radical change. Very similar, but even more extreme, to what happened in Georgetown.

    This is why people are so upset. Their previously peaceful lives, homes and farms have been devastated and they come to learn that their only possible legal recourse to this NextGen sneak attack expired years before the change was even implemented! It’s exposing the federal government in general, and the FAA in specific, as a Kafka’esque dystopian nightmare to citizens spread across the country, everywhere NextGen has been implemented.

    David Goebel

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