Soundscapes: Overhead Flights

Aircraft overflights are one of the most common human-caused sources of noise in Grand Canyon National Park and can be heard almost everywhere within its boundaries.

Noise from aircraft can have a significant adverse impact on natural quiet, wildlife interactions, and visitor experiences. In 1975, the Grand Canyon National Park Enlargement Act recognized natural quiet as being an important part of visitors' experiences and sought to understand the impact of overflights on the park's soundscape. Subsequently, the 1987 Overflights Act required the "substantial restoration of natural quiet." The data collected by the Soundscape Program helps Grand Canyon National Park (GCNP) determine how well it is meeting the objectives of preserving natural sounds.

 

One of the criteria for substantial restoration of natural quiet (SRNQ) is that no more than 50% of the park may exceed 25% time audible.

The following rose diagram image below from 2024 demonstrates the percentage of park area exceeding this 25% time audible threshold, plotted on a polar axis. The length of the line indicates the maximum daily value of park area for that week, and the 52 weeks of the year are arranged clockwise on the plot. The daily maximum park area was chosen in place of a weekly average to prevent the situation where a peak value gets averaged down by the other days in the week. The limit of the plot is 50%, to show that no single day ever exceeds the SRNQ threshold.

 

 

 

The above graphics summarize the area of the park with audible overflight sounds greater than 25% of the time throughout the year for 2024.

• Substantial restoration of natural quiet” threshold (50% or more of the park achieving natural quiet, i.e., no aircraft audible, for 75% to 100% of the day, each and every day) has never been exceeded for any of the years in which the National Park Service has analyzed those data (beginning in 2021).
• During 2020, the park had the fewest overflights in the Dragon-Zuni flight corridors since we’ve been tracking this data (6,990 total operations), attributed to reduced visitation due to the COVID-19 pandemic. The sound level (LEQ12, or the average sound pressure level during daylight hours) was the lowest during 2020 in our reporting history, 50.3 dB (compared to a high of 58.0 dB in 2018), which is consistent with the lower numbers of air tours.
• Quiet Technology aircraft (which is based on a per passenger noise average and not a per aircraft noise reduction) usage by percent of flights was 74.7% for 2023 (highest percentage recorded to date), compared to 55.5% in 2019 (lowest percentage recorded to date).

Air tour activity for the Dragon-Zuni air tour operations for 2021 - 2024 remain lower than 2015 - 2019 but continue in an upward trend from 2020 (6,990 tours reported that year): 14,839 air tours in 2021, 23,372 air tours in 2022; 23,047 in 2023 and 25,738 tours in 2024, compared to an average of 42,131 air tours between 2015-2019.

• The average noise level (LEQ12) was 53.6 dB for 2021, 55.3 dB for 2022, and 54.9 dB for 2023 - down from the high of 58.0 dB in 2018 and under the baseline of 58.1 dB. However, missing daily operator data for years 2021-2022 restricts full analysis of the substantial restoration of natural quiet calculations for those years.
• Due to the amount of air tour activity for both years 2021 and 2022, we are reasonably certain that SRNQ continues to be met due to comparisons with years with higher air tour activity. In 2023, there was 100% reporting among the existing air tour operators with a noise level below the 58.1 dB baseline. The National Park Service is working on noise level calculations for the 2024 air tour numbers, although again we are reasonably certain that SRNQ continues to be met based on total tour numbers.

Related Resources

• Grand Canyon National Park Overflight Documents
• Learn more about Air Tour management at the National Park Service Natural Sounds Subject Site.