In a year-long study of the noise impacts of commercial flights coming into San Francisco International Airport by crossing the Santa Cruz Mountains in the vicinity of Woodside and Portola Valley, the Federal Aviation Administration found that 83 percent of the flights crossed over a Woodside navigation beacon at or near the required minimum altitude of 8,000 feet above sea level.

"A small portion" of the flights cross over the Woodside beacon at 6,000 feet, most of them arriving from the eastern Pacific, according to the recently released study on noise impacts in four Bay Area counties, including San Mateo.

Rep. Anna Eshoo, D-Menlo Park, extracted a noise-reduction agreement from the FAA in 2001 that requires aircraft to stay above 8,000 feet when crossing the Woodside beacon. That can be cold comfort for residents living along Skyline Boulevard, which tracks the crest of the mountains. The beacon's location at 2,270 feet above sea level is 5,730 feet below an aircraft crossing at 8,000 feet, and just 3,730 feet below a jet crossing at 6,000 feet.

The FAA report is quite a contrast to a three-year analysis, ending in 2012, conducted by Portola Valley residents Dr. Tina Nguyen and Vic Schachter and Woodside resident Jim Lyons. They reported that 88 percent of commercial flights over the Woodside beacon crossed at altitudes below 8,000 feet, and about 28 percent below 6,000 feet.

"I can tell you that Mid-Peninsula noise abatement groups are disappointed with the FAA's result," Dr. Nguyen said via email. "The FAA was tasked with finding solutions to our worsening problem, but there is nothing in their plan to help our mid-Peninsula communities."

The methodology of the FAA study, referring to the Woodside beacon, involved an assessment of "what opportunities exist to modify operations ... including looking at the possibility of adjustments during reduced-volume night operations, even if day operations cannot be changed." The conclusion: changes to current patterns are "not feasible" and "no further action" is planned.

In 2015 the Portola Valley Town Council authorized a budget of up to $13,000 to pay an airline industry consultant to evaluate the FAA's proposed noise analysis. The consultant, Williams Aviation Consultants of Gilbert, Arizona, replied in a January 2016 letter that criticized the proposal on several counts, including a lack of specificity and a lack of a commitment to engage with the public.

Glide paths

In referring to flights over the Woodside beacon, the FAA study notes that flights crossing at 6,000 feet do so for safety reasons and because air traffic controllers prefer putting these flights on a glide path to the airport, with their engines at or near idle.

Glide paths are a relatively new practice intended to save fuel and reduce noise, but are not without criticism, particularly from people who live under them.

Traditionally, arriving aircraft track paths determined by air traffic controllers, descending gradually by stepping down in altitude and noisily using the engines for braking — a practice called vectoring. Controllers will still have that option over the Woodside beacon, which was troubling to Dr. Nguyen.

"The FAA characterizes vectoring as a form of equitable distribution," she said, a characterization she called misleading and distressing to some communities — such as Woodside, Portola Valley and Ladera and parts of Menlo Park — more than others. "Vectoring procedures create tremendous noise since engines are at full throttle to keep the planes at low altitudes," she said.

A committee of elected officials from San Mateo, Santa Clara and Santa Cruz counties formed recently to engage with the FAA on noise issues that impact the South Bay. Portola Valley Councilwoman Ann Wengert is one of two members representing San Mateo County.

Asked to comment on the FAA's Woodside-related conclusions, Ms. Wengert replied via email: "The lack of feasible solutions related to the Woodside (beacon) is hugely disappointing to all Portola Valley and Woodside residents who have experienced the negative impact of aircraft noise on their quality of life," she wrote. "While I appreciate the difficulty the FAA had in addressing the community input and suggestions in the Feasibility Study, the lack of 'feasible' solutions to the South Bay Arrivals issues was disheartening and predictable. We had all hoped for more."

One positive note, Ms. Wengert noted: the study called "feasible" a suggestion to raise elevations for arriving aircraft flying to SFO from the direction of Santa Cruz, a change that might be noticed in Menlo Park and Atherton.