Mayors, city managers and public safety officers from a dozen San Gabriel Valley communities expressed serious concerns Friday over a plan to alter flight patterns at Southern California’s 11 airports and reroute airplanes over local skies.

City representatives from Monterey Park to La Verne who gathered at the Glendora Police Department wondered whether the changes would shift noise and air pollution to their communities.

They also complained they were kept in the dark about the plan, now six months after the Federal Aviation Administration closed the public comment period.

“This is government operating in their own little sphere. It is opaque. So we don’t know the impact on our residents of noise, air pollution and property values,” said La Puente Mayor Dan Holloway.

The so-called Next Generation Air Transportation System will be implemented in Southern California, changing flight paths in phases starting November 2016.

Planes arriving at LAX from the east normally follow the 60 Freeway. Under the new plan, they would fly above La Puente, Avocado Heights, Valinda, Industry and the hills near Walnut 50 percent of the time in the day. From midnight until 6 a.m., planes would fly over La Puente, West Covina and Industry 100 percent of the time under the plan.

The FAA says planes would fly at an altitude of 10,000 to 13,000 feet, the same as current flights.

LAX departures to easterly destinations currently fly over Alhambra, Los Angeles and the San Gabriel Mountains. Under the plan, the new trajectory skews more toward the San Gabriel Valley floor, directing commercial airliners over El Monte, Baldwin Park, West Covina, Covina, San Dimas, Glendora and above the San Gabriel Mountain community of Mt. Baldy north of Upland. Planes would fly at altitudes of 8,000 to 10,000 feet in these areas, according to information from the FAA and Los Angeles World Airports, the Los Angeles department that operates LAX and other airports.

San Gabriel Valley city leaders were invited to hear more about the plan by Rep. Grace Napolitano, D-El Monte, who insisted the FAA did not inform her of the plan until after public hearings were held and a draft environmental assessment was published on June 10.

NextGen affects 21 metropolitan areas in the United States by upgrading communication and directional services from air traffic controllers to pilots in the sky. The switch from ground-based communications to GPS will streamline flight paths, said the FAA’s Steve May, senior adviser to Glen Martin, the regional administrator.

Under the plan, airplanes would no longer zigzag from takeoff to landing but follow a smoother path. The FAA could place airplanes closer together and add capacity to airports. In Southern California, the plan is estimated to save 2.7 million gallons of fuel and $7.8 million a year.

“The bottom line is you are changing flight plans over our areas and now it affects our communities,” said La Verne Councilman Tim Hepburn. “I’m concerned about noise. I’m also concerned about fuel burn.”

But changes would not cause a significant noise impact and would reduce fuel burn, May said.

“By burning less fuel, you are putting out less emissions,” May told the group.

During the meeting, exchanges between May and Napolitano became tense. The congresswoman, who sits on the House Transportation Committee, said the FAA needs to form a task force with San Gabriel Valley cities to work through these issues. If not, she said she would author an amendment to delay implementation of the plan.

NextGen has raised the ire of residents in San Diego, New York and Phoenix. In Palo Alto, where flight path changes have already been implemented, noise complaints reportedly have greatly increased.