Photo: JEWEL SAMAD / AFP/Getty Images

The SkyWest Airlines passenger jet began descending below the 500-foot level toward San Francisco International Airport’s runway 28R when the pilot saw an alarming sight: what looked like a drone passing just 20 feet directly below the plane’s nose.

The pilot landed the plane safely without needing to take evasive action, according to a newly released Federal Aviation Administration report. But the Jan. 23 incident, one of at least 27 pilot-reported drone sightings so far this year around Bay Area airports, highlights the emerging worry drones pose for airports.

Although the FAA and airports are examining the use of emerging technologies to detect or take down drones, there’s not much they can do right now except educate drone pilots to fly safely or send law enforcement officers to try to find the offenders.

But it’s difficult for aviation officials to determine that what the pilots saw was a drone, much less provide information such as size and type that would lead police back to the operator.

“We’re concerned about every drone sighting, regardless of the details,” said SFO spokesman Doug Yakel. “The reports only capture what the pilots said they observed and are not corroborated by any other source of data. We evaluate these reports collectively to identify trends and patterns of reporting activity.”

According to the FAA’s latest report, more than 400 sightings of unmanned aircraft were reported to the agency nationwide from January through March. About 1,200 sightings were reported in 2015 and 1,800 in 2016, according to FAA spokesman Ian Gregor.

The reports mainly come from aircraft pilots, although some are from private citizens on the ground. Some pilots reported drones that were so far away that they posed no immediate danger to the aircraft, but were still close enough to the 5-mile restricted airspace around airports or far above the 400-foot height limit for drones.

A Feb. 3 drone sighting, one of three reported in the first three months of this year to Oakland International Airport, concerned a device flying about 1,000 feet over the Oakland Coliseum.

And, as The Chronicle reported July 23, pilots of two jets reported a drone hovering 3,500 feet over Hunters Point.

This year, 11 sightings have been reported to SFO, almost equaling the 15 during all of last year. None has caused problems or delays for passenger flights, including the close call Jan. 23, Yakel said.

In that incident, the pilot of SkyWest Flight 5233, operating for United Express from Redding, reported his plane was descending toward runway 28-right about 6:35 p.m. when he saw “a drone passing below the nose of the aircraft,” the FAA report said. A SkyWest spokesman declined to comment.

At Mineta San Jose International, one of nine sightings reported in the first quarter came March 12, when the pilot of a passenger jet approaching the airport at 5,000 feet said a circle-shaped drone with four blades came within about 100 feet. No evasive action was taken, and the incident was relayed to the Santa Clara County sheriff’s office, the FAA report said.

Photo: Liz Hafalia / The Chronicle 2016 Photo: John Stillwell, Associated Press

Aviation officials and drone experts say it’s difficult to determine whether the sighting is of a drone or another object. One pilot’s report filed to SFO described a drone flying a half-mile away as “gold and red and shiny.”

Drone sightings are bound to rise as sales soar. The FAA expects the number of drones to increase in the U.S. from 2.5 million in 2016 to 7 million in 2020.

A 2015 study by the Center for the Study of the Drone at Bard College in New York found that only one-third of reported drone sightings near airports could be considered serious incidents. The rest of the reports had inconclusive details, or suggested the object was too far from the aircraft to pose an immediate threat, said Dan Gettinger, the center’s co-director.

Most often, the pilots reporting drones are taking off or landing. “These pilots are doing a lot of things at the same time, and they’re not always able to report with absolute certainty that the things they saw are drones,” Gettinger said.

In 2016, a British Airways jet initially thought to have hit a drone in flight actually hit a plastic bag, according to Gettinger.

“It’s not clear how serious of an issue it is even though we see all these reports,” he said.

The severity of a collision would depend on several factors, including the relative speeds of the aircraft, the size and weight of the drone and whether the airplane’s cabin was fully pressurized, said Javid Bayandor, director of the Crashworthiness for Aerospace Structures and Hybrids Lab at the State University of New York at Buffalo.

The lab is conducting studies on drone collisions. Since drones are made of plastic or metal, they have the potential to cause more damage than birds if sucked into a jet engine, Bayandor said. That’s particularly true on takeoffs and landings, when the pilot has less time to avoid obstacles.

“The answer is nothing really good comes out of it,” Bayandor said of a potential collision. “At any cost, it has to be avoided.”

Analysts say the burgeoning drone industry, including hobbyists, photographers, racers and potentially package shippers, could come crashing down if there’s a tragic incident.

“All it’s going to take is one major fatality and the downside for the industry at large will be incalculable,” said Chris Carr, a drone law expert and partner at the law firm Baker Botts in San Francisco.

SFO has tried to streamline its reporting system so that when a pilot reports a drone, the information gets to a local law enforcement agency as soon as possible, according to Yakel. “Part of what we’re trying to achieve is to be able to identify and locate the drone operator,” he said.

But that’s difficult because the flight paths leading to and from the airport stretch for miles, over many cities, counties and the bay. Dispatching police to one location to find a single drone operator can be like looking for a needle in a haystack.

The FAA prohibits shooting a drone out of the air because it might cause more harm if it fell to the ground, although the U.S. Department of Defense recently authorized the military to shoot down drones over bases.

SFO is trying to map drone flights to determine whether there are patterns that will help find illegal operators. It also is examining various tech options for tracking and detecting drones, Yakel said.

For example, San Francisco startup Dedrone offers technology to track the unique communication signatures for individual drone models. But Yakel said the airport is still far from deciding what type of technology to use. One option is training its current systems, like radar and security cameras, to find drones.

NASA researchers in Langley, Va., recently developed an electronic GPS system that can force a drone to the ground before penetrating a designated zone. SFO has an electronic fence around its physical property to track the movements of ride-hailing services like Uber and Lyft, but installing a similar electronic fence covering flight paths would be difficult, Yakel said. Still, “we want to understand what’s available,” he said.

The FAA, the Association for Unmanned Vehicle Systems International, which represents commercial drone operators, and the Academy of Model Aeronautics, which represents hobbyists, have launched a campaign to educate drone pilots.

“Drone operators have to understand that as soon as they start flying outside, they are pilots with the responsibility to operate safely, just like pilots of manned aircraft,” the FAA’s Gregor said in an email.

Benny Evangelista is a San Francisco Chronicle staff writer. Email: bevangelista@sfchronicle.com Twitter: @ChronicleBenny

Drone sightings

by pilots near

Bay Area airports

City 2016 2017* San Francisco 15 11 San Jose 22 9 Oakland 3 3 Napa 2 1 Fairfield 1 Santa Rosa 4 1 Pacifica 1 Palo Alto 2 Concord 1 Danville 1 Hayward 1 Fremont 1 Livermore 1 Woodside 1

*Through March, except for San Francisco