A new Federal Aviation Administration report details 583 drone sightings in the nation's airspace, a dramatic increase, the agency said.
Ten of the nearly 600 incidents were in airspace around Philadelphia International Airport. None of these was described as a "near miss."
Nationwide, the sightings came from pilots, citizens, and, air traffic controllers from Aug. 21, 2015, to Jan. 31, 2016.
Each month, the FAA receives more than 100 reports of remotely piloted drones in the skies, many in restricted space around airports.
While the majority of the nearly 600 incidents were not close calls, some pilots spotted drones 50 to 100 feet from aircraft, next to runways, and hovering over crowds at sports stadiums.
"I am very surprised, honestly, that we haven't had a major" accident, said David Thirtyacre, a retired U.S. Air Force fighter pilot who teaches courses in unmanned aircraft systems at Embry-Riddle Aeronautical University based in Daytona Beach, Fla.
"We've got to get our arms around these people who are out there flying illegally and doing just plain stupid things," he said. "They are endangering people and property, and it's against the law."
The rules for drone enthusiasts are clear: Fly no higher than 400 feet, always within sight of the operator, and at least five miles from airports and crowds. The drones can weigh no more than 55 pounds.
Those rules have been broken near Philadelphia International Airport on several occasions:
On Nov. 20 a small private Lancair aircraft climbed to 6,000 feet to avoid a drone aloft at 4,000 feet 15 miles west of Philadelphia airport.
On Oct. 29, a US Airways Dash-8 passenger plane came within 20 feet of a "large black four-bladed quadcopter" at 4,000 feet while descending to Runway 35 at Philadelphia International.
On Oct. 8, a Grumman aircraft while on one-half mile final approach to Northeast Philadelphia Airport saw a drone within 20 feet of the plane.
On Sept. 25, a medical transport helicopter had landed at Abington Hospital when the pilots saw two drones "just west of the landing pad" 500 to 600 feet in the air.
A crew member on a British Airways flight from London to Philadelphia reported that a possible drone passed 100 feet of the aircraft at 6,000 feet in the vicinity of Blackwood, N.J., on Aug. 21.
"The FAA wants to send out a clear message that operating drones around airplanes, helicopters, and airports is dangerous and illegal," the agency said. Although unauthorized drone operators can face stiff fines, criminal charges, and possible jail time, catching them remains a challenge.
"Most of these reports cannot be verified, so the only thing we have to go on is what the pilot reported," an FAA spokesman said.
About a dozen encounters nationwide required pilots to take evasive action or were described as near collisions.
California, Florida, the New York metro area, and Texas reported the most drone sightings.
In Pennsylvania, pilots sighted drones in Monongahela, Allentown, Wilkes-Barre, Butler, Scranton, and Monroeville. In New Jersey, drones were reported near Newark, Princeton, Teterboro Airport, Sandy Hook, Colts Neck, Robbinsville, and Morristown.
As technology has advanced, unmanned aerial vehicles have been able to fly higher and have become cheaper, anywhere from less than $100 to more than $4,000
Last year the FAA launched a registry for noncommercial, recreational drone owners. About 406,000 people have signed up since the registry went live in December.
The FAA said it has 24 ongoing enforcement cases and had settled 12 about a month ago.
"There are definitely more drones out there," said Thirtyacre, at Embry-Riddle. The FAA expects annual drone sales to hit 2.5 million this year and grow to 7 million by 2020.
Most professional drone operators follow the rules, he said. Recreational users fall into three categories: those who have no idea what the rules are and just go out and fly, drone hobbyists who register their drones and generally follow the safety rules, and "the most dangerous ones who understand the rules and disobey them."
"People open up the new drone they get for Christmas, and it says it can fly up to 15,000 feet, and they say, 'Oh, cool. Let's go do it,' " Thirtyacre said.
The first time a serious accident happens, "and it's going to happen," Thirtyacre said, the FAA and the U.S. Transportation Department will level hefty fines and civil penalties. "There's no way you can get away by saying it wasn't gross negligence. If you are flying at those altitudes next to an airport, against all the rules, there's no way you can say 'Oh, I didn't know.' "