WASHINGTON -- Sunil Gupta and his buddies were on top of a parking garage at Dulles International Airport one recent Saturday morning, practicing one of the more dubious hobbies of the new terrorist-wary age: plane spotting.
Chatting in the sunshine, while checking flight schedules on laptops and listening to hand-held scanners tuned to air traffic control signals, the carefree guys, all computer specialists, looked as if they were throwing a Dilbert-theme tailgate party.
Someone heard a far-off engine and spotted a streak in the breezy sky. Banter ceased. The smokers dropped their cigarettes and grabbed cameras, binoculars and notepads for the approach of an airliner they thought would be a special-edition jet from Ethiopia.
"That's not Ethiopian," said one man, totally engrossed. "I think it's a JetBlue."
"No, it's a Delta," said another, hoisting binoculars into position. But as it neared, they recognized it as the Ethiopian Airlines plane they were waiting for, its red, green and yellow tail colors a dead giveaway.
With heightened security since Sept. 11 at airports worldwide, this nerdy yet innocent hobby has had its setbacks, and spotters have found themselves in a tense relationship with security officers.
Police at an airport in Bangkok, for example, detained Gupta last October while he was plane spotting on a business trip. He said they confiscated his film and asked whether he knew Osama bin Laden.
Gupta, a resident of suburban Gaithersburg, Md., said he was released after he explained his peculiar hobby. The next day, he was featured in Bangkok's media because of the security scare.
The hobbyists rattle off airplane trivia, scribble down registration numbers found on the tail of aircraft, take photos that they collect and trade, examine their plane data on spreadsheets and generally enjoy deja vu when a jet they saw years ago arrives.
"It is pretty much a phenomenon around the country where people position themselves and watch the planes land," said Arlene Salac, a spokeswoman for the Federal Aviation Administration's eastern region.
Nothing illegal about it
Except these are not leisurely enthusiasts. Dozens of hard-core spotters spend as much as 20 hours a week at Dulles, Reagan National and Baltimore-Washington International airports and take vacations to other cities for whole weekends spent watching aircraft. They pore through aviation guidebooks, get excited about interesting color schemes on planes and share sightings on group e-mail lists.
The FAA says plane spotting and photography are legal, but boundary enforcement is up to local airports.
"Anywhere that is public is open to plane spotting," said Marty Clarke, a duty operations chief at Dulles. "If there is something we don't particularly like, we will investigate."
Area spotters said they are approached by airport police regularly and often ordered to leave.
"Sometimes, I think they overstep their bounds," Gupta said.
Tara Hamilton, spokeswoman for the Metropolitan Washington Airports Authority, which operates Dulles and National, said spotters should expect to be questioned.
"Our police have to use their best judgment," she said.
Heavy Internet traffic
Plane spotting has been around since aviation began, but it really developed after World War II, when civilians recorded enemy plane movements. Some say enthusiasts are multiplying, as evidenced by heavy traffic on Internet sites such as www.airliners.net.
Most spotters are men. Some bring their children along; others have tried, with little success, to bring their wives or girlfriends.
Many spotters are involved in technical careers such as computers or engineering.