Can you tell me why there has been a small airplane circling the Bethesda-Kensington area for the past couple of months? In the back of the plane there is an attachment that looks like a car spoiler. Is the plane conducting aerial surveillance?

Jennifer Boger, Kensington

Yes, it is conducting surveillance, and it has been for years, but unless you're in a fender bender on River Road or broken down on the shoulder of Interstate 270, the people inside that plane aren't looking at you.

How does Answer Man know this? Because one morning this month, he stood in a hangar at the Montgomery Air Park as pilot Eddie Vendetti prepared this very plane for takeoff. Eddie took out a yellow aerosol can and sprayed it along the leading edge of the plane's wings.

"This is Pledge," said Eddie. "We get a lot of bugs. This just helps the cleaning process." (Now there's something you won't read in "Hints From Heloise.")

While Eddie went through his preflight check, John Lottes looked over his radio equipment. John is a traffic engineer with the Montgomery County's Transportation Management Center. Either he or his colleagues Chris Leight or Greg Windham go up in the plane, whose call sign is Mike Charlie 10.

The county has been monitoring traffic from the skies for the past 15 years. You probably didn't notice the earlier planes they used. They weren't that noticeable. But the current plane -- a Cessna 337 Skymaster in use since 2001 -- is memorable. It's what's known as a "push-pull" aircraft because it has two propeller engines, one facing forward, the other backward. At the rear of the plane is that split tail that looks, to reader Jennifer, like a car spoiler.

"We picked it because we fly low and slow over congested areas," Eddie said. "Having more than one engine is advantageous in a situation like that." Meaning: If one engine goes out, you've got another one to limp home with. A military version of the plane, known as the O-2, was used in Vietnam to call in air strikes and to drop pamphlets as part of psychological warfare operations.

The Skymaster has a unique sound to go with its odd look. The two engines are rarely in sync and thus produce a sort of fluctuating waa-waa-waa-waa thrum. If you're ever in Montgomery County during rush hour, you've probably seen it or heard it or both.

In these anxiety-filled times, the plane can look a little menacing. I once saw it circling downtown Silver Spring, and I wasn't sure whether it was up there keeping us safe or up there dumping anthrax out the window.

The plane flies twice every weekday, from about 6:30 to 9 a.m. and again from 4 to 6 p.m. It's the only plane in the area flown by a local traffic agency.

"When the program first started, I was one of the people who screamed it was a waste of money," said John, a radio mouthpiece clipped to his shirt collar.

Then he took his first flight and saw what could be accomplished from 1,500 feet up. If he spots a bag of trash in the middle of the road, he can call down for it to be picked up. If he notices unusual clots of traffic at intersections, he can ask that traffic signals be retimed.

"When there's something bad going on, this becomes a command center," John said.

And he doesn't rely on just his eyes. A big ball hangs off the underside of the airplane. Inside is a video camera and an infrared camera. A microwave antenna beams the images to the ground.

We took off at 6:53 a.m. (from Runway 14/32, for those keeping score), and by 6:56 we were sweeping over Montgomery Mall.

"We're out of Gaithersburg and here in three minutes," John said. "It's a nice capability."

It's a capability the poor schmoes on the ground probably wish they had. We didn't spot anything too bad the day I went up. John called a tow truck for a charter bus broken down on Colesville Road. He noted some traffic signals that needed work.

Mostly, it was just the normal sludgy progression of a morning rush hour. It was definitely more fun being above it than in it.

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Traffic engineer John Lottes, left, and pilot Eddie Vendetti watch the Montgomery County commute from on high in a Cessna 337 Skymaster.