The US Coast Guard, which monitors the airspace above the district, scrambled helicopters as a precaution.

The US military also dispatched F-16 fighters from Andrews Air Force Base as a precautionary measure.

Such airspace violations are not uncommon.

There are two concentric restricted zones over Washington, D.C., the outermost called the Air Defense Identification Zone or ADIZ.

Many pilots unintentionally violate the ADIZ as they move out of the zone's outer fringes.

The ADIZ was reshaped on August 30, 2007, from a "Mickey Mouse" shape to a circular shape to help reduce accidental incursions.

The innermost restricted airspace is the Flight Restricted Zone, or FRZ – a smaller and more restrictive zone. With a few exceptions, only scheduled airlines and governmental flights are allowed in the FRZ.

The Federal Aviation Administration added four air traffic controller positions last summer to help pilots transit the airspace.

Pilots had complained about having trouble getting permission to enter the airspace, forcing them to circle outside, which sometimes caused safety issues.

(Xinhua News Agency March 13, 2008)