WASHINGTON -- A new system of lasers designed to warn pilots they have entered restricted airspace over Washington can't be used on planes flying in or above the clouds.

Trouble is, clouds cover most of the sky almost half the time in the nation's capital.

The limitations of the laser warning system were evident during an airspace violation Monday, when military F-16s escorted a small plane from a restricted area to a nearby airport.

The laser system wasn't engaged because it couldn't penetrate the layer of clouds over which the pilot was flying, said 1st Lt. Lisa Citino, a spokeswoman for the North American Aerospace Defense Command, or NORAD.

"We know we can't use the system 100 percent of the time, but, remember, the system isn't a stand-alone one," Citino said. "It's just one of the other systems we have in place."

Though NORAD won't disclose the cost of the red-and-green laser warning system, its price tag is at least $1 million. NORAD acknowledged that each set of lasers costs $500,000, and two have been sighted by The Associated Press.

Between sunrise and sunset, clouds covered at least 88 percent of the sky over Reagan Washington National Airport for 162 days last year, according to the National Climactic Data Center.

Most small planes, though, fly below the clouds, said Chris Dancy, spokesman for the Aircraft Owners and Pilots Association, which represents private pilots. "The laser would still be effective," Dancy said.

During Monday's incident, the plane had permission to fly through the restricted airspace en route to Gaithersburg, Md., from Knoxville, Tenn., because the pilot filed a flight plan and maintained radio contact with air traffic control, said Federal Aviation Administration spokesman Greg Martin.

But lightning struck the small Canadian aircraft and he lost his radio, Martin said Tuesday. The pilot never got closer than 10 miles from the White House, he said.

Government officials who spoke on condition of anonymity said the pilot's one mistake was that he didn't switch his transponder code to indicate he had no radio.

Once the plane's communications went out, fighters took off from Andrews Air Force Base in suburban Maryland and escorted the plane to the Gaithersburg airport.

The rationale for the system of warning lasers, which started operating Saturday, was to avoid the expense of such scrambles, according to NORAD.

Hundreds of planes have entered the restricted area -- about 2,000 square miles radiating from the three commercial airports around Washington -- since it was established in February 2003.

Most planes that transgress do so by skirting the edge of the zone by mistake, said Dancy.

Citino said fighters are scrambled when planes enter the zone and don't communicate with the ground.

On May 11, a Black Hawk helicopter, a Citation jet, and two F-16s were scrambled to intercept a plane that flew unusually far into the restricted zone -- about three miles from the White House. Thousands of people were evacuated from government buildings.

The FAA revoked the license of the May 11 pilot, Hayden L. "Jim" Sheaffer, on Monday after determining he was so careless that he "constitutes an unacceptable risk to safety in air commerce."

Sheaffer's lawyer, Mark McDermott, said Tuesday that he will appeal the revocation.

"They almost got shot down because the government didn't know its helicopter radio was not receiving," McDermott said.

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