As mini-copter slips through Washington's defences, many ask: How?

WASHINGTON (AFP) - By brazenly flying a mini-helicopter through Washington's no-fly zone and landing it on the US Capitol lawn, a 61-year-old pilot stunned the security world and prompted the question: Why wasn't he detected?

The city essentially became fortress Washington after the 9/11 attacks of 2001, when the military added Humvee-mounted Avenger missile launchers, intensified aircraft detection systems and integrated air security operations.

A huge hovering radar blimp casts a protective eye over the National Capital Region, and monthly night-time dummy flights test the city's air defence system.

Fighter jets can scramble at a moment's notice when alerted to danger.

Yet none of these defences kicked in to prevent pilot Douglas Hughes from flying his gyrocopter through layers of restricted air space Wednesday, past the White House and down the National Mall, apparently unnoticed except by stunned tourists, to the domed building where Congress was in session.

The daring feat prompted an area shutdown and left security officials flummoxed.

"This individual apparently literally flew in under the radar," Homeland Security Secretary Jeh Johnson told reporters Thursday.

Johnson admitted he had the same reaction as many Washingtonians, one that suggests authorities are not cognisant of every possible airborne threat: "What's a gyrocopter?"

In January, a man inadvertently crashed an unmanned hobby drone into the White House gardens. President Barack Obama afterwards called for closer regulation of drones, as debate swirled over whether to ease restrictions on their use.

Hughes, a Florida mailman who planned the stunt as a way to draw attention to campaign finance reform, is suspected of flying his ultralight aircraft from an airport in Gettysburg, Pennsylvania.

Flight experts say he would have had about 90 minutes in the air, including half an hour flying through the Special Flight Rules Area ringing Washington and which requires a flight plan and radio contact with air traffic controllers.

He cut a path through the smaller Flight Restriction Zone, which has tighter flight constraints and is open only to commercial planes flying in or out of Ronald Reagan National Airport or aircraft with federal authorisation.

Hughes, armed only with 535 letters addressed to every lawmaker in Congress, then entered the Secret Service-controlled no-fly zone immediately surrounding key buildings like the White House.

"Norad (North America's air defence command) was not made aware of the gyrocopter until after it landed on the Capitol grounds," Jamie Humphries, a Norad spokesman, told AFP.

'LOW-SLOW' THREAT

The flight by Hughes, who was promptly arrested and was to appear in court Thursday, set lawmakers on edge, with hawkish Republican Senator Lindsey Graham citing the threat of Islamist terrorism as justification for using potentially deadly force against Hughes.

"He should have been subject to being shot out of the sky. I don't know why he wasn't," Graham said on the Hugh Hewitt radio show.

Washington's air defences are largely focused on traditional threats such as incoming missiles or larger planes, including hijacked jets.

Norad has responded to more than 5,000 "possible air threats" nationwide since the 2001 attacks. Some have turned out to be nothing more than a flock of geese.

"Detecting and tracking low-slow fliers - and differentiating them from weather, terrain, birds, and other radar clutter - is a technical and operational challenge," Norad's Michael Kucharek acknowledged.

In August 2010, commanders came close to shooting down an unmanned US Navy helicopter when it flew out of control from neighbouring Maryland towards Washington, but operators managed to regain control of the aircraft.

Norad says the blimp over Washington, known as Jlens, has the potential to help detect threats like low-slow fliers, but it remains in test phase and has yet to be integrated to the region's air defenses.

That system's coordination is under review.

Jennifer Orsi, managing editor of the Tampa Bay Times which documented Hughes' flight preparation, said the paper called the Secret Service and US Capitol Police 30 minutes before Hughes landed to ask if they were aware of his flight. They said they were not, the paper reported.

A spokesman said the Secret Service interviewed Hughes in 2013 about his apparent plans to land a small aircraft on the Capitol grounds.