Police officers and other officials lift a gyrocopter that landed on the Capitol South Lawn on to a trailer on April 15. (Paul J. Richards/Agence France-Presse via Getty Images)

DOUG HUGHES, the letter carrier who piloted a featherweight gyrocopter over the treetops to alight on the Capitol lawn Wednesday, was acting out a one-man “voter’s rebellion.” In the name of campaign finance reform and “honest government,” he managed to slice through the world’s best-protected airspace no more molested than the birds who wheel and soar in the breeze.

Mr. Hughes turned out to be harmless — not to mention right that current campaign finance rules are a blot on U.S. governance — so thank goodness he wasn’t shot down.

But why, with all the billions spent on homeland security and the special attention lavished on the no-fly federal core known to aviation officials as Area 56, could the combined agencies — Secret Service, Federal Aviation Administration, Capitol Police, FBI — not shut down Mr. Hughes before he took off from Gettysburg Regional Airport in Pennsylvania or divert him before he reached the Capitol?

Federal officials may offer some boilerplate explanation about no security regime being truly airtight, no matter its level of resources and funding. Maybe it is undeniable that the Capitol remains a magnet for the unhinged, the unbalanced and the unconventional — and not only for those elected to serve there.

Still, it’s not as if the authorities were blindsided by Mr. Hughes. In 2013, the Secret Service interviewed him the day after it was informed by “a concerned citizen” that Mr. Hughes wanted “to land a single manned aircraft on the grounds of the United States Capitol or the White House,” agency spokesman Brian Leary told The Post.

Then, a year ago, when Mr. Hughes announced his plan on his Web site, the Secret Service again took notice. This time an agent interviewed not only him but also friends and co-workers at the post office where he works in Riverview, Fla.

That information was reported by the Tampa Bay Times, which received explicit advance notice of his plans but failed to bring them to the attention of authorities until it posted an article and video on its Web site as Mr. Hughes was taking off, just an hour’s flight from the Capitol.

In 1987, a German teenager and novice pilot named Mathias Rust crossed through Soviet airspace in a small plane and landed a stone’s throw from Red Square, to the world’s amazement. That act of plucky lunacy furnished then-Soviet leader Mikhail Gorbachev with a pretext to purge the security forces of hard-liners opposed to his reforms.

No one expects Mr. Hughes’s flight to play such a historic role. But the White House and congressional leaders should be asking hard questions about whether the nation’s capital is safe from small aircraft, which next time might be piloted by someone more malevolent than Mr. Hughes.