Normally, a small civilian aircraft can fly fast enough that there is not enough time to get F-16s into the air and in position for an interception. There are said to be Stinger crews in the White House (or personnel trained to use Stingers.) But firing the Stinger from the White House is a tricky business, as there is less than a minute between the time the approaching aircraft comes into range and it hits the White House (unless the missile takes out an engine and forces the aircraft to land somewhere else.) There are some UH-60s stationed in the area, with people on board armed with M-16 assault rifles. These choppers can be airborne in minutes. These choppers intercept several aircraft a week that have entered the no-fly zone. But these helicopters are there mainly to protect the White House from Congress (as in "what are you doing about all these unauthorized aircraft over Washington.")
The helicopters could also be equipped with Sidewinder air-to-air missiles. The Sidewinder has a range of six kilometers and has been mounted on helicopters for years. U.S. Army Avenger systems (a hummer armed with Stinger, a .50 caliber machinegun, radar and laser range finder) have been seen in the area, as well as Stinger missile teams. The Stinger has a range of only 4.5 kilometers. Effective range of the .50 caliber machine-gun is more like two kilometers. There could also be teams of snipers equipped with .50 caliber rifles (firing armor, or engine block, piercing incendiary bullets). These could also take down one of these aircraft, or at least disable the engines and ruin the attackers aim.
Before you can knock down a suicide aircraft, you have to be able to track the intruder, and if the pilot comes in at tree-top level, that will be difficult. Flying like that within the DC area suburbs is also dangerous, for there are numerous office parks within that area, many with buildings over a hundred feet high. But the no-fly zone is 1800 square kilometers, and each Stinger team is only covering about 14 square kilometers. The Avenger can move around, if it doesn't get stuck in traffic, but is unlikely to be able to cover more than a kilometer (or two) a minute, while trying to get in position for a shot. Because of their short range, the Stingers could only be used as a last line of defense, and would probably be stationed within 5-10 kilometers of the White House. While the attacker could further complicate the situation by coming in at night, this makes it more likely to collide with some high rise obstacle that is not well lit.
The May 11th intruder was traveling slow enough to allow the UH-60s helicopters and F-16s to get airborne and intercept. The helicopters were next to the aircraft in about ten minutes, while the F-16 showed up about five minutes later. At the same time, five minutes after noon, someone on the ground finally made contact with the intruding aircraft, and got it to turn around. The F-16 was ready to fire a missile at the civilian aircraft, and the helicopter was pulling away to allow that.
But a faster civilian aircraft could have made it to the White House in under eight minutes. Air defense above Washington DC is still a risky, and unpredictable, business.
50 AM on May 11th, a single engine civilian aircraft, because it was lost, entered the forbidden zone around the While House. Such aircraft are not supposed to fly within 25 kilometers of the White House (which is close to many other important government buildings in downtown Washington.) The intruding aircraft kept coming, but not at maximum speed. The nightmare scenario has a faster, twin engine, aircraft, piloted by a suicide bomber and carrying about 500 pounds of explosives. This gives you the equivalent, in destructive power, of a Tomahawk cruise missile. Such aircraft are fast enough to breach the no-fly zone and hit the White House in eight minutes. The government recently released details of this incident, including the times various events took place.