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It's not explosives that scare the Secret Service, it's a WMD type attack.

Suppose that plane was carrying a pea size sample of radioactive Cesium 137 along with say 10lbs of explosives to disperse it. The WH and the surrounding area could be walled off for 30 years.

http://www.wired.com/news/conflict/0,2100,53110,00.html

If a relatively tiny "dirty bomb" -- one containing only ten pounds of TNT and pea-sized amount of cesium-137 -- were detonated in Washington, federation scientists recently told Congress, "The initial passing of the radioactive cloud would be relatively harmless, and no one would have to evacuate immediately."

"However," the scientists continued, "residents of an area of about five city blocks ... would have a one-in-a-thousand chance of getting cancer. A swath about one mile long covering an area of forty city blocks would exceed EPA contamination limits, with remaining residents having a one-in-ten thousand chance of getting cancer. If decontamination were not possible, these areas would have to be abandoned for decades."

As cesium-137 "cools" from its radioactive to its normal state, the isotope emits gamma radiation, waves of ultra-high electromagnetic energy. These rays, while not as toxic as the heavier, alpha particle emitted by uranium, travel further, and are extremely difficult to contain. Only concrete, steel or lead can keep gamma radiation in check.

What's worse, cesium is the most "reactive" metal there is -- in nature, cesium's always found combined with another element. So the isotope becomes easily attached to roofing materials, concrete, and soil, said Fritz Steinhausler, who led the International Atomic Energy Agency's environmental assessment of the disaster at Chernobyl.

"The Russians tried to clean it up for years, and they eventually gave up. It just wasn't economically viable," said Steinhausler, who's currently a physics professor and visiting scholar at Stanford University's Center for International Security and Cooperation.