And then there's the criminal underground angle. The drug smuggling business uses a lot these aircraft to move their merchandise. Terrorists often work with criminal gangs to obtain false identifications, weapons and other illegal services. At the moment, legitimate aircraft rental operations are on their toes, and any foreigners seeking to rent a twin engine aircraft would be carefully scrutinized. But al Qaeda's top operatives (and hundreds of these guys are unaccounted for) were trained in the techniques of working around such obstacles. Latin America has a lot of aircraft and weapons suppliers who deal with local rebels and drug gangs. South America and Africa are also the home of many small air freight operations using long range B-707 or DC-8 aircraft. There aircraft fly into and out of the United States, and the place they fly from are known for corruption and an attitude of "money will buy anything." A four engine DC-8 jet freighter can be bought for less than a million dollars.
Since September 11, 2001, aircraft rental and sales companies that deal in twin engine aircraft have been alerted to the possibility of this kind of attack. But if someone, that doesn't look Middle Eastern comes in with the right documents (to allay suspicion), the aircraft could be obtained. Bribes could obtain the right paperwork, and a long range aircraft, in South America or Africa. The cargo could even be legit, although a load of explosives would enhance the attack. Flying into New York or Los Angeles, two areas that see a lot of international air freight traffic, nothing would be amiss until the aircraft deviated from it's flight plan and headed for a large building. Would there be enough time for the F-16s to get airborne?
If you have the money, you can get what you want and fly a suicide mission without worrying about passenger security. It's not quite as easy as it sounds, but it is possible.
Despite all the efforts to prevent terrorists from hijacking commercial aircraft for suicide attacks, there is still a substantial vulnerability. While passenger aircraft are now pretty secure, the same is not the case for commercial freighters and private aircraft. It is quite possible that a smaller aircraft, or long range transports from foreign nations, could be used for suicide attacks. This scenario has terrorists renting a small two engine aircraft (like the Piper Aztec or Cessna Businessliner) and flying off to any target within several hundred miles. These aircraft rent for about $250 an hour (with a 3-4 hour minimum). They have a cargo capacity of about half a ton, and that could be filled with explosives. This would give the terrorists the equivalent of an American cruise missile (which has a one ton warhead.) These aircraft have a maximum take off weight of about three tons and only carry about 500 pounds of fuel. Probably would not bring down a large skyscraper, but would do a lot of damage to the White House or most other government buildings in Washington. You can buy these aircraft second hand for $200-300,000.