in San Diego-Muted to a pale disk, the morning sun glimmers behind a thin fog that hangs over Montgomery Field, a general aviation hub in the heart of San Diego. On a normal day, the cacophony of man-made flight would fill the air here: Cessna propellers gurgling to life; sleek Beech twin-engine commuters buzzing across the tarmac; the reverse-thrust of small jets landing to dispense overnight packages and busy executives. Today, though, a lone Cessna 182 cracks the quiet, its propeller kicking clouds of red dust past a sea-green hangar and across the deserted runways. Since Sept. 11, general aviation traffic here has plunged by about 75 percent. Although 19 terrorists on that day slammed jumbo jetliners with full fuel tanks into American landmarks thousands of miles away, some people at Montgomery Field saw evil up close. Two of the 9/11 suicide hijackers, Nawaf Alhazmi and Khalid al-Midhar, took flying lessons here in May 2000. But the pair didn't want to learn to fly small planes, according to their instructor, Rick Garza. Instead, they yearned to leap straight into the driver's seats of Boeing passenger jets. "It's not happening," Mr. Garza told them, and gave them six lessons apiece in a single-engine plane. Alhazmi and al-Midhar flunked out of pilot school, but not out of jihad. And the ease with which they and other 9/11 terrorists acquired American flight training, coupled with the exposed-throat nature of U.S. general aviation, raises new questions about the next terrorist weapon of choice. For years, homeland-defense experts have focused on four major categories of potential terrorist threats: chemical, biological, nuclear, and cyber attacks. This left Islamic extremists to exploit the security weakness of commercial aviation while experts focused on other, more high-tech threats. As a result, commercial air security is now drum-tight. But general aviation security isn't, and that raises a question: Could death soon rain from the sky again? More than 345,000 general aviation aircraft are registered with the Federal Aviation Administration. That includes helicopters, small aircraft, private jets-pretty much everything except commercial airliners. Approximately seven in 10 of the 170,000 flights that crisscross the nation daily are flown by pilots who don't file flight plans, and who aren't required to check in with air traffic control (ATC) except to obtain takeoff or landing clearance. Many operate from fields with no control tower at all: Pilots activate navigational aids and airport lighting via radio signals. That means more than 100,000 general aviation aircraft fly unmonitored over American cities every day. Asked whether a terrorist could conceivably pack a light plane or small jet with explosives, take off legally (or undetected), and fly unmonitored to destroy a target of choice, pilot and Arizona State University professor Danny Peterson said yes. "The threat is certainly there," said Mr. Peterson, a former military base security expert who teaches a class called Terrorism and Weapons of Mass Destruction. "I used to live in Victorville, [Calif.,] and fly to Catalina Island," he said. "I'd fly near Disneyland and Anaheim Stadium and wonder why someone hasn't done that before." The Department of Transportation recognizes the potential threat of small aircraft to targets of opportunity. Earlier this month, the agency grounded all U.S. crop-dusters to prevent possible bio-terror strikes. And, until last week, Ronald Reagan National Airport remained closed to all air traffic. Federal law-enforcement agencies were concerned about the airport's proximity to plum terrorist targets, such as the Pentagon and the White House, which are just seconds from the departure end of Reagan National's runways. In 1994, Frank Eugene Corder, 38, of Perry Point, Md., crashed a small plane onto the White House's South Lawn. Federal officials have considered a permanent ban on general aviation at Reagan National. Although federal and local law-enforcement agencies have buttoned up security at other airports that host commercial passenger traffic, the same is not true of general aviation fields. "There is absolutely zero security there," said Jim Thompson, an aviation attorney and licensed pilot who flies a Cessna Citation-a small jet often used for corporate travel-owned by his law firm. Mr. Thompson said that before Sept. 11, he and other flight crew used to leave the Citation parked overnight with the doors open because of high temperatures at some airfields in the Southeast. Such fields provided no security personnel or even full-time airport employees to keep an eye out for thieves. Since the 9/11 attacks, Mr. Thompson's firm has tightened security to ensure no one steals the company jet. But a terrorist might not need to go to the trouble of stealing a plane. Asked to peg the most exploitable weakness in general aviation, Mr. Thompson cited the ease of simply renting one. "You could go right over here to [the airport], show a valid FAA medical certificate, a pilot's license, and a logbook showing current flight hours," he said. "Give 'em 50 bucks and you've got yourself a plane you can then use to crash into a building or throw lethal chemicals out the window." An estimated 20,000 new pilots earn licenses each year in the United States, according to the FAA. A sobering question: How many terrorists who did not die on Sept. 11 have earned theirs in recent years? But neither Mr. Thompson and nor Arizona State University's Mr. Peterson believe a light plane such as a Cessna 172 or Piper Cherokee would pack enough wallop to interest a serious terrorist. On the other hand, a larger corporate jet might. Some major firms own and maintain fleet aircraft, such as Leer and Gulfstream jets, and even 50-passenger Airbus jetliners, and park them at poorly secured general aviation airports. "It's a lot easier to hijack something like that," Mr. Peterson told WORLD. Without flight attendants to warn of a hijacker's takeover of a cabin, without cockpit/cabin intercom systems, and without much of what Mr. Thompson calls "standup fighting room," a terrorist could easily commandeer what would in effect be a piloted missile loaded with enough jet fuel to cause significant damage. Still, while the destructive power of a light plane, or even a larger corporate jet, cannot match the architectural demolition power of a Boeing 757 or 767, what about the impact of such a craft on a large gathering of people? Islamic terrorists have declared themselves at least as interested in exacting high "infidel" body counts as they are in the symbolic significance of buildings toppled. Many large cities are home to airports that lie in close proximity to venues where large groups gather. Montgomery Field, for example, is located on Aero Drive off Interstate 15, about 2 miles from 70,000-seat Qualcomm Stadium. That's not a far hop for a determined terrorist aiming to wipe out a few thousand American football fans on a Sunday afternoon. A pilot could take off under visual flight rules (VFR), request to fly south along the interstate, veer west over the Qualcomm parking lot, and nosedive into the crowd. Controllers would deny a request for such a flight today, because the Federal Aviation Administration (FAA) has quashed all VFR flights in "Class B" airspace, a type generally surrounding airports in heavily populated urban areas, and the type in which Montgomery Field is situated. But the restriction is probably temporary. As of last week, the FAA had lifted VFR flight strictures across the nation, Class B airspace excepted, according to FAA Flight Service Station Supervisor Orrin Kelso. The FAA's Class B exception is meant to protect populous areas from any rogue VFR terrorists who might soon attempt a 9/11 follow-on attack. The idea is that aircraft operating in such airspace must be radar-identified, receive an ATC clearance, and enter a discrete numeric code that shows up on air traffic controllers' radar screens and simplifies flight tracking. But even the requirement to contact ATC can't deter a pilot determined to fly wherever he wants. In 1999, the pilot of a multi-engine Beech aircraft reported spotting what appeared to be a Pitts Special airplane performing aerobatic loops over Dallas, Texas. According to an FAA incident report, the Pitts Special was operating without ATC clearance in Class B airspace. Meanwhile, general aviation may also provide opportunity for terrorist strikes on U.S. military assets. At Marine Corps Air Station Miramar, for example, which shares the southern half of its airport traffic area with Montgomery Field, lost civilian pilots fly over (and have even landed on) military runways several times each year. In 1999, at Key Field, an Air Force Reserve base in Meridian, Miss., a Cessna Centurion breached airport airspace and nearly collided with a military aircraft operating near the runways. Such aircraft are presumed lost and never challenged. A "lost" terrorist could easily crash his general aviation aircraft into a military aircraft parking area and destroy millions of dollars worth of tactical aircraft. Despite the number of ways terrorists could further exploit American aviation, experts like Mr. Peterson believe terrorists are more likely to release biological agents in any new attack than they are to crash more planes. Still, he admits his "likelihood" formula has changed since Sept. 11. "What was the likelihood of the WTC attack?" he asks rhetorically: "Up until three weeks ago, I would've said extremely small."