TAMPA A military cargo plane that typically requires 3,500 feet for takeoff landed unexpectedly Friday at Peter O. Knight Airport, where the longest runway is 95 feet short. Work began immediately to lighten the load of the 174-foot-long aircraft so that it might leave Davis Islands safely. The drama ended at 8:27 p.m., when the C-17 Globemaster III took a hop over Hillsborough Bay to MacDill, the original destination. It landed just a few minutes later. It was unclear why the plane, headed to MacDill, made an unscheduled landing at the small airport near downtown Tampa. Master Sgt. Bryan Gatewood, a spokesman for MacDill Air Force Base, said authorities are investigating. Hillsborough County Aviation Authority spokeswoman Janet Zink said the plane landed "inadvertently" at Peter O. Knight Airport. The plane, arriving from U.S. Central Command operations in southwest Asia with 23 passengers and a crew of 19, touched down on Davis Islands about 1:20 p.m., authorities said. "It was so loud, it woke up my sister who was sleeping at the time," said Chelsea Alper, 23, a Stetson University College of Law student who was in a convenience store on E Davis Boulevard when she heard the roar of the engines. Minutes later, witnesses saw a caravan of military vehicles respond to the runway, retrieve the crew and begin to haul away cargo. According to an Air Force fact sheet, the bulky plane has a wingspan of nearly 170 feet. At 55 feet high, it appeared twice as tall as a nearby hangar, and from certain angles it eclipsed two blue-and-white buses that pulled onto the runway shortly before 5 p.m. Peter O. Knight is a general aviation airport operated by the Aviation Authority. It has two runways, including a smaller one that is 2,688 feet and a larger one that is 3,405 feet. The longest runway at MacDill is 11,421 feet. Over time, the runways of MacDill, Peter O. Knight and Tampa International Airport have occasionally been confused with one another, though unscheduled landings at MacDill have most often made the news. One week in 2004, two planes mistakenly landed there. In 1984, a commercial pilot mistook the base for Tampa International Airport and landed a jet loaded with passengers. This time, all the excitement happened at Peter O. Knight. "This is the second time this has happened," said spectator Gary Garrett, 71, who has a real estate office on Davis Islands. "The last time, it was a 727 in the '80s," he recalled, "and they took that plane apart to get it out of here." All traffic at Peter O. Knight Airport was grounded for seven hours. "I was supposed to leave about five minutes after that plane landed," said Ryan Gucwa, 29, a corporate pilot from Tampa. He was scheduled to pick up passengers at Tampa International and get them to Georgia on Friday afternoon. Instead, he caught a cellphone video of the C-17's amazing landing. "It stopped about 6 feet from the end of the runway; any farther and it would have been grass," Gucwa said. Hours later, Alper thought the takeoff would be impossible. "It was going so slow I didn't think it was going to make it," she said. Dozens of people stood outside the gated perimeter, forming an impromptu tailgate party, as the engines roared and the plane inched toward the sky. With about 400 feet to spare, the nose pulled up and the C-17 was back on course. Marti Smith, 58, a nurse from Tampa, came just in time for takeoff. "When he started moving, I started praying out loud," she said. "It was quite a show." Times news researcher John Martin contributed to this report.