San Francisco is one of several airports around the country that border bodies of water that have walls at the end of their runways to prevent planes that overrun a runway from ending up in the water.Since the plane was about to land, its landing gear would have already been down, Barr said. It's possible the landing gear or the tail of the plane hit the seawall, he said. If that happened, it would effectively slam the plane into the runway, he said. Noting that some witnesses reported hearing the plane's engines rev up just before the crash, Barr said that would be consistent with a pilot who realized at the last minute that the plane was too low and was increasing power to the engines to try to increase altitude. Barr said he could think of no reason why a plane would come in to land that low. Belding was out jogging just before 11:30 a.m. on a path across the water from the airport when she noticed the plane approaching the runway in a way that "just didn't look like it was coming in quite right." "Then all of a sudden I saw what looked like a cloud of dirt puffing up and then there was a big bang and it kind of looked like the plane maybe bounced (as it neared the ground)," she said. "I couldn't really tell what happened, but you saw the wings going up and (in) a weird angle." "Not like it was cartwheeling," she said, but rather as though the wings were almost swaying from side to side. The flight originated in Shanghai, China, and stopped over in Seoul, South Korea, before coming to San Francisco, airport officials said. The airline said there were 16 crew members aboard, and the 291 passengers included 77 South Koreans, 141 Chinese, 61 Americans and one Japanese citizen. The nationalities of the remaining passengers weren't immediately known.