SAN FRANCISCO : A Boeing 777 aircraft crash landed at San Francisco International Airport on Saturday, US aviation authorities said, with no immediate reports of casualties.Pictures and video showed the plane, an Asiana Airlines passenger aircraft, engulfed in a large plume of smoke on the runway.One dramatic photo tweeted by someone claiming to be a survivor showed people streaming out of the jet, with the aircraft's belly on the ground and its tail missing. An inflatable slide was visible at the front entrance."I just crash landed at SFO. Tail ripped off. Most everyone seems fine. I'm ok," the survivor, David Eun, wrote.News reports said the airport had been closed as firefighters and emergency personnel headed to the scene.