A passenger jet flipped onto its roof while landing in Toronto, Canada, the fourthmajor aviation accidentin North America in the past three weeks.While at least 18 people were injured, all 80 people on board the Delta Air Lines flight from Minneapolis survived the crash Monday.Here are some things to know about the crash:What caused the airplane to flip?Communications between the tower at Torontos Pearson International Airport and the pilot were normal on approach and right now its not clear what went wrong when the plane touched down.Were strong winds a factor in the crash?Toronto Pearson Fire Chief Todd Aitken has said the runway was dry and there was no crosswind conditions.Audio recordings indicate that the control tower warned the pilots of a possible air flow bump on the approach. Winds were gusting up to 40 mph (65 kph) during the day at the airport, according to the Meteorological Service of Canada.But airplanes and pilots should be equipped to handle those kind of winds while landing, said John Cox, CEO of aviation safety consulting firm Safety Operating Systems in Florida.Were the passengers badly injured?Those hurt had relatively minor injuries, the airports chief executive said.The airport fire chief said 18 passengers were taken to the hospital. An air ambulance operator said it had transported one pediatric patient and two adults to hospitals. Delta said Tuesday that some of those injured had been released.What happened inside the plane?One passenger told toldCBC Newsthat he found himself upside down and still strapped in his seat after a forceful landing.Peter Carlson said he crashed onto the ceiling when he took off his seat belt and smelled gas. He and another man helped a mother and her young son out of the plane before getting out.Who is investigating?The U.S. Federal Aviation Administration said the Transportation Safety Board of Canada will lead the investigation. The National Transportation Safety Board in the U.S. said it was sending a team to assist.Is it safe to fly?The fourth major aviation accident in North America in less than a month has many people concerned aboutthe safety of flying. Fatal crashes remain rare and the track record of U.S. airlines is remarkably safe.But there have beendeadly crashes recentlyaround the world and U.S. officialshave been raising concernsabout an overtaxed and understaffed air traffic control system for years.By JOHN SEEWER Associated PressAssociated Press writers John Wawrow and Michael Casey contributed to this report.