The alert system on a Delta Air Lines jet that flipped upside down and burst into flames as it tried to land in Toronto last month indicated a high rate of descent less than three seconds before touchdown, according to a prelimi nary report released on Thursday.The Transportation Safety Board of Canada, which issued the report, continues to investigate the Feb. 17 crash-landing in which 21 people were hospitalized.All 76 passengers and four crew members survived when the Delta plane arriving from Minneapolis burst into flames after flipping over and skidding on the tarmac.RELATED STORY | Truth Be Told: Flying is still exceptionally safeThe TSB of Canada report says that when the planes ground proximity warning system sounded 2.6 seconds before touchdown, the airspeed was 136 knots, or approximately 250 kph (155 mph). It says the planes landing gear folded into the retracted position during touchdown and the wing detached from the fuselage, releasing a cloud of jet fuel, which caught fire.The safety board says its ongoing investigation is focusing on several key areas, including metallurgical examination of the wing structure, landing techniques, pilot training and the passenger evacuation process.RELATED STORY | Remains of all 67 victims of the deadly plane-helicopter crash recoveredAll of those who were hospitalized were released within days of the crash.At least two lawsuits have been filed in the United States, and a law firm in Canada has said that its been retained by several passengers.Delta declined to comment on the preliminary report.We remain fully engaged as participants in the investigation led by the Transportation Safety Board of Canada. Out of respect for the integrity of this work that will continue through their final report, Endeavor Air and Delta will refrain from comment, the airline said. ...read more read less