Investigators are probing the causes of an unusual plane crash at Canada's largest airport on Monday, when a regional jet flipped upside down upon landing during windy weather, sending 21 of the 80 people on board to hospital.
Video shows the Delta Air Lines plane belly up and missing its right wing at Toronto's Pearson Airport, and of the crash that involved no fatalities, circulated widely on social media. The Transportation Safety Board of Canada said on Tuesday that parts of the plane -- a Bombardier-made CRJ900 -- separated after impact and the fuselage came to rest slightly off the right side of the runway, upside down, facing the other direction.
The TSB said it is too early to know what happened and why. Here is what we know about this accident and similar crashes.
HOW DOES A PLANE LAND UPSIDE DOWN?
US aviation safety expert Anthony Brickhouse said aircraft are normally designed to land first on the two main landing gear, and then the nose gear. While the cause of the accident is unclear, the type of impact on the runway likely damaged the landing gear, leaving the plane imbalanced.
Brickhouse said that the plane ending up pointing in the opposite direction speaks to the amount of force and speed that led it to change direction.
"With all the forces and everything going on, if that wing is not there to support the aircraft it's going to go over," Brickhouse said. "It's not something that we see regularly, but when structures start failing, they can't do their job and the aircraft is going to react to the different forces on it."
HOW DID EVERYONE SURVIVE?
Passengers say they were hanging upside down in their seats after the crash.
"All of the passengers were wearing the safety belts. This prevented more serious injuries from occurring," said Mitchell Fox, director of the Asia Pacific Center for Aviation Safety.
Airplane seats are designed to withstand the force of 16 times the normal pull of gravity, or 16Gs, in a crash, whereas wings and fuselage are designed to handle 3-5Gs.
"In an impact-survivable crash, it's more important for the seats to hold up, giving passengers the best chance of survival," said Raj Ladani, a program manager for aerospace engineering at Australia's RMIT University. Good evacuation is key to air accident survivability, as witnessed last year when all 379 people escaped a burning Japan Airlines plane after a runway collision.
"The crew did a remarkable job of evacuating all of the passengers expeditiously," Fox said of the Delta crash.
HAS THIS HAPPENED BEFORE?
While rare, there have been cases of large jets flipping over on landing, including three accidents involving McDonnell Douglas' MD-11 model.
In 2009, a FedEx freighter turned over on landing in windy conditions on the runway at Tokyo's Narita airport, killing both pilots. The left wing was broken and separated from the fuselage attaching point and the airplane caught fire.
In 1999, a China Airlines flight inverted at Hong Kong while landing during a typhoon. The plane touched down hard, flipped over and caught fire, killing three of 315 occupants.
In 1997, another FedEx freighter flipped over at Newark in the United States, with no fatalities.
Brickhouse said it is too early to draw any conclusions from these earlier cases, especially as the MD-11 is a three-engine aircraft and the CRJ900 has two engines mounted toward the back of the aircraft, producing different flight dynamics.
HOW WILL THE INVESTIGATION PROCEED?
Unlike other investigations in which parts of the plane have gone missing, and there are mass fatalities, investigators will be able to interview all 76 passengers and four crew.
Investigators have access to the fuselage and wing, which are on the runway, and the black boxes -- the flight data and cockpit voice recorders -- have been sent for analysis.
"This is going to be a textbook investigation," Brickhouse said. "Some accidents, a lot of the pieces of the puzzle are missing. But right now looking at this accident, all the puzzle pieces are there. It's just you piecing them back together at this point."
