Feb 19 (Reuters) - 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 ( DAL ) 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?

U.S. 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 Centre 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."