The pilot was just as surprised as the passengers when a propeller plane skittered off an Edmonton runway and partially collapsed, says a federal investigator.

"He didn't have any idea that anything was wrong at all in the air," said Barry Holt, regional senior investigator with the Transportation Safety Board in an interview with CBC.

The aircraft's 71 passengers and four crew members braced themselves as the plane's right side crashed to the ground on the landing.

The force of impact sheared the propeller's blades right off and launched them through the cabin. Three people were hurt, but not badly, by pieces of the plane wall that broke and flew into the air.

No one knew exactly what happened to cause the crash that night in November 2014 — until now.

This map of the inside of the plane shows where the propeller crashed through the cabin. (Transportation Safety Board) a "total fluke." It's something Holt calls

The plane blew a retreaded tire on takeoff from Calgary, which is not ideal but not uncommon, Holt said. Crews arranged to change planes in Edmonton, and went in for what was expected to be a routine landing.

Instead, the vibration from a blown retreaded tire somehow matched the harmonics of the landing gear, triggering a sensor that made that gear collapse.

High-flying mystery

"It wasn't likely to happen. No one could foresee this kind of thing," said Holt.

A mystery. One that took several months to figure out. Investigators found nothing mechanically wrong with the plane. And the flight crew had the right training, they were well-rested, and did everything they should, Holt concluded.

He and his team spent a year and a half doing hundreds of tests, recreating the conditions of the landing. Called "dynamic vibration testing," Holt said it's a type that "had never been done anywhere in the world" before.

And his report suggests that needs to change.

"If there are no specific requirements for dynamic vibration testing of components or completed airframes, there is a risk that similar or other aircraft systems could fail during high-vibration conditions," the report states.

How safe is it to fly, really?

The International Air Transport Association (IATA), a trade organization that represents 260 airlines, or 83 per cent of total air traffic, can give a better idea of just how rarely accidents like this happen.

The agency says accident numbers overall are lower than ever.

For the year 2014, IATA recorded "the equivalent of one major accident for every 4.4 million flights," which the agency said is the lowest rate in the history of aviation.

The agency says runway excursions like the one in Edmonton, where an aircraft departs a runway during landing or takeoff, account for 22 per cent of accidents over five years from 2010 to 2014. They caused less than seven per cent of deaths in that same five-year span.