When the pilot of an Air Georgian business jet flying to Toronto started to lose cabin air in 2003, he did what crews are trained to do — he shoved the plane into an emergency descent.

With just minutes of oxygen available in the emergency supply, he had to get the plane down to breathable air, and fast.

But in a nearly catastrophic twist, the manoeuvre meant to save lives instead sent the seven-passenger Cessna Citation plane swooping into the path of a Delta MD-80 airliner.

Only a sophisticated collision avoidance system prompted the Delta pilot to take evasive action, saving up to 140 lives.

The cause of the depressurization was later discovered to be a faulty door seal — a problem that had been showing up sporadically in joint U.S.-Canadian data on the Cessna since 1996, yet had never been addressed by Canada's airline regulator, Transport Canada.

A joint investigation into airline safety by the Toronto Star, the Hamilton Spectator and The Record of Waterloo Region shows that aviation mechanical malfunctions have been rising, even though the number of takeoffs and landings has decreased.

The investigation has also shown that more than 80,000 passengers have been placed at risk over the past five years when planes came too close together in the sky.

Experts say many recurring safety problems have only been addressed after they killed people, leading to so-called "tombstone improvements."

The industry needs to do a better job identifying trends in mechanical problems that, however small, can lead to tragedy, they say.

Transport Canada, which regulates the airline industry, says it doesn't see any disturbing overall trends in mechanical problems since 2000.

However, the newspapers' investigation found that, in 2004, there were close to 1,800 mechanical incidents reported. Incidents of depressurization were up 23 per cent from 2000 to 2004, the last complete year of data available.

The most serious mechanical incidents — forcing emergency descents like in the Air Georgian case — are rarer, with 10 last year involving commercial planes, according to the Transportation Safety Board (TSB), which investigates airline incidents.

The air leak in the Air Georgian plane was first noticed by a passenger who heard a whistling sound coming from the door, says a TSB incident report. The man was barely back in his seat when the whistle turned to a roar, with pressurized air sluicing past a faulty door seal.

The noise was so loud the pilots couldn't hear the urgent calls from air traffic controllers to stop losing altitude.

The Transport Canada incident report later said the plane had originated in the Turks and Caicos Islands where it was exposed to high humidity.

Then, at a stopover in Georgia, it was raining. All that water seeped into the door seal and froze at a high altitude, leading to the depressurization and the Cessna's emergency landing.

Neither the Mississauga-based Air Georgian nor Delta would comment on the incident.

Cessna indicated to Air Georgian that "they have had a few like incidents reported before" and recommended checking the door seals in high humidity conditions, the incident report says.

U.S. data show as early as 1996, there had been sporadic reports of Cessna Citation door seals failing because of water, including three that forced emergency descents.

All of the emergency descents involved Canadian planes, although the data do not reveal who the operators were.

Loss of cabin pressure can quickly lead to unconsciousness for crew and passengers, turning aircraft into ghost planes that careen to the ground when they run out of fuel.

Professional golfer Payne Stewart was killed in just such a crash in 1999 and a depressurized Helios Airways 737 crashed into a mountainside in Greece last year killing 121.

Halifax aviation consultant Alex Richman says if there had been a crash between the Cessna and the Delta airliner, the door seal problem would have got a lot more attention than it did. The system to fix mechanical defects is flawed, he says.

"As much attention has to be paid to rare events as is paid to frequent events," Richman said. "Eighty-eight per cent of unscheduled maintenance is not predicted by the maintenance manual or by the airworthiness directives," he said.