Brazilian search planes spotted possible debris from Air France Flight 447 about 650 kilometres northeast of the Archipelago of Fernando de Noronha. (CBC)

Brazilian ships and French and Brazilian military planes began the search hours after an Air France passenger jet disappeared from radar screens, Monday, June 1, 2009, on a flight to Paris from Rio de Janeiro.

The jet is presumed to have crashed in the Atlantic Ocean, somewhere off the coast of Brazil.

But the last known location of Air France Flight 447 makes finding the plane a difficult task, said a former senior investigator with the Transportation Safety Board of Canada (TSB).

Nick Stoss, the past director of air investigations with the TSB, said locating the Airbus 330-200 aircraft or any of the 228 people who were on board is complicated by the fact that the plane was outside the reach of radar and over deep ocean waters.

"It's a significant search area," he told CBC News.

Brazilian military ships and planes are searching the area near the archipelago of Fernando de Noronha, more than 300 kilometres off the most eastern point of Brazil, while French planes that took off from Senegal look farther east.

France has also called upon the United States to aid with satellite imagery. Ground radar stations are usually able to track a plane's whereabouts within 400 to 500 kilometres from the coast, said Stoss, with higher altitude planes easier to detect.

The Air France jetliner was said to be hundreds of kilometres from the Brazilian coast when it sent out an automatic message signalling a malfunction of the plane's electrical circuitry about four hours into the flight. The plane reportedly hit a severe electrical storm.

Without the assistance of radar, air officials rely on automatic messages like the one sent at 10:36 p.m. ET from Flight 447 to not only keep them informed of the technical information of the aircraft but also to let them know the aircraft's location.

Air crews also routinely check in with status updates, but these are infrequent, often once an hour, whereas the automatic transmissions can occur about every five minutes or when there is an issue, said Stoss.

Aircraft also have the ability to send a distress signal using an Emergency Locator Transmitter, or ELT. These transmitters can send a signal at 406 MHz that satellites can pick up and send to ground control stations, but it's unclear whether the Air France plane sent out such a signal.

Lacking data from the ELT, search and rescue operations typically extend about 30 kilometres on either side of the planned flight route, starting from the aircraft's last known position, according to the Transport Canada website.

To conduct the actual search, planes and ships rely on sonar signals from the flight data recorder, or black box.

Stoss said the black box, if functioning, can send out a ping that can be detected as far as 100 kilometres. Unless visual signs of the plane turn up, the flight data recorder is likely the best clue for investigators trying to uncover what happened to the plane.

Tracking it down, however, won't be easy, as the Atlantic Ocean extends to depths of around 3,900 metres.

The head of investigation and accident prevention for Brazil's Civil Aeronautics Agency, Douglas Ferreira Machado, said as much when he told the Associated Press the search could take a "long time."

"It could be a long, sad story," he said Monday. "The black box will be at the bottom of the sea."