Chief Air France spokesman Francois Brousse suggested the plane could have been struck by lightning.

But most experts say lightning doesn't usually bring down a modern airliner unless it coincides with other factors.

About four hours after taking off and flying through the night over the mid-Atlantic, the pilots of the Air France Airbus reported they had encountered an area of intense cumulonimbus activity, part of the massive thunderstorms that regularly batter the world's equatorial belt.

It remains unclear whether Flight 447 took evasive action to avoid the area of heavy turbulence.

Air France reported that the aircraft's ACARS (Aircraft Commu-nications and Addressing System), a digital datalink that automatically transmits service messages from the aircraft to ground stations, messaged the company's headquarters regarding a problem with the aircraft's electrical and pressurisation systems.

Locate the black boxes

Former NTSB Chairman Jim Hall said that since the A330 is widely used in international travel, it was vitally important to locate the black boxes as quickly as possible and analyse what happened to Flight 447.

Although aviation experts stressed it was much too early to speculate about the causes of the disappearance, they noted that the incident was most likely caused by various factors that combined to cause a catastrophic chain of events.

"It sounds like something that evolved into a problem, not something that happened instantly," said Bill Voss, president and CEO of Flight Safety Foundation, in Alexandria, Virginia.

Smith said that that if the crew was forced to ditch the A330 in the ocean at night in stormy weather, "the outcome would not likely have been a good one."

"It would be nothing like landing that Airbus at midday in the Hudson river, a completely different scenario," Smith said, referring to the successful Jan. 15 water landing of a US Airways Airbus A320 in New York.

- AP