Air France flight flew well beyond range of radar

BRUSSELS – Before it vanished, Air France Flight 447 was flying hundreds of miles beyond the scope of the nearest radar station, just as scores of commercial flights do every day over the world's oceans.

Above those vast waters, pilots follow different rules for navigation and safety because they are so far from land that air traffic controllers may not be able to pinpoint their precise positions.

Much of what happened to Flight 447 is still unknown, largely because the plane was soaring in a remote zone between Brazil and West Africa. Air crews in that region are never out of radio contact with the ground, but radar cannot track them until they draw closer to shore.

The route was not unusual. Pilots of long-haul flights are often beyond the reach of radar for many hours at a time. Radar coverage over oceans is largely limited to coastal areas extending no more than a couple of hundred miles out to sea.

Because of this radar-free void, crews aboard many transoceanic flights must observe safety procedures that are significantly different from those for flying over land.

Land overflights are normally separated by 5 to 10 miles. But long-range oceanic flights are spaced 20 minutes apart — the equivalent of 80 nautical miles — to minimize the possibility of midair collisions in places far beyond radar.

Oceanic flights also use different navigation techniques. A land-based flight typically follows aerial pathways marked with radio beacons that crisscross the continents. But those paths do not exist over water.

Instead, flight controllers determine specific flight tracks each day — one eastbound and one westbound — on the basis of weather reports and other information. Thousands of airliners follow each other along these tracks.

Pilots can still speak with ground controllers, but standard VHF radios do not work on transoceanic routes because of the earth's curvature. Long-haul pilots must use less reliable HF voice communications which are more susceptible to interference.

Modern airliners also have a digital datalink that automatically transmits and receives messages between the aircraft and ground stations. Those messages are then relayed to air traffic control centers or the airline's own dispatch center, and are used by controllers to determine the aircraft's approximate position.

On Tuesday, Brazilian aircraft located debris from the Airbus A330 in two areas about 410 miles beyond the Brazilian island of Fernando de Noronha.

The only other clue to the plane's fate was an automated message received by an Air France dispatch center in Paris that reported an electrical failure and loss of cabin pressure.

There was no mayday or distress call, but an Air France spokesman has mentioned that a lightning strike in an area of heavy turbulence may have sparked a chain of events that led to disaster.

Air traffic controllers lost contact with the jet just as it was entering a band of violent thunderstorms and heavy turbulence that stretched along the equator.

All modern planes like the A330 are equipped with weather radar that displays a multicolored map showing hazardous weather in yellow or red colors.

Since thunderstorms can tower to altitudes of more than 60,000 feet, where passenger planes cannot climb over them, pilots will often weave left and right to find a route that avoids the worst of the weather.

"I've been on flights that have had to divert hundreds of miles to avoid a wall of thunderstorms," said Gideon Ewers of the London-based International Federation of Air Line Pilots Association.

Some analysts have speculated that the pilot may have been trying to return to Fernando de Noronha, about 220 miles off Brazil's northeastern coast, when disaster struck.

The airport there, built by the U.S. military during World War II, has a runway that is more than 6,000 feet long — sufficient for an Airbus A330 to land safely in case of emergency.

As the crash investigation progresses, analysts will zero in on the flight data recorder and the cockpit voice recorder — if they are recovered from ocean depths up to 9,800 feet.

"The aircraft was cruising at 35,000 feet," Ewers said. "Wreckage could have dispersed over a wide area of ocean and then drifted even further apart while sinking to the ocean floor a couple of miles down."

Despite the lack of radar data, officials from the U.S. National Transportation Safety Board and from Eurocontrol, Europe's aviation agency, say that investigators have many ways to begin investigating the accident even before they recover any wreckage or the black boxes.

"Investigators will have to do a forensic analysis, by piecing together all available information as best they can," said Jim Hall, a former chairman of the NTSB.

They will review the maintenance records of the aircraft, interview the crews who flew the plane in the last few weeks and go to the locations where recent maintenance was done to interview mechanics.

They will also study the personal histories of the crew members and reconstruct what they did in the last 36 hours before the crash.