Aviation experts have cited multiple possible reasons for the problems in the multi-country effort to locate the Malaysia Airlines jetliner that dropped off the grid over the South China Sea four days ago.

First, startling as it may be in an age of sophisticated satellite tracking technologies, some aircraft still depend solely on radar that at times can lose contact with air traffic controllers. Even if the latest GPS technology is on a plane, it may be damaged -- or unused, experts say.

"The challenges of locating a lost aircraft ... can be enormous, and could be made worse by weather, currents, accuracy of last known position, etcetera," said Frank Graham, Jr. president of AeroVox Forensics, an aviation security consulting firm. "Besides, it's a big ocean out there."

Malaysia Airlines Flight MH370 was en route from Kuala Lumpur to Beijing with 239 passengers and crew onboard, when air traffic controllers suddenly lost contact with the aircraft while it flew somewhere over the sea between Malaysia and Vietnam. Efforts to locate the Boeing 777-200ER jetliner have so far been unsuccessful for reasons that remain unclear.

A Boeing 777 aircraft (Photo: Boeing Co.)

On Tuesday morning, the search took a new twist as Malaysian Air Force officials told cable television network CNN that the aircraft may have veered hundreds of miles off course in the opposite direction just before it vanished.

The most recent similar incident came in 2009 when an Air France Airbus flight from Rio de Janeiro to Paris crashed over the mid-Atlantic. Search and rescue teams didn't find debris for several days, while the fuselage wasn't discovered for almost two years.

Finding the exact location of a downed plane can be tricky if it was beyond radar coverage and lacked equipment that could transmit updated positional data back to the airline, Graham said.

"Some planes have that capability and some don't," he said. "There are still instances where controllers and airlines rely solely on position reports via radio from the crew to know where the airplane is."

Loss of radar contact isn't uncommon, especially over long stretches of ocean, Graham said. "While there are a variety of ways an aircraft can transmit positional data, the equipment must be installed in the plane, properly maintained, and actually in use."

Requirements, procedures, and practices, as well as equipment configurations varies from plane to plane, airline to airline, and country to country. "The whole world isn't like the continental US where you can track every flight yourself unless the operator requested and received a block on the [publicly transmitted] ASDI (Aircraft Situation Display to Industry) data," Graham said.

Aviation authorities around the world are starting to implement plans to supplement radar with GPS technologies, but that won't happen everywhere for another 10 years or so, he said. Eventually, all position data will come from the plane. "We're not there yet," Graham said.

The major aircraft tracking technologies include Automatic Dependent Surveillance-Broadcast (ADS-B), said Ric Peri, vice president of government and industry affairs at the Aircraft Electronics Association. Rather than relying on a radar ping, ADS-B uses a GPS signal and an aircraft's navigation system to determine the position of a flight and then broadcast that information, he said.