Messages sent by a maintenance alert system cannot provide the "physical location" of a missing plane but indicate whether there are system or engine failures, a U.S. expert said Thursday.

The Wall Street Journal reported earlier in the day that the communication system of the missing Malaysia Airlines plane, a Boeing 777with 239 people on board, continued to "ping" a satellite for a number of hours after the plane disappeared off radar early Saturday.

The signal came from the jet's "airplane health management system," or Aircraft Communication Addressing and Reporting System (ACARS), which provides a flow of data on the airliner's operations, said the report.

The newspaper later retracted one detail in its original report, which had incorrectly stated that U.S. investigators were looking at data automatically downloaded and sent to the ground from the plane's Rolls-Royce Trent engines.

"I've heard conflicting versions of exactly what the WSJ was reporting. That may have been the ADS-B system, Automatic Dependent Surveillance-Broadcast system, which sends a signal twice per second broadcasting the aircraft position and altitude," Bill Waldock, an expert on aviation accident investigation from U.S.-based Embry-Riddle Aeronautical University, told Xinhua.

"It's a newer airplane-based GPS system that air traffic control uses near land in addition to the radar transponders to track the airplane," Waldock said.

If the ADS-B was still working properly, air traffic control should have been able to track the aircraft within 150 to 200 miles (240 to 320 km) of land, he said.

"The ACARS system should't be sending location or speed/altitude data unless someone was entering them manually," Waldock said, adding that its primary use is as a maintenance planning tool for the airline.

"The automatic feature periodically transmits 'bursts' of condensed system condition and function data," said the expert.

He said that before Air France Flight 447 crashed into the Atlantic on June 1, 2009, the French airline's maintenance headquarters received four bursts of such data.

"These indicated multiple system failures and the last indicated a loss of pressurization. The main problem is that the data in a single burst isn't time-sequenced, so it is difficult to determine the order in which they occurred," Waldock said.

He said it is also possible for the pilots to send short messages through the system using a manual console in the cockpit.

Asked whether all airlines are equipped with the system, he said ACARS is operated by an independent organization as an add-on cost to the airline.

"Most modern airliners have the system in various configurations," he said. 

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