Communications satellites picked up faint electronic pulses from Malaysia Airlines (MAS) Flight 370 after it went missing on Saturday, but the signals gave no information about where the stray jet was heading and little else about its fate, two sources close to the investigation said on Thursday.
But the "pings" indicated that the aircraft's maintenance troubleshooting systems were switched on and ready to communicate with satellites, showing the aircraft, with 239 people on board, was at least capable of communicating after it lost touch with Malaysian air traffic controllers.
The system transmits such pings about once an hour, according to the sources, who said five or six were heard. However, the pings alone are not proof that the plane was in the air or on the ground, the sources said.
While the troubleshooting systems were functioning, no data links were opened, the sources said, because the companies involved had not subscribed to that level of service from the satellite operator.
Boeing Co, which made the missing 777 airliner, and Rolls-Royce, which supplied its Trent engines, declined to comment.
Earlier Malaysian officials denied reports that the aircraft had continued to send technical data and said there was no evidence that it flew for hours after losing contact with air traffic controllers early Saturday after taking off from Kuala Lumpur en route to Beijing.
The Wall Street Journal had reported that US aviation investigators and national security officials believed the Boeing 777 flew for a total of five hours, based on data automatically downloaded and sent to the ground from its engines as part of a standard monitoring programme.
Sources familiar with the investigation reiterated that neither Boeing nor Rolls-Royce had received any engine maintenance data from the jet after the point at which its pilots last made contact.
There is still no evidence that demonstrates the plane's disappearance five days ago was related to foul play, U.S. security sources stressed, though the officials said they still have not ruled out the possibility of terrorism.
Modern aircraft can communicate with airline operations bases and sometimes with the headquarters of its manufacturers automatically to send maintenance alerts known as Acars messages.
It was this system that sent out the hourly pings, apparently over several hours, the sources said.
But MAS had not signed up for an expanded service that is based on the system and can send information such as updated flight plans and position reports, people familiar with the matter told Reuters this week.
In the past such data was sent via radio links, but in recent years, airlines have begun using satellites to transmit the information more reliably.
Oliver McGee, a former senior US Transportation Department official and professor of mechanical engineering at Howard University in Washington, said the pings by themselves would not necessarily help locate the plane.
Data about the engine's fuel burn, weight and other aspects of its performance is needed to help determine how far the airplane had traveled, he said.
"It depends on the data coming from the engines," McGee said. "If you have no reliable source of what information you are reading, you can not get the range, air speed or time traveled."
He cautioned that the aircraft could continue to generate the signals, even if it had crashed, depending on any damage to the aircraft and its engines.
Mark Rosenker, a former chairman of the National Transportation Safety Board, said the aircraft would continue generating the signals even if it was on the ground, unless the system had shut down.
Honeywell International Inc makes the components that go into the Acars system on Boeing 777s, but a different service provider sets them up to communicate with the airlines, said one industry source familiar with the system.
Each airline determines how it wants the system to work and under what circumstances, said the source, noting that some carriers receive messages when the plane is using auxiliary power, while others want updates only when the aircraft's engines are running.