Missing plane: How airlines use tracking technology

A version of this story was originally published in March 2014, after Malaysia Airlines Flight 370 disappeared.

As another plane has gone missing in Southeast Asia, attention is paid to the tracking systems that commercial airlines use. AirAsia Flight QZ8501, carrying 162 people, lost contact with air traffic control.

Planes typically relay maintenance information in flight to their manufacturers or airlines. This data can also help track a lost plane like Air France Flight 447, which crashed in the Atlantic in 2009.

In the case of Malaysia Airlines Flight 370, the technology at issue is called ACARS, for Aircraft Communications Addressing and Reporting System. The system sends different sorts of information between the plane and airlines and manufacturers on the ground. Depending on the sophistication of the information, the airline might receive the information itself or rely on the manufacturer to relay it for a fee.

John Hansman, director of the International Center for Air Transportation at Massachusetts Institute of Technology, estimates 90% of U.S. planes have some version of the system because it saves airlines money.

At first, the precise monitoring of when brakes were engaged or when planes took off or landed helped airlines track personnel costs more accurately, Hansman said. Maintenance reports can show when an engine is running hotter than usual, which signals wear, he said.

"The reason why people do this is because if something breaks in flight, if maintenance gets the message they can actually be at the landing point with the replacement part and fix the airplane and turn it around quickly," Hansman said.

In general, the messages can be sent cheaply over land by VHF signal, like the Apollo astronauts used, Hansman said. Or they can be sent over water by satellite, which is a more expensive service to send messages, he said.

The sophistication of the service depends on what the airline would like to spend. For example, Boeing promotes a service called custom alerting and analysis, which is available for 777, 747 and 787 aircraft with high-speed Internet connections. It monitors fuel, flight controls, landing gear, hydraulic power and communications.

"The major carriers collect it themselves because they have the capability of processing it and deciphering it and so forth," said David Greenberg, who worked 27 years at Delta Air Lines and is now an airline consultant as president of Compass Group. "My understanding is that Malaysia doesn't subscribe to the Boeing program and that they collect the data for their own use."

While Malaysia Airlines and its manufacturers aren't saying what sort of service was used on Flight 370, the Airbus A330 in the Air France crash had sophisticated messaging that reported problems with airspeed and altitude that helped track down the missing plane.

"In Air France, that's how they knew where to look," Hansman said.