The matter is that commercial airliners do actually transmit some data. Radio transponders identify them while scanning by radar, and most of them are fitted with an Aircraft Communications Addressing and Reporting System (ACARS).
The latter periodically relays text-message like snippets of data about the aircraft’s status. Those transponders seem to have stopped transmitting data about Flight MH370, and the airline refused to provide any comments about ACARS signals while the incident is being investigated.
According to the computer scientist Krishna Kavi, this data can be streamed to cloud storage, in a system called the "glass box". However, transmitting information via satellites is not cheap, especially and if such a system operates continuously.
Apparently, it would cost billions to implement flight data streaming across the airline industry.
However, most of the information is based on the maker of the existing black box technology L-3. The latter spun a false premise that all flight information would need to be streamed, all of the time. According to a safety and insurance director of an aviation consultancy, systems could be designed to be triggered by unusual flight events, and only after this start streaming flight information.
These devices already exist on the market, fitted to about 350 aircrafts run by 40 operators and they transmit information helping airlines plan maintenance and minimize fuel consumption.
The company producing the system revealed that it transmits information via Iridium satellites and can be programmed to start streaming flight data after a plane deviated from its flight plan, or there are suspicions something is going wrong.
In case an aircraft is blown out of the sky by a bomb, or if it suffers a sudden catastrophic structural failure at cruising altitude, such devices won’t be much help. However, in those rare cases, conventional black boxes are viable technology.