[curtis]

Air France 447 actually had such a system, although it was not designed to fulfill the same role as the black box.

Here's Wikipedia on the subject (http://en.wikipedia.org/wiki/Air_France_Flight_447#Automated...):

An Air France spokesperson stated on 3 June that "the aircraft sent a series of electronic messages over a three-minute period, which represented about a minute of information. "[32][33][Note 2] These messages, sent from an onboard monitoring system via the Aircraft Communication Addressing and Reporting System (ACARS), were made public on 4 June 2009.[34] The transcripts indicate that between 02:10 UTC and 02:14 UTC, 6 failure reports (FLR) and 19 warnings (WRN) were transmitted.[35] The messages resulted from equipment failure data, captured by a built-in system for testing and reporting, and cockpit warnings also posted to ACARS.[36] The failures and warnings in the 4 minutes of transmission concerned navigation, auto-flight, flight controls and cabin air-conditioning (codes beginning with 34, 22, 27 and 21, respectively).[37]

[austinz]

Interesting. From the linked article, it looks like coordinate data was transmitted as well as telemetry. Given the high speed of airliners, ocean currents, and travel time for rescue ships, even minute-by-minute transmissions wouldn't solve the problem of pinpointing the exact location of wreckage/survivors.

[curtis]

In the case of AF447 the flight control system knew something was wrong. In a situation like that increasing the rate of messaging automatically might be a good idea, and could probably be implemented entirely in software. It probably wouldn't help in the case of catastrophic air frame failure out altitude however.