Apparently they almost do: the emergency transponder beacon at the back of the plane can be triggered by G force, and has its own battery. http://www.airliners.net/aviation-forums/general_aviation/re... That leads to the question of why this particular dog wasn't heard to bark when (and if) MH370 hit the water in the Indian Ocean, but that could probably be explained by assuming that the tail section sank fairly quickly: the beacon doesn't work underwater. But interestingly it also highlights what the Law of Unintended Consequences could do to self-powered, tamper-resistant systems: http://money.cnn.com/2013/07/18/news/dreamliner-fire-report/