121.5, and (243.0 militar, are the standard aviation emergency frequencies used to transmit distress calls. They are monitored by air traffic control facilities. 121.5 is also the frequency used to transmit emergency locator transmitter signals when an aircraft goes down, although that frequency is slowly being replaced and will eventually be in the 800MHz band. ELT's are activated upon hard impacts and begin transmitting a warbling tone that can be triangulated using an ELT locator either on the ground or by search and rescue aircraft flying overhead near the crash site. ELT's can also be activated by a hard landing. At our airport, we occasionally have to get our locator and track down an ELT after a plane has landed hard. It takes a while, but we eventually find the ELT that is going off and reset it.