American Airlines Flight 370 had two beacons , or emergency locator transmitters . One was designed to activate on demand , but satellites did not transmit a distress signal . Once activated , the device simultaneously transmits `` bursts '' -- short , digitally coded signals -- on three frequencies . The lack of a distress signal raised concerns of passengers ' families .
American Airlines Flight 370 had two beacons , or emergency locator transmitters . One was designed to activate on demand , but satellites did not transmit a distress signal . The lack of a distress signal raised concerns of passengers ' families .