What indicates that an aircraft emergency exists?

An Emergency Locator Transmitter (ELT) signal being heard or reported is a strong indicator that an aircraft emergency exists. ELTs are specifically designed to activate automatically in the event of a crash or emergency situation and send distress signals to alert search and rescue teams. When an ELT signal is detected, it typically means that the aircraft is either in distress or has suffered an incident requiring immediate attention, thereby confirming the existence of an emergency.

While flying at an unusually low altitude may arouse suspicion of an emergency, it isn't definitive evidence. Similarly, a pilot changing to an emergency frequency could suggest an emergency, but this action might also be taken for precautionary reasons without confirming that an actual emergency is occurring. Deteriorating weather conditions can suggest difficulties for a flight but do not necessarily signal an emergency. Therefore, the detection of an ELT signal is the most definitive indication of an aircraft emergency.