Under what conditions is an ELT required on board an aircraft?

An ELT, or Emergency Locator Transmitter, is required on board an aircraft under specific conditions to ensure that it can be effectively located in the event of an emergency. The correct option indicates that an ELT must be present at all times unless the aircraft is operating within 25 nautical miles of the departure aerodrome.

This requirement exists because the risk of needing to locate an aircraft quickly is significantly high soon after departure, particularly in the vicinity of the departure airport. The ELT helps search and rescue operations find aircraft that may have gone down or encountered emergencies, significantly increasing the chances of survival for the occupants.

When an aircraft is operating beyond this proximity, the likelihood of an incident occurring away from the departure area increases, necessitating the presence of an ELT.

The other conditions described, like only needing an ELT for night flights, international flights, or only for flights over water, do not align with the broader safety regulations designed to cover all flight operations effectively, which is why they are not correct. ELTs play a vital role irrespective of the time of day, geographic borders, or flight paths, as their primary function is to enhance safety across all possible flight scenarios.