Under what circumstances is an Emergency Locator Transmitter (ELT) not required on board?

The correct answer indicates that an Emergency Locator Transmitter (ELT) is not required on board when operating within 25 nautical miles of the departure aerodrome. This regulation is designed for safety and operational convenience, especially in situations like training flights or short local flights where the likelihood of needing an ELT is significantly lower due to the proximity of the airfield.

The rationale behind this regulation is that when flying close to an aerodrome, search and rescue operations can be conducted more efficiently and quickly. The assumption is that the aircraft can be easily located or that any emergency would generally not be prolonged, given the relative proximity to the airfield.

In comparison, other scenarios mentioned in the choices, such as flying at night, conducting flight training exercises, or operating below a certain altitude, generally do not exempt an aircraft from the ELT requirement due to the increased potential risks associated with these conditions. In most environments, the presence of an ELT is crucial in enhancing safety and ensuring a timely response in case of an emergency, making its requirement crucial outside of the specified 25 nautical mile radius.