When must the batteries in an ELT be replaced or recharged?

The correct choice regarding when to replace or recharge the batteries in an Emergency Locator Transmitter (ELT) is indeed that both scenarios are valid. The first point to note is that ELT batteries must be replaced after 100 hours of use. This is a clear maintenance standard established to ensure the device operates effectively in an emergency situation.

In addition, ELT batteries must also be addressed when 50% of their useful life has expired. This means that if the batteries are rated for a certain lifespan, they should be replaced or recharged once they have used up half of that capacity. This is critical because it ensures that the ELT remains a reliable resource when needed.

Both conditions highlight the need for regular testing and maintenance of an ELT to ensure it remains functional and ready for use. Adhering to these guidelines maximizes the chances of a successful rescue operation in an emergency, underscoring the importance of maintaining aviation safety standards. Thus, the answer encompasses the essential practices for battery management in ELTs.