When must crew members carry a spare set of spectacles?

Crew members must carry a spare set of spectacles whenever they fly to ensure their vision is adequately supported in the critical operating environment of aviation. The safety and effectiveness of flight operations greatly depend on the ability of crew members to see clearly and perform their duties without hindrance.

Having a spare set of glasses mitigates the risk associated with potential loss or damage to their primary eyewear during a flight. Conditions in-flight can sometimes lead to unexpected issues; thus, being prepared with a backup enhances safety measures. This precaution is vital not only for the crew's ability to maintain operational effectiveness but also for the safety of all on board.

This requirement applies universally to all flights, not just specific types or durations, reinforcing the understanding that safety measures should be comprehensive and always in place.