What indicates that passengers should brace for impact?

The indication for passengers to brace for impact is effectively communicated through a specific announcement from the captain or lead flight attendant. This announcement serves several critical purposes: it provides clear and authoritative information, ensures that all passengers receive the same message, and delivers instructions on how to position themselves for potential impact.

While other factors like a sudden cabin pressure change, loud alarms, or flight attendants standing by their exits may suggest an emergency situation or signal the need for readiness, they may not provide the precise and necessary direction on how to brace for impact. These other events might create heightened alertness but lack the clarity necessary to directly inform passengers of the specific action they need to take. Therefore, the most reliable method of communication in such a scenario is the direct announcement from the flight crew.