Under what condition must flight attendants repeat all preflight safety checks?

The condition under which flight attendants must repeat all preflight safety checks is when the entire crew deplanes and leaves the aircraft unattended. This requirement is crucial because when the crew leaves the aircraft, they are no longer able to monitor or ensure that all safety measures are in place. Returning to a situation where the crew has not been present can introduce variables that may have changed during their absence, such as the airline's safety equipment, cargo configurations, or passenger conditions.

The preflight safety checks are designed to confirm that all emergency equipment is available and operational, that evacuation procedures are ready, and that all passengers are informed. If the crew has left the aircraft unattended, they must verify these critical safety aspects again to ensure the aircraft is safe for flight.

In contrast, while scenarios such as having someone new board the aircraft or a delay might warrant additional considerations, they do not necessitate a full repeat of all preflight safety checks. Similarly, emergency landing procedures focus on specific protocols for handling emergencies, rather than the comprehensive review of safety checks required under unattended conditions.