How long must an aircraft on an IFR flight plan be unreported before it is considered missing?

An aircraft on an IFR flight plan is considered missing if it has not been reported for 30 minutes past the estimated time of arrival (ETA) at its destination or a designated reporting point. This timeframe is based on the guidelines established by the applicable authorities to ensure a proper response can be initiated for search and rescue operations. The 30-minute threshold allows for the reasonable expectation that problems may arise or that delays may occur, and it strikes a balance between providing sufficient time before classifying the aircraft as missing while also ensuring timely action in the event of an emergency.

If the aircraft has not communicated its position or status during this time, search efforts can begin more promptly, which is crucial for locating the aircraft and potentially assisting crew and passengers. Other times mentioned in the choices do not align with this standard timeframe and would either delay necessary actions that could be crucial for ensuring safety.