Under what condition must you file an IFR flight plan?

Filing an IFR (Instrument Flight Rules) flight plan is required when weather conditions are below VFR (Visual Flight Rules) minimums in controlled airspace. This is crucial because at these lower visibility levels, pilots cannot navigate or control the aircraft safely by visual references, which is the primary method during VFR operations. Under these conditions, IFR procedures ensure that pilots can operate safely by relying on the air traffic control system for guidance and navigation.

In controlled airspace, IFR flight plans provide systematic separation between aircraft, which is essential when visual references are compromised due to weather. This maintains safety and efficiency in often congested airspace.

While there may be specific cases in various classes of airspace or at different times of the day that require IFR flight plans, the overarching rule necessitates IFR flight plans primarily when VFR minimums are not met, especially in controlled environments where numerous aircraft may be operating simultaneously.