According to ICAO procedures, when must you be flying IFR?

Flying under Instrument Flight Rules (IFR) is necessary in certain conditions and scenarios, primarily to ensure safety and compliance with regulations that govern airspace management. The correct scenario in which IFR must be used is when flying in conditions unsuitable for Visual Flight Rules (VFR), such as in controlled airspace, in poor weather conditions, or when operating above certain altitudes.

The reference to supersonic flight also ties into operational requirements dictated by ICAO and national regulations. While there aren't universal ICAO rules explicitly stated for supersonic flight concerning IFR, supersonic operations typically occur in controlled airspace or over specific routes that may require IFR adherence due to the complexities of managing such flight profiles.

In contrast, flying under daylight hours, managing inoperative equipment, or flying below 10,000 feet does not mandate IFR by ICAO standards. Daylight operations can typically be conducted under VFR, while managing inoperative equipment has other specific guidelines. Additionally, VFR is often permissible below 10,000 feet in good weather, depending on altitude restrictions and the specific airspace being navigated.