What is required for VOR checks according to regulations?

For VOR checks, the regulation states that aircraft operators must perform a VOR receiver check every 30 days when operating under Instrument Flight Rules (IFR). This frequent checking ensures that pilots rely on accurate navigation data, which is critical for safe instrument flying.

The reasoning behind the 30-day requirement for IFR operations lies in the increased reliance on navigation systems during these flights, where precise location tracking is necessary. Regular checks help to identify any potential issues with the VOR equipment, ensuring that pilots can navigate accurately and safely.

In contrast, the longer intervals required for visual flight rules (VFR) operations and for non-commercial purposes differ because the navigation reliance and risk are not as great in non-IFR conditions. Thus, while the other options suggest less frequent checks, they do not meet the standard set for IFR operations.