In IFR conditions, when should the weather forecast be good enough to proceed to an airport without an alternate?

The correct answer highlights the importance of having a sufficient weather forecast that ensures the safety and viability of proceeding to an airport without an alternate within a specific time frame. Specifically, the forecast should indicate good weather conditions one hour before estimated time of arrival (ETA) and continue to show these favorable conditions for one hour after ETA. This time frame provides a buffer to account for any changes in weather that may occur as the flight approaches the destination.

Being assured of good weather both before and after ETA is crucial for IFR flights as it allows adequate time for any necessary go-around procedures or hold patterns if the weather unexpectedly changes. It reflects the regulatory requirements designed to mitigate risks associated with IFR operations where visibility and weather conditions can be variable.

In contrast, the other choices provide narrower or less effective time frames that do not offer the same level of assurance needed for safe IFR operations. For example, only considering the 15 minutes around ETA may not provide enough time to assess sudden changes in weather, while limiting the forecast to a shorter window of 2 hours only before ETA misses important considerations for post-ETA conditions. Furthermore, choosing to look only at 30 minutes after ETA neglects what may happen before arriving at the airport.