When should the altitude preselector be set to MDA during an RNAV approach?

Setting the altitude preselector to Minimum Descent Altitude (MDA) at the Final Approach Fix (FAF) is a critical step in the RNAV approach procedure. The FAF is where the approach transitions from the en route phase to the final approach phase, indicating that the aircraft is nearing its minimum descent altitude. By setting the MDA at this point, pilots ensure they are prepared to conduct the approach safely and are aware of the altitude they cannot go below if visual references are not established.

This action aligns with standard practice in instrument flying, allowing the flight crew to stay aware of altitude during the vital phase of the approach. This ensures that there is a clear plan in place as they prepare to descend to MDA if needed, which is crucial for maintaining safety and compliance throughout the approach. The other options would not provide the timely awareness needed at the crucial stage of the approach flight path.