What are the typical altitudes for IFR flight levels?

The typical altitudes for IFR flight levels start at 18,000 feet MSL (Mean Sea Level) and above. This altitude marks the transition from visual flight rules (VFR) to instrument flight rules (IFR) in the United States. Once aircraft reach 18,000 feet, pilots are required to operate under IFR guidelines, which involve adhering to a predefined set of regulations and procedures that help ensure safe navigation and control in various weather conditions.

At this level, aircraft are typically flying within the National Airspace System (NAS), where air traffic control manages aircraft separation, traffic flow, and safe operation, particularly in congested areas. The altitude aids in providing a more structured and controlled environment for both commercial and general aviation flights, which is critical for safety once the atmosphere becomes less stable and visibility decreases.

Lower altitudes like those mentioned in the other options do not represent the standard horizons where IFR operations are mandated; for instance, starting at 10,000 feet or 12,500 feet typically pertains to different operational tolerances or flight rules rather than defining the IFR boundary. Thus, the answer correctly identifies where IFR protocols become essential in flight operations.