What must be true for Mode C to be accepted for an aircraft below assigned altitude?

For Mode C to be accepted for an aircraft operating below its assigned altitude, the requirement is that the altitude deviation must be 300 feet or more. This is established to ensure that there is a significant altitude discrepancy that requires attention and management to mitigate potential conflicts with other aircraft. A deviation of this magnitude indicates a discrepancy that is significant enough to warrant a review of the aircraft's altitude reporting and adherence to assigned altitudes.

This threshold helps air traffic controllers determine whether the Mode C altitude reporting is still reliable enough to use for separation responsibilities. If a deviation is less than 300 feet, it may be due to minor variances in aircraft altitude reporting that are deemed acceptable under normal operations and would not necessarily trigger further scrutiny or action from air traffic control.

Thus, the 300-foot deviation is a critical standard in managing air traffic safety and ensuring proper oversight of aircraft altitude compliance.