What feature is essential for submitting a flight plan when using an airway?

The essential feature for submitting a flight plan when using an airway is a valid instrument rating. This is because airways are typically used for instrument flight rules (IFR) operations, which require pilots to have an instrument rating. This rating ensures that the pilot is trained and certified to fly an aircraft in weather conditions where visual references are limited, navigating using the aircraft's instruments instead.

While being at a certain altitude, such as over 25,000 feet, may be pertinent to specific regulations, it does not specifically relate to the ability to file a flight plan on an airway. Weather conditions play a significant role in flight operations but do not necessarily impact the ability to submit a flight plan if the pilot is instrument-rated and operates under IFR. Similarly, advanced autopilot systems may enhance flight management but are not a prerequisite for submitting a flight plan on airways. Thus, having a valid instrument rating is the critical requirement in this context.