When planning IFR enroute routes, which factors determine how altitudes are selected?

• A. Pilot's personal schedule
• B. Aircraft type and range
• C. Terrain, airspace structure, and navigation coverage
• D. Seasonal weather patterns

Answer: (C)

When planning IFR enroute altitudes, the key idea is to ensure obstacle clearance and reliable navigation along the route. Terrain and obstacles set the minimum safe altitude for each segment, so tall mountains or structures push you to higher enroute altitudes to maintain clearance. The airspace structure then determines the appropriate altitude to stay within the designated airways and ATC separation requirements, balancing route design with controlled airspace rules. Navigation coverage matters because you need to be able to navigate the leg using the available nav aids or GPS; if signals aren’t reliable at a chosen altitude, you can’t safely fly the route. So you pick altitudes that meet obstacle clearance, fit the airway/ATC structure, and ensure continuous navigation guidance. The other options—pilot’s schedule, aircraft type and range, or seasonal weather—aren’t the factors that define standard enroute altitude selection.