For IFR operations off of established airways below 18,000 feet, how far apart should VOR navigation aids be to describe the 'route of flight'?

For IFR operations below 18,000 feet, the Federal Aviation Administration (FAA) requires that VOR navigation aids be spaced no more than 80 nautical miles apart in order to effectively describe the route of flight. This spacing ensures that aircraft flying under IFR can reliably receive navigational signals from the VORs and maintain situational awareness while navigating.

When VORs are spaced this distance apart, it allows pilots to switch between navigation aids in a timely manner, minimizing the chances of signal loss and ensuring that they can accurately follow their intended flight paths. Additionally, the 80 NM spacing rule helps provide adequate coverage for pilots flying in various areas, ensuring that there are navigational resources readily available even in less densely populated airspace.

Other options, such as 40 NM, 70 NM, or 100 NM intervals, do not align with the FAA's prescribed minimum spacing requirement for effective route navigation under IFR below 18,000 feet.