Definition
Federal airways established in the low and medium frequency radio band (190–535 kHz) and defined by non-directional beacons (NDBs). They are charted in brown or shown as colored airways (red, green, amber, blue) and are designated by a color name and number, such as Green 6 or Amber 1. L/MF airways exist primarily in remote areas of Alaska where VOR coverage is limited.
Plain English
Older low-altitude airways that are flown using NDB radio beacons instead of VORs. They still exist mainly in parts of Alaska where VOR signals don't reach, and they're named by color and number rather than by a 'V' prefix.
Context Anchor
Seen in instrument flight rules (IFR) enroute chart discussions, especially when the FAA describes colored Federal airways and older ground-based route systems.
Derivation
L/MF refers to the low and medium frequency portions of the radio spectrum used by NDBs, the underlying navaid for these airways. The 'colored airway' naming convention comes from the early days of U.S. airway charting, when airways were literally drawn in different colors on aeronautical charts.
Why Pilots Care
They provide a navigation network in regions where VHF coverage is limited or unavailable.
Intuition Check
Do not read “L/MF” as low or medium altitude. Here it means low-frequency and medium-frequency radio signals. An airway is not just any path through the sky; in this context it is a published route with defined navigation guidance.
Example Sentence 1
Planning the IFR route through interior Alaska, the pilot included a segment along an L/MF airway because no VOR airway covered that stretch.
Example Sentence 2
Older charts still show several L/MF airways that pilots can follow using beacon signals.