Definition
Federal airways defined by low frequency and medium frequency non-directional radio beacons (NDBs), shown in brown on IFR en route low altitude charts and identified by a color and number (for example, Amber 1 or Green 3). They form designated routes between NDBs in areas where VOR coverage is limited, primarily in parts of Alaska and a few other remote regions.
Plain English
These are airway routes that follow older-style ground radio beacons instead of the more modern VOR stations. On the chart they show up in brown and are named with a color and a number. They mostly exist in places like Alaska where VOR coverage is thin.
Context Anchor
Seen on IFR en route low altitude charts, especially when reviewing colored airway routes and older ground-beacon navigation routes.
Derivation
LF stands for low frequency (30–300 kHz) and MF stands for medium frequency (300–3000 kHz). These are the radio bands used by NDBs, which is why airways built on those beacons are called LF/MF airways.
Why Pilots Care
If a pilot is flying IFR in an area served by LF/MF airways, they need an ADF or equivalent to track the NDBs that define the route. Recognizing the brown color and color-name labels on the chart tells them at a glance which airway system they are using and what equipment is required.
Intuition Check
Do not read “airway” as a physical lane or tube in the sky. Here it means a published instrument route shown on a chart and defined by radio navigation signals.
Example Sentence 1
Filing an IFR route through interior Alaska, the pilot used Amber 2, an LF/MF airway, and made sure the ADF was tuned and identified before departure.
Example Sentence 2
When the VOR signal was unavailable, the crew switched to an LF/MF airway segment for the next leg of the flight.