Definition
The protected lateral airspace of a low-frequency or medium-frequency airway, extending 4.34 nautical miles either side of the airway centerline up to and including the point at which the 4.5-degree angular outward expansion from the navigation aid exceeds those limits. Beyond that point, the airway boundaries diverge at 4.5 degrees from centerline until the changeover point or the next navigation aid is reached.
Plain English
An LF/MF airway is a corridor of protected airspace centered on the airway. Close to the navigation aid, it is a fixed-width corridor 4.34 nautical miles wide on each side of the centerline. Farther from the station, the corridor fans outward at 4.5 degrees on each side, because the signal becomes less precise with distance.
Context Anchor
Seen when reading about older low frequency/medium frequency airways and radio-beacon-based instrument route design in the Instrument Procedures Handbook.
Derivation
LF stands for low frequency and MF stands for medium frequency, the radio bands used by the older ground stations (such as NDBs) that define these airways. The width rule reflects how those signals behave: accurate close to the station, less precise farther out.
Why Pilots Care
Knowing the width lets the pilot stay inside the area where navigation signals and obstacle clearance are guaranteed.
Grounding Statement
Picture the airway as a radio-guided corridor: the centerline is the intended path, and the width is the protected space on both sides of it.
Intuition Check
Do not picture LF/MF airway width as a physical lane painted in the sky. It is a protected area of airspace measured outward from the airway centerline.
Example Sentence 1
On this leg the LF/MF airway width is still the standard 4.34 nautical miles either side of centerline, so we have plenty of protected airspace for the turn.
Example Sentence 2
At greater distances from the facility the LF/MF airway width expands to keep the protected corridor intact.