Definition
An allowance applied to en route obstacle clearance area dimensions to account for the small angular error that exists between a published VOR radial and the actual signal path a pilot tracks in flight. Because the radial a pilot follows can be slightly offset from the true magnetic bearing the chart depicts, the protected airspace around the route is widened to absorb this error and keep the aircraft clear of obstacles even when tracking is not perfect.
Plain English
Extra room built into the protected airspace along an airway to cover the fact that a VOR signal is never perfectly aligned with the line shown on the chart. The aircraft might be a bit off the drawn course without the pilot knowing, so the safe corridor is made wider to keep obstacles at a safe distance.
Context Anchor
Seen in en route obstacle-clearance discussions, especially when explaining how protected airspace is built around routes based on ground navigation facilities.
Derivation
Radial comes from Latin radius, meaning a ray or spoke -- a line going outward from a centre point, in this case a VOR station. Alignment displacement simply means how far the actual line is shifted from where it is supposed to be. Together the phrase describes how far the real signal track can sit away from the charted radial.
Why Pilots Care
It ensures the published minimum altitudes provide a real safety buffer even when the navigation signal is not perfectly aligned with the charted radial.
Analogy
Think of drawing a straight line with a ruler that is turned just a tiny bit. The line still points in the intended direction, but farther away it can be noticeably off to one side. Procedure designers account for that possible offset.
Intuition Check
Do not read “alignment displacement” as a pilot intentionally flying off course. Here it means a built-in allowance for small navigation-signal alignment differences when protected airspace is designed.
Example Sentence 1
The width of the en route primary area accounts for navigation facility radial alignment displacement, so a small tracking error still leaves the aircraft inside protected airspace.
Example Sentence 2
Accounting for navigation facility radial alignment displacement kept the minimum en route altitude safe despite the VOR's slight signal offset.