Definition
A flight visibility of one-half statute mile, meaning the pilot can see prominent unlit objects by day, or prominent lighted objects by night, at a horizontal distance of 2,640 feet. On certain RNAV (GPS) approaches using WAAS with LPV minimums, this is the lowest visibility minimum that may be authorized when the runway has the required approach lighting and other infrastructure.
Plain English
The pilot must be able to see at least half a mile ahead. If the official weather report or actual flight visibility is less than that, the approach minimums aren't met and the landing can't be made from this approach.
Context Anchor
Seen on instrument approach charts, especially in the minimums section for an RNAV (GPS) approach using WAAS.
Derivation
SM stands for statute mile, the standard 5,280-foot mile used on land in the United States. Aviation visibility in the U.S. is reported in statute miles rather than nautical miles, even though most other distances in flying use nautical miles. This is a holdover from how surface weather observations have always been reported.
Why Pilots Care
It sets the legal limit for continuing the approach to a landing instead of executing a missed approach.
Grounding Statement
At 1/2 SM visibility, the reported seeing distance is about half of a land mile horizontally through the air.
Intuition Check
Do not read “visibility” here as a general feeling that you can see well enough. It is a measured or reported distance, and “SM” means statute miles, not nautical miles.
Example Sentence 1
The LPV minimums for the approach showed a 200-foot decision altitude with 1/2 SM visibility required.
Example Sentence 2
With reported visibility at 1/2 SM we can legally continue to the decision altitude.