Definition
The distance a pilot can see along the angled line of sight from the cockpit on approach down to the runway environment, as opposed to the horizontal visibility measured at ground level. Because the pilot is looking through the atmosphere at an angle, slant visual range is often shorter than the reported runway visual range (RVR) when fog, haze, or low cloud is present.
Plain English
How far the pilot can actually see when looking down and ahead toward the runway during an approach. It is the view along the angle of descent, not straight ahead at ground level, so it can be less than what the airport instruments report.
Context Anchor
Seen in instrument approach and landing minimums discussions, especially when comparing reported visibility with what the pilot can actually see from the cockpit.
Derivation
Slant' here means tilted or sloping. The term describes visibility along a sloping line — from the cockpit down through the air to the runway — rather than a level, horizontal one.
Why Pilots Care
It determines whether required visibility minimums are met for landing, directly affecting go/no-go decisions in marginal weather.
Grounding Statement
Imagine descending toward a runway with a thin fog layer below you: looking straight down you can see the lights clearly, but as you angle toward the threshold you are looking through more fog, and the view washes out.
Intuition Check
Do not treat slant visual range as the same thing as reported ground visibility. Ground visibility is measured mostly across the surface; slant visual range is what the pilot can see along the angled path from the airplane toward the runway.
Example Sentence 1
Even though the tower reported visibility well above minimums, the pilot's slant visual range through the haze made the approach lights difficult to pick up until short final.
Example Sentence 2
In the fog the pilot could see the runway lights at a slant visual range of about 1800 feet, just meeting the minimums.