Definition
The distance a pilot can see along an angled line of sight from the cockpit down to the ground or runway environment, as opposed to the distance measured horizontally along the surface.
Plain English
How far you can actually see when looking down and forward from the airplane toward the ground, rather than how far someone on the ground can see across the airfield.
Context Anchor
Used in weather and approach discussions, especially when a pilot is trying to see the runway, runway lights, or ground features through haze, fog, rain, or snow.
Derivation
Slant comes from Middle English meaning to slope or angle. Here it describes the diagonal line of sight from a cockpit at altitude down to a point on the ground, rather than a level look across the surface.
Why Pilots Care
It directly affects whether the runway environment becomes visible on approach even when surface visibility reports indicate otherwise.
Grounding Statement
On approach, you may be looking diagonally through miles of air, so the runway can be hidden even when visibility on the ground sounds acceptable.
Intuition Check
Do not assume slant visibility is the same as the airport’s reported horizontal visibility. Slant visibility is the angled view from the airplane to what the pilot is trying to see.
Example Sentence 1
Although tower reported three miles visibility, slant visibility through the haze layer was poor and the runway didn't come into view until short final.
Example Sentence 2
Although the METAR showed one-mile visibility, the slant visibility from the cockpit was sufficient to complete the landing.