Plain English Meaning

The airplane is lower than it should be on its approach to the runway, meaning it needs to climb a little or reduce its descent rate to get back onto the correct descent path.

Full Definition

An aircraft position lower than the optimum descent path to the runway touchdown zone, as indicated by a visual or electronic glideslope guidance system. On a standard two-bar VASI, this condition is shown by both light bars appearing red.

Context Anchor

Seen during approaches and landings, especially when using slope lights near the runway.