Definition
The minimum spacing measured between an aircraft on one final approach course and an aircraft on an adjacent parallel final approach course, taken along a slanted (diagonal) line rather than directly side-by-side or directly in trail. During simultaneous dependent approaches to closely spaced parallel runways, ATC staggers aircraft on the two approach courses so that a required diagonal distance is maintained at all times.
Plain English
When two aircraft are landing on parallel runways at the same time, controllers don't put them right next to each other. They keep one a bit ahead of the other, so the gap between them runs at an angle. That angled gap is the diagonal separation.
Context Anchor
Seen in instrument approach procedures for airports using simultaneous approaches to parallel runways.
Derivation
Diagonal comes from the Greek 'diagonios,' meaning 'from angle to angle.' Here it describes the slanted line between two aircraft on neighboring approach paths -- not abreast, not in line, but offset at an angle.
Why Pilots Care
Allows higher arrival rates while still meeting safety standards when full lateral or longitudinal separation cannot be provided.
Analogy
Think of two cars in nearby lanes on a road. They are safer when one is slightly ahead of the other, instead of directly beside each other with no room for error.
Intuition Check
Do not read diagonal separation as simply “side-to-side distance.” In this context, it means a required stagger between aircraft on adjacent approach paths.
Example Sentence 1
During simultaneous dependent approaches, ATC staggered the two arrivals so the required diagonal separation was maintained throughout the final segment.
Example Sentence 2
Diagonal separation of 1.5 nautical miles was maintained between the lead and trailing aircraft on the parallel runways.