# Why do simultaneous-ops parallel runways need to be so far apart?

In the answers to this question about Denver's widely-scattered pinwheel of runways and this question about Schiphol's runway out in the middle of nowhere, it's mentioned that parallel runways are required, by both European and U.S. regulations to be over a kilometre apart in order to be able to have simultaneous departures or simultaneous arrivals on both runways. In contrast, if one runway is departing an aircraft while a parallel runway is landing an aircraft, the runways need only be a few hundred metres apart.

Why is there such a large runway-separation requirement for simultaneous arrivals or simultaneous departures from parallel runways? It can't be for aircraft separation, since an aircraft is allowed to land on a runway quite close to a parallel runway that is simultaneously departing an aircraft, despite the possibility of this resulting in a head-on collision if one or both aircraft drift off track, so what, in fact, is the reason for this?

• Aircraft on parallel runways both go in the same direction, so that a departing plane and arriving plane would both be flying in the same direction. There would be no head-on collision. Commented May 1, 2018 at 16:37

(YouTube) Much closer than 1 km at SFO.

1 km (~3,300') is not the minimum (see this illustration from the FAA AIM). The centerline spacing for parallel approaches can go as low as 750' (~230 m), a prime example is San Francisco:

(Google Earth) Centerline spacing at SFO.

But getting very close like at SFO requires special training, procedures, charts, controllers, and ground equipment (namely the precision runway monitor system - PRM). In lack of PRM and the associated items, the centerline separation allows sufficient reaction time against any transgression (e.g., pilot deviation).

Wake turbulence is a factor for the trailing plane whether on the same runway or a parallel one, but RECAT has helped reduce the over conservative separation (ICAO).

For your last point on departures and arrivals on parallel runways, a minimum centerline spacing is required:

(FAA)