Basically what
rhombus said: It really doesn't seem (to my reading) like 5-8-4 applies to intersection runways. In fact I'm not 100% certain it applies to closer-than-2500-feet parallels either, but I can see the argument there. This is based on the wording in the note, which references "
the runway."
But say I'm wrong—say the note is just worded poorly, and actually you are able to use the rule with intersecting runways. In order to do it properly, aircraft speeds and performance have to permit an increase to three miles' worth of airborne separation "within 1 minute after
takeoff." Frustratingly they don't specify whether that means the moment the aircraft begins takeoff
roll or the moment that it
rotates, and even more frustratingly the .65 is not very consistent:
- 3-9-6e says that radar-based WT sep has to exist when the #2 aircraft becomes airborne.
- 3-9-6e Note 2 (which really applies to 3-9-6f/g/h) says that the timer starts when the #1 aircraft begins takeoff roll.
- 3-9-7a uses the wording "has taken off" and I was taught (at the academy and in the field) that that means rotation.
- Pilot-controller glossary defines the term "takeoff roll" but doesn't define "takeoff."
- Pilot-controller glossary defines "cleared for takeoff" as ATC authorization for an aircraft to depart. Nothing we didn't know already, not very useful.
But on the balance I would say the "within 1 minute after takeoff" timer has to start when the departure begins rolling—that's how runway separation works in all other cases except intersection-departure-WT. And if the intersection is anywhere except the very beginning of the runway that means you're using up a lot of your one minute with the departure moving
towards the arrival instead of away from it... although you are helped by the departure moving at cross paths to the arrival once it does finally pass the intersection, which increases the separation faster than if it was departing straight-out the same runway (thanks, Pythagoras).
(Again, to be very clear, as soon as the departure crosses the intersection you now have diverging separation—the departure has crossed the projected flight path of the arrival. But that doesn't matter when you're trying to apply 5-8-4 prior to the intersection. If they're not going to be three miles apart within 1 minute, you had a deal; just because you later re-gained approved separation doesn't mean the deal didn't happen.)
But... maybe I'm wrong about all that. Maybe the one-minute timer starts when the departure rotates and you'll end up having the three miles so you can properly apply 5-8-4. Or maybe your airport layout is such that the intersection is only 500' downfield from where the departure starts their roll, so the "airborne before the intersection" situation doesn't come up; 3-10-4 plus 5-5-7 is all you need. I can be persuaded, perhaps.
Between the time the departure rotates and the time the arrival crosses the intersection, you need three miles of airborne separation between the two aircraft. So you have to launch the departure in time for them to cross the intersection when the arrival is no closer than three miles from that intersection.