I work near Levi Stadium just north of SJC airport. Like many Silicon Valley folks, I don't come into the office much. When I do, I normally see flights taking off and turning right above our parking lot. This morning I saw a flight on final approach. Then later in the afternoon, a couple more. Tonight on my way home, I saw several more as I passed the airport. Yes the wind was reversed from normal, coming out of the South but only at 13 MPH. That seems hardly enough to justify the reversed approach/takeoff directions.
In a smaller market, there might not be much concern reversing the pattern. But in the Bay Area the mountains north of SFO mean they can't do the same reverse. So instead of the approaches being nice parallel conga lines, today they had to make a bunch of sharp turns over the Peninsula to stay away from each other. Must have made for a wild ride instead of the normal gradual straightline approach to both airports.
I can see why an airport would reverse things. In the name of safety when winds are higher, a 30 minute delay makes sense. So how do they undo it? Harder to justify further delays just to make things normal again. Do they just wait until things come down overnight? Even if that means leaving things extra busy for hours afterward it's no longer necessary?
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Re: When/why do airports reverse approach/departure directions, and how do they switch back?
MRYapproach wrote:I work near Levi Stadium just north of SJC airport. Like many Silicon Valley folks, I don't come into the office much. When I do, I normally see flights taking off and turning right above our parking lot. This morning I saw a flight on final approach. Then later in the afternoon, a couple more. Tonight on my way home, I saw several more as I passed the airport. Yes the wind was reversed from normal, coming out of the South but only at 13 MPH. That seems hardly enough to justify the reversed approach/takeoff directions.
13 miles an hour is just a little over 11 knots. The tailwind limit on most planes is 10 knots. I don't know the airport layout or the winds, but if it was a direct tailwind, it would have exceeded the tailwind limit of most planes.
Re: When/why do airports reverse approach/departure directions, and how do they switch back?
Tailwinds catch up with you pretty quickly, because your braking distance once on the ground scales as roughly the square of your ground speed. Winds aloft are also typically 5 kts higher than at the surface, so if you need to fly a given vref, your GS might be vref+15 with a 10-kt surface wind. As you descend into less of a tailwind, you're now 5 kias fast, so you'll float over your spot, and THEN still have 15 kts of extra speed to dissipate. For these reasons, a lot of aircraft have performance documents that say things like "reduce ground roll 2% for each 2 kt headwind", and "add 10% for each 2 kt tailwind."
Another challenge with tailwinds is meeting departure restrictions: SIDs require a certain climb gradient in feet/nm. If you've got a tailwind, your GS is higher, but your climb rate is the same, so your gradient is worse and it gets harder to hit those constraints.
To address the last point about the delays: when you hit that magical 10-knot limit, lots of crews will refuse clearances, which will gum things up a lot worse than just 30 mins. If they're being proactive, ATC will see the tailwinds climbing, and plan a switch time. At SJC, they might start sending departures to the 12s, then clear out the last few 30s, and switch. In busy areas like the bay, they'll coordinate with Norcal Approach and the other big fields to switch things at similar times to minimize disruption. How exactly that sequence works I don't know, but they manage.
Another challenge with tailwinds is meeting departure restrictions: SIDs require a certain climb gradient in feet/nm. If you've got a tailwind, your GS is higher, but your climb rate is the same, so your gradient is worse and it gets harder to hit those constraints.
To address the last point about the delays: when you hit that magical 10-knot limit, lots of crews will refuse clearances, which will gum things up a lot worse than just 30 mins. If they're being proactive, ATC will see the tailwinds climbing, and plan a switch time. At SJC, they might start sending departures to the 12s, then clear out the last few 30s, and switch. In busy areas like the bay, they'll coordinate with Norcal Approach and the other big fields to switch things at similar times to minimize disruption. How exactly that sequence works I don't know, but they manage.
Re: When/why do airports reverse approach/departure directions, and how do they switch back?
A 13 mph tailwind is going to get the airport turned around. Often, even less of a tailwind will turn an airport, but winds in busy airspace with a design around prevailing winds will usually need to go to the 10 knot number. Busy airports don't just turn around for nothing, they do it because it is operationally necessary. When LAX is on the 6/7s, it absolutely ruins the airspace for hundreds of miles, so they aren't turning if they don't have to. Turning SJC, especially if SFO and OAK are standard, is absolutely awful for their arrivals, and the entire area, so they aren't doing it just to do it.
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