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Joined: Fri Sep 17, 1999 7:43 am

ILSs That Cross

Fri Oct 11, 2002 1:00 am

Thinking about the ILS to 13L at JFK, it finally occurred to me that the situation here in the Bay Area is unusual.

When the winter weather gets rainy the winds are out of the S or SE or E, so the custom is all SFO arrivals on the rwy 19L ILS while OAK arrivals use the rwy 11 ILS. The two approach courses cross at 37.74932 N, 122.28606 W, which is 8.23 nm from the SFO 19L threshold and 2.82 nm from the OAK 11 threshold. The published approach to OAK requires crossing PLAZA (west of the crossing) at 1800 ft, while SFO arrivals are required to cross SHAKE at or above 2900 ft. I had always assumed SHAKE was north of the crossing, but now I see it's 8.82 nm from the 19L threshold. Close enough, I guess.

(When controllers clear aircraft for the ILS approaches they invariably state those crossing restrictions-- they don't just trust the pilots to read the chart.)

So: where else in the world do ILSs cross like this, with a stream of aircraft using the two ILSs simultaneously? Anybody think of one where the difference in distances from the two thresholds to the crossing point is less than 5.4 nm?

RE: ILSs That Cross

Fri Oct 11, 2002 1:28 am

One of the reasons for putting St. Louis' new runway so far west of the field (its east end will be at the current west end of the airport) is to allow simultaneous ILS approaches. Basically, with the termination of the approaches being a couple miles apart, the difference in altitude will allow their relatively close lateral proximity. The current two runways are too close together for simultaneous ILS approaches. One would assume at the future St. Louis, as well as the current SFO/OAK situation you describe, there is little danger of aircraft on different approaches plowing into each other unless vectored that way by ATC. With the altitude differential, for one aircraft to cross that intersection point with the other approach, they would have to be at such a great glideslope deflection their company procedures would call for a missed approach.
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RE: ILSs That Cross

Fri Oct 11, 2002 6:36 am

LGA's ILS RWY4 and JFK's ILS RWY13L cross each other...
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Joined: Thu Mar 14, 2002 4:10 pm

RE: ILSs That Cross

Fri Oct 11, 2002 9:39 am

So long as they are arranged that one is far enough below the other so as to keep separation, it's fine. I don't think either of these cases have the locolizer AND glideslope intersecting at the middle.

Also, that's what the approach controller's there for, to keep the planes separated.
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Joined: Fri Sep 17, 1999 7:43 am

RE: ILSs That Cross

Sat Oct 12, 2002 2:11 am

I certainly assume they never use the LGA rwy 4 ILS while they're using the JFK 13L ILS, but if anybody knows better let's hear it.

And I assume the approach controller's job is not to separate the aircraft on one ILS from the aircraft on the other. Spacing the aircraft into one airport is quite enough to keep him busy; the aircraft arriving the other airport are talking to a different controller who has his own worries. If the controller has time he might mention the "conflict" just so they know what's going on on their TCAS screens (and what will TCAS show in that situation, anyway?) but my guess is each arrival stream is spaced entirely without regard for the aircraft in the other stream.

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