Conducting parallel dependent ILS

ZBWPL Newcomer

?a. Apply the following minimum separation when conducting parallel dependent ILS, MLS, or ILS and MLS approaches:

1. Provide a minimum of 1,000 feet vertical or a minimum of 3 miles radar separation between aircraft during turn on.

2. Provide a minimum of 1.5 miles radar separation diagonally between successive aircraft on adjacent localizer/azimuth courses when runway centerlines are at least 2,500 feet but no more than 4,300 feet apart.

3. Provide a minimum of 2 miles radar separation diagonally between successive aircraft on adjacent localizer/azimuth courses where runway centerlines are more than 4,300 feet but no more than 9,000 feet apart.?

This is my question:

Why would you just provide 1.5 miles to aircraft on more closely spaced LOC's vs. more. I would have though 2 miles for 2,500 - 4,300 and 1.5 for 4,301 - 9,000 would be 1.5 ? When does the stagger end, until they are over the threshold or until they are transferred over to tower or something else? If visual separation is applied (to aircraft on adjacent LOC?s) do you still have to stagger them?


Thanks,

-PL

AlphaLima Senior Member Florida

I have very limited experience with parallel ILS's, only at Baghdad while i was deployed and things there were kinda winged at times. But we always followed 1.5 miles diagonally on parallel approaches (ILS to 33R, TACAN, VOR, or GPS to 33L). We had ~5000 feet between the runways i think, been awhile. Never made sense to me either but thats what the book said.

As far as visual goes, i'm not sure. You need approved separation before and after, not sure if the lack of required separation after landing counts.

MikeATC Retired FAA, NATCA Member Nashville TN

The 1.5 stagger is to the thresshold, if visual is applied then the stagger is no longer required.
Now to answer your question why would you apply 1.5 vs more, more is wasted space. In all honesty you probably will see staggers of 2 miles since it is easier to judge two miles vs. 1.5

FM_Weasel Trusted Member

I think the OP's question is: Why does it require MORE separation (2 mi) when the runways are FARTHER apart. Seems weird to me also. Can't offer any opinion though, my airport lacks parallels. I'm sure there's some technological restriction that requires the additional separation.

Matt_CTI Junior Member South Florida

It’s hard to show without a picture or a diagram but think about the diagonal line between the aircraft on final. If the runways are closer together (at least 2,500 but no further than 4,300) the diagonal line will be at more of an angle to assure the 1.5 mile staggered separation. If the runways are further than 4,300 feet apart, then the diagonal line will start to become horizontal and the aircraft will be staggered closer together on final, so that’s why they need 2 miles instead of the 1.5 for the closer runways. I’ll try to come back and edit this post with a quick diagram or picture to help show the point.

atcbrownie Trusted Member Warrenton Va

This is correct. If the centerlines are further apart there is less in-trail sep when 1.5 miles diagonal is applied so 2 miles is required to provide more in-trail sep. It kinda doesn't make sense unless you see it drawn out or done in person.

Roddy_Piper Resident Knucklehead Vegas baby

the in trail stagger will be the same distance behind. if u have STARS and put a ghost on the parallel localizer u will see the distance behind is the same for both configurations. seems weird i know, but it is so.

Roddy_Piper Resident Knucklehead Vegas baby

to clarify, the aircraft on the closer parallels will be closer together on a diagonal because of the stagger, but the DME distance to the parallel thresholds would be similar.

plus, hopefully u have a PRM if the WX goes to $h!t.

scott523 Junior Member Philadelphia (Upper Darby), PA, USA

Okay, I'm still a rookie when it comes to this but you would think closely spaced parallel runways would require more separation diagonally just to be safe. However, I don't think it's even possible to have 2 miles diagonal AND 2.5 miles behind for closely spaced runways if the point is to have a plane on runway A laterally in between two planes on runway B. Think of it as a right triangle (forumula). Here's the only diagram I could find on Google, numbers are wrong:

Matt_CTI Junior Member South Florida

There's a good picture ^^... If the >4300 runways had the 1.5 mile staggered separation the trailing a/c on final would have less than 2.5mi separation.