Monday, April 28, 2014

Pop quiz explained

Just so we're all together here, I'll start with a recap of the pop quiz photo (above). We have a beautifully-centered shot of one of the many antennas that adorn the control tower. Lurking in the background is Terminal Two, and beyond it the north runway complex (24 left & right). On Taxiway E is a short line of aircraft on their way out for departure: a pair of A380s, with an A320 sandwiched between.

Although several of you commented about how it could be gotten out from behind the Singapore A380, the A320 is actually a red herring. To address your points, though: Yes, the A320 could access the runway via E-8 if the local (tower) controller chose to go that route; we do it all the time. It's to our advantage to do this, as it keeps the A320 out of the wake turbulence as well as keeping the A320 from having to wait for the runway inspection that will follow the A380's departure. To answer the question that I know some of you are asking: Yes, each A380 departure off Runway 24 Left (or 6 Right) requires a runway inspection -- Even if the very next departure is another A380. The reason is that the A380s regularly destroy runway signs with their jet blast, and the debris fouls the runway. And to address the second point raised in the comments: Yes, the A320 can turn at D-8, but the first A380 must have turned the corner onto Taxiway V. There really wouldn't be any advantage to that, however, unless the A320 was then run around the approach end of 24 Left (via E-7) to access the runway from the north side -- or to use Runway 24 Right. A lot of effort to accomplish, and it will require coordination with the ground controller. The only scenario I can easily conceive that would call for this would be if the first A380 reached the runway without being ready for departure, but the pilot thinks that it will be ready soon. Remember that once the A380 reaches the position where we see Singapore in the above shot, the only place for the airplane to go is onto the runway, even if it is unable to take off. I've personally had this happen, and at the time the only way out was for the A380 to take the runway and taxi all the way down to exit at Taxiway AA. Since then, Taxiway Z has been approved for an A380 to exit the runway, but even so it requires a long taxi down the runway to get there. Of course, the other reason the A320 might turn at D-8 would be because it was an inbound aircraft on the way to its gate. If that was the case, though, in most circumstances it wouldn't have been put behind the A380 in the first place. But I digress . . .

The issue at hand is the pair of A380s. Specifically, the new requirement that an A380 on Taxiway E must be east of Taxiway D-9 if another A380 is using Runway 24 Left for departure. This is because the runway and taxiway are sufficiently close together that there isn't much wingtip clearance if two A380s pass wingtip-to-wingtip. The logic is that it is safer for the two A380s to pass at the low-speed part of the takeoff roll. This makes sense when you consider that if the departing A380 strayed off the runway centerline, it will be moving at a slower speed and is likely to have a smaller deviation toward the other A380. At higher speeds, a deviation of the same time duration will involve a greater distance, and a corresponding greater likelihood of striking the other A380. What this means is that the Singapore A380 may not commence its takeoff roll until the Air France A380 has made it past Taxiway D-9. In the shot above, Air France is in the intersection of E and D-9. Here is how things looked when Singapore actually rolled:

In your comments, some of you referenced an earlier post in which I included a diagram of A380 restrictions. Here is the current one for the north side of the airport:

And here is the corresponding diagram for the south side of the field:

It's hard to see on the diagram, but we also have a similar restriction on the south side of the airport. An A380 on Taxiway A must be east of Taxiway A-3 if another A380 is taking off on Runway 25 Left. 

If we are east traffic, then the A380 zones are at the other ends: An A380 on E must be west of BB in order for another A380 to depart on Runway 6 Right, and an A380 on A must be west of T if another A380 is rolling on 7 Right.


  1. I swear, it took me half an hour ( not continuously viewed) before the penny dropped and I understood that on the pic of the South complex, South is up! I guess this makes this an easier reference to you, looking down from the Tower, but I must admit it is a pet peeve of mine to discover maps with North in another direction than usual, without any note or symbol on the map itself. On that pic, the map maker was not even consistent, since the three small pics in the left lower corner that show ADG-IV aircraft access to ramps south of TWY A ( West Imperial, KAL Cargo and ICC) are shown 'right side up', that is to say, North is up!

    On the subject of runway inspections, is the other runway ( say RWY 24R when the A380 departed from RWY 24L ) available for landing and/or take-off during this inspection? And is it possible for an aircraft to cross during such inspection? I suppose departing aircraft would have access to 24 Left via Echo 7, but there is no end-around taxiway at the other end.
    How long do those inspections last? Do you have to do them for other ADG-IV aircraft?

    Can you tell us the distances of the runway centrelines to the taxiway centerlines? There was a proposed project a while ago to move runway 06L/24R to allow the installation of a centre taxiway, but I heard nothing about this project going further. I remember that there was a Senator (possibly a State Senator ) who entered a bill preventing the runway move, on the ground of noise nuisance ( the aircraft would be hundred feet or wathever closer to inhabited buildings). I also found a website of a lobby group that was against this, on the grounds not of noise, but because the additional traffic of the construction vehicles would snare traffic all over the area north of LAX!

    Finally, I was wrong about the ILS holdback line in my earlier comment, it is more recent. It is clearly visible on pic Pcv1390022.JPG on this page, behind the holding point right in front of the A320.

    1. It's a great "Quick Reference" guide, isn't it? A number of our local-use maps are depicted as seen from the tower. Airborne radar displays, on the other hand, are standardized with North at the top.

      Yes, Runway 24R continues in use while the inspection takes place on 24L, and we can cross the runway as long as we sequence the aircraft crossing and the vehicle performing the inspection. The A380 is the only ADG-VI aircraft that requires the post-departure runway inspection on 24L. The wider runway 25L doesn't require an inspection. The other ADG-VI aircraft seen at LAX are the B747-8, the An-124, and the C-5.

      I'm sure we've got the distances between centerlines somewhere, but off the top of my head I don't know. The easiest way to get a good estimate would probably be to go to Google Earth. Our new mayor pretty much put the kibosh on the north side runway relocation project, so for the time being Runway 24R is going to stay where it is.