Another Reason CDAs Won’t Work

I got into a rather heated discussion with an airline pilot/friend about CDAs (Continuos Descent Approaches) the other day. In the process, I hit on another reason CDAs won’t work. And let me clear that up while I’m here. They’ll work -- for some of the people, some of the time -- but they won’t cure what ails the system.

If you take one group of airplanes inbound to one airport -- and consider nothing else -- CDAs will save that group of airplanes at that one airport a ton of fuel. In other words, it’s the same old story. It’s a valid theory -- on paper. Trying to integrate it into a system with hundreds of variables is another problem altogether. We can “make it work” (at a very limited number of places) but only at the expense of some other portion of the system.

With that caveat out of the way, let’s get down to that other “reason”.

I think of CDAs as unmolested descents. The aircraft reaches a certain point, and after that point, ATC won’t “molest” the aircraft (turn, level off or assign different speeds) any further until the aircraft lands. There is some wiggle room in all that, but that is the general idea. In order for CDAs to work, all the aircraft have to be perfectly spaced at their cruising altitude. That is “perfect” in human terms. With computers calculating the spacing and flying all the aircraft (i.e. everyone is using the autopilot) that sort of perfection is possible. The issue then becomes attaining the desired spacing of the various aircraft at their respective cruising altitudes.

It is that location -- the typical cruising altitudes -- that presents another reason CDAs won’t work.

Ask any enroute controller (ARTCC or Center controller) to describe the busiest airspace and they’ll have to think on it for a minute or two. If you’re at Atlanta Center (my old workplace) they will probably think in terms of sector names. “Spartanburg High is tough.” “The Pulaski sector is a nightmare if the ATL and CLT arrivals are there at the same time.” You get the idea. But for this argument you need to ask them which altitudes are the busiest. Once they (and you) start thinking in that frame of reference, the problem becomes clearer.

As a generalization, most aircraft spend 50% or more of their time at their cruising altitude. If you’ll ask the same Center controllers where most of the real spacing work takes place -- in terms of arrivals -- they’ll quickly come up with the altitudes of FL230 to 3,000 AGL (Above Ground Level). In other words, it takes place below the typical cruising altitudes right down until the arrivals are lined up with the runway.

Think of it as processing a piece of wood. The high altitude sectors do little more than knock the bark off the logs and rough cut them into boards. The low altitude sectors at the Centers -- typically FL230 to 11,000 MSL (Mean Sea Level) -- cut them into standard lengths and give them a rough sanding. The Approach controllers cut them into the custom lengths needed and sand them smooth.

A CDA tries to make that entire process happen at the cruising altitudes. After the aircraft starts down (begins its descent), controllers aren’t supposed to use anything but fine sandpaper to smooth out some slight blemishes.

If the problem isn’t obvious to you, that would be because you aren’t a Center controller. The high altitude controllers are already busy chopping down trees. Remember, that is where most of the aircraft spend most of their time. They already have their hands full just keeping the enroute aircraft separated. You’re asking them to monitor the cabinet shop (the computers for CDAs are supposed to do the work) while they harvest the trees and operate the saw mill.

To put a finer point on it, suppose the enroute controller turns aircraft B 20 degrees right to go behind aircraft A. Just when it looks like it is going to work, the computer trying to accomplish the spacing for the CDA says to slow aircraft A down. If the controller does that, the vector will no longer work. It’s either ignore the CDA computer instruction or turn aircraft B another 10 degrees so that it will still go behind aircraft A when aircraft A slows down.

The airline flying “aircraft A” will think CDAs are great because of the fuel they are saving. The airline that is flying “aircraft B” will quickly figure out that CDAs are costing them money. It isn’t hard to figure out that A and B are interchangeable so, in the end, the process is a wash. Except for the high altitude controller. He’ll now have three jobs to do, instead of one or two.

You might be asking yourself what the low altitude Center controllers will be doing -- assuming CDAs work (which they won’t.) That question (I hope) will bring some understanding, if you think about it. The reason the majority of the spacing work at the Centers occurs in the low altitudes (FL230 down to 11,000 MSL) is because those altitudes have the capacity to handle the work. There aren’t that many airplanes enroute at those altitudes. The structure of a sector (geographical boundaries and altitudes) comes about for reasons. Form follows function. There is a reason most of our low altitude sectors working airline hub arrivals look like funnels.

It really is hard to see the forest for the trees sometimes. Just remember, this is only one reason that CDAs won’t work any better than the current system works. There are others.

Don Brown
May 4, 2009