Get 'um down!

Altitude is the most basic form of separation in the 3D world of Air Traffic. It doesn't require radar, and is often the most efficient way to keep two airplanes from colliding. However, aircraft eventually run low on gas and want to land. How do we safely allow planes to descend through the maze of other traffic and what considerations are necessary to accomplish this? It's funny you should ask...

Compression

The wind is in effect 24/7. In general, aircraft with a headwind climb, descend and turn faster relative to the ground, because they are moving over the ground slower than the aircraft with a tailwind, who fly faster, but climb, descend and turn relatively slower.

In the en-route world, the winds vary greatly at altitude, and this causes aircraft with a higher tailwind component to fly faster, when assigned the same speed, as aircraft with less of a tailwind. Wind tends to be stronger at higher altitudes. Thus, as aircraft descend into the airport, the first airplane will be at a lower altitude first, with less of a tailwind then the aircraft following it down from behind. Even though they will be at the same altitude at the same point in space, the trailing aircraft are higher relative to the current time. So, if you have 2 identical aircraft at FL270, 6 miles in trail, and both are assigned 300 knots, the first aircraft will start down 6 miles (or about 1 minute)before the second one wants to. The first plane will already be one or two thousand feet lower than the back one until a minute after the first one levels at the lower assigned altitude. However, the first aircraft will gradually lose part of the tailwind sooner than the back aircraft, causing the front plane to slow down relative to the ground and the plane following. This is called compression. We need 5 miles, and your 1 mile buffer you had at FL270 will cease to exist by the time they're both down to FL180.

The vast majority of descending aircraft in my area flow eastbound, so they almost always have this tailwind compression issue. The compression for westbound traffic and headwinds is less of an issue (there would still be some compression due to the thinner air aloft), but I don't have any extensive experience with lines of aircraft doing this. This is my blog, so we'll stick with what I'm familiar with.

There are a few techniques controllers use to prevent this from happening. The first solution is the assign the back aircraft a speed at least 20 knots slower than the front aircraft. The first plane would be assigned 310 knots or greater, the second 290 knots, the third 270 knots, and the fourth would be assigned 250 knots. As they reached their lower assigned altitude they would then be reassigned 310 knots to keep up with the first guy.

We can't assign slower than 250 knots, so we'd have to vector any other planes out to allow room for the compression to happen. Often times, we'll vector everyone a little, except the first one, of course, just keep the speeds up with about 10-15 miles in trail. This would depend on the difference in wind speed at the starting and ending altitudes.

In other cases, aircraft will approach the crossing point from different angles. This is effectively the same issue, since one aircraft will have more of a tailwind component than the other. We will either assign different speeds in attempt to keep them matched up, or we can vector the back airplane more towards the route of the first one so they both have similar tailwinds.

Another common solution is starting the back aircraft down first, or assigning a restriction that keeps both aircraft at the same altitude at the same time. And that brings us to our next chapter.

Restrictions

There are a bunch of ways we instruct aircraft to descend. Some require the pilot to descend immediately after accepting the clearance, while others allow the pilot some discretion. A few techniques are actually not totally "legal", but we'll get into that.

The following sequence will assume the same situation depicted above. The aircraft on the left is at FL270, and is the aircraft we will be communicating with. Our standard traffic flow requires the plane to enter the next sector (which is just to the right of the VOR) at FL180. Any subsequent aircraft shown, (such as the aircraft to the right of the VOR) are considered traffic in that airspace, but that traffic isn't moving, per se. There will be a plane there when our plane gets there. We better have altitude separation, if you know what I'm saying...

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The most basic clearance is "Descend and maintain FL180." The aircraft will descend at a "normal" rate, which varies greatly. Some will descend at 500 feet per minute (fpm), most will use 1000fpm, but others will descend as much at 3000fpm. Controllers tend to expect 1000fpm. Since this clearance is unpredictable, this would not be a good clearance in this situation, as you can see below.


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The best, and most common, clearance would be "cross VOR at FL180." This is called a pilot's discretion descent. The pilot could maintain FL270 (with a nice tailwind and thin air, very fuel efficient) as long as he/she would like and then descend rapidly using idle thrust. Or, the plane could start down early if they were in turbulence. It is up to them.


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Now lets say there is traffic crossing in front of the VOR at FL270 as well. We can't let the plane stay at FL270 as long as it would like, but we don't need the plane going all the way down yet, either. "Descend now to FL260, then cross VOR at FL180" This tells the pilot to start down to FL260 right away, but then they have discretion to cross the VOR at FL180 as before.


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Another common occurrence is to issue minimum descent rates to planes. This is often used when there isn't a convenient place to issue a crossing restriction, or when there are multiple airplanes in the way and we just need to get the plane going down quick. "Descend and maintain FL180, descend at 2000fpm or greater through FL190". This is NOT included as an approved way to ensure separation according to our rule book, and so, we are taking our careers in our own hands when we use this. When issuing this type of clearance, we have to watch very carefully to make sure that the plane will still be OK, as opposed to a crossing restriction which we can assume will work. Nothing a hard left turn wouldn't fix! And that isn't to say we don't keep an eye on planes with crossing restrictions....

My personal preference is to use descent rates when there isn't really anyone in the way, but a fast rate of descent would benefit me in other ways, such as preventing compression or if other future descent clearances to other aircraft are predicated upon the first plane getting out of the way.

Also, starting an airplane down early, faster than normal, may only serve to over-restrict the airplane and cause it to burn more fuel.


When there is specific traffic involved, I prefer a slightly less illegal "descend to reach FL180 in 3 minutes or less". I say "less illegal" because the correct way is to say "descend to reach FL180 at or before 1323 zulu, time now 1320 zulu." I hope to get some brownie points by using half correct phraseology (descend to reach FL180), I just don't give an exact time or a time check. They both mean the same thing, but the legal way is MUCH more confusing and time consuming. I don't care what time it is, just hit your stop watch and be level in 3 minutes please. Again, if I use a descent rate or don't use the time check appropriately, I need to keep an eye on it, have a good backup plan (vectors mostly), and hope I don't lose my radios. I could always just assign a different altitude that is safe. Yeah. That's a good idea too.

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Speaking of confusing, lets change the traffic a little. Also, assume we don't need the airplane AT FL180 for the next sector, just descending to FL180.


In one fell swoop, we can issue "descend now to FL250, cross VOR at or below FL210, descend and maintain FL180". If you get a good readback the first time around, tip your hat to the pilots, especially if they're in the middle of their descent briefing or trying to copy down the ATIS on the other radio. Perhaps "descend at least 2000fpm for traffic" would be better after all....

Also, many pilots forget that they DO NOT have pilots discretion after the VOR, and they often level at FL210 until prompted by us to continue down.

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You'll notice how I always have the blue "idle" line up there. That is the ideal descent rate for the aircraft based on the weight of the aircraft, the winds aloft, the speed being flown, temperature, and countless other factors, I'm sure. That blue line, obviously, is never the same for each plane and the time of day, etc. We don't know what it is. But we've grown accustomed to aircraft descending quite rapidly into their restrictions. I've seen up to 7000fpm. That would mean that a plane that is a little more than 1 minute from the VOR can still make the crossing restriction. Excellent. Every once in a while (and more often recently, it seems), something goes astray and they don't make it. And by the time the pilots realize they aren't going to make it, it is too late. They're descending too fast to stop the descent for other traffic, and they're too close to the VOR to turn them out a little and give them more room.


I assume the main reason for this error is when the wind at lower altitudes is stronger than the pilots/flight computers expect. The ideal profile gets pushed out a little at the lower altitudes and thus they miss the restriction. I'm not sure what I'm supposed to do about this as a controller except be extra cautious and start giving "cross 5 miles west of VOR at FL180" to give them a buffer. Yet that is what I gave the trailing plane to prevent compression! So much for that.

Descending on idle thrust also prevents us from issuing a speed adjustment until the plane is level if we want them to make the altitude crossing restriction. This isn't a big issue as long as you've given yourself enough room for compression.

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Its very annoying when I see a plane approach their restriction, still at their original altitude, and when challenged "verify you will cross VOR at FL180?", they respond "affirmative, we'll make it, we're starting down now" and then they miss it by ALOT. That happened to me more than twice last week. Did I mention its annoying. Perhaps unsafe?

I could recommend that pilots stop descending so fast. I could claim the problem is greed in the airline industry trying to save money in an unsafe manner. But in some situations when we, as controllers, get busy and can't give the crossing restriction until the last minute, the pilots drop the anchor and make us look good.

I guess I just want to know when you can't make your restriction...in time to do something about it. Thank you.

Till next time,

DM