Wake Turbulence: Part 2 - Controller Requirements

This is the second part in a series on wake turbulence that I'm doing in response to a reader's inquiry. Link to Part One.

As I discussed in the first section, the wake turbulence an airplane creates is invisible under most circumstances. So how can pilots avoid getting caught up in one of these vortices? The primary avoidance techniques rely on predicting the presence of wake turbulence by being aware of the locations and actions of preceding aircraft. The pilot's first concern is, of course, his own aircraft, and keeping track of others, especially ones that may not even be on the same radio frequency or the same runway, is not an easy task. In most situations, air traffic controllers are thus in a better position to protect an aircraft from the wake turbulence of the airplanes ahead because it's our job to know where all of our airplanes are.

For air traffic controllers in the United States, the FAA has set out wake turbulence rules for controllers in our rulebook, officially known as FAA Order 7110.65. Most controllers in the US are FAA employees; many of the rest are either military or Department of Defense. There are non-federal civilian controllers who work for private companies at some control towers, often referred to as "contract" towers. In all cases, the same separation standards and rules are applied.

The 7110.65 is hundreds of pages long, and reading it is a guaranteed cure for the worst case of insomnia. I will attempt to not replicate that experience here. Hardy souls are welcome to peruse it to your heart's content. Meanwhile, I'll paraphrase and generalize here to cover the main points; nuances and details will come later (if at all).

In a nutshell, the idea is this: The bigger the airplane in front, the greater the distance required between it and the next aircraft. Put differently, the smaller an aircraft, the more space required between it and the aircraft that it is following. To that end, aircraft are broadly broken down into three wake turbulence groups: Small, Large, and Heavy. The determination is based on the aircraft's maximum weight. It is part of the controller's job to know the weight classification of the aircraft under his control. We'll go into the weight classes, with photo examples (You knew that was coming!) in a later part of this series.

Let me start out by explaining that, not counting wake turbulence, a tower or approach controller normally needs three miles between aircraft. A center controller, who uses a different (long range) radar system, normally needs at least five miles. There are all sorts of exceptions and loopholes that I won't get into here because they're not germane to the topic at hand. However, the wake turbulence separation requirements apply to all phases of flight: take-offs and landings, as well as just flying along between the two.

When we have to consider wake turbulence behind an aircraft, the standard practice is to add additional miles between it and the next one. This is to allow some time for the wingtip vortices to spread out away from the runway and begin dissipating:

In some situations, we can't use miles-in-trail and instead must use minutes; in other words, time is substituted for distance. The most common application of minutes would be for a tower controller. When an airplane is going to depart from a runway intersection behind a larger airplane that started from a point prior to that intersection or from the other (opposite) end of the runway, there must be a three-minute interval between the two departures. Example:

At LAX, this is the most common scenario that produces a 3-minute wait for wake turbulence: A heavy jet (red arrow) has departed from the end of Runway 25 Right. A subsequent departure (green arrows) will go from Taxiway F or off Runway 25 Left. Because the two runways are only a few hundred feet apart, the runway 25 Left departure is treated just like an intersection F departure on runway 25 Right. This happens to be one of the traps for controllers training at LAX; if the controller expects to need to do this, he has to make note of the heavy's departure time so that the three minute interval can be applied.

Most of the time, however, we use mileage instead of minutes. This does require the use of radar however, so if the radar is out of service (or at towers where there is no radar available), we have to revert to using minutes for establishing wake turbulence separation. That said, at a minimum, we are required to have five miles behind a heavy jet. This applies in either departure/arrival or enroute scenarios; the following airplane should be at least five miles behind the heavy jet. The pilot of the following aircraft has to be told that he's behind a heavy jet and given a wake turbulence advisory. For example, I might say:

"Delta 492, five miles in trail of a heavy Boeing 747, caution wake turbulence, runway 25 left cleared to land."

"Southwest 1132, caution wake turbulence departing heavy Tristar, runway 24 left position and hold."

As I mentioned before, in a radar environment we can use miles in lieu of minutes, but the pilot has the option to request additional spacing if he deems it necessary. This sometimes happens with departures; I've had a pilot tell me that he'd like 'the full two minutes' or 'an extra minute' behind a larger departing aircraft. The two minutes that the pilot asked for is what we have to use behind a heavy jet if we can't determine the mileage and the following aircraft is departing from the same point and in the same direction as the heavy jet (in other words, not from an intersection).

Besides the five miles behind a heavy, there are a number of other wake turbulence rules for controllers. One of these is particularly simple: Any time we're talking to, or about, a heavy jet, we have to use the term "heavy". Examples:

"Northwest One Heavy"
"Speedbird 283 Heavy"
"Heavy DC-10"
"Heavy Boeing 767"

The point of this is to increase everyone's awareness of the wake turbulence potential created by the heavy jet. Controllers are required to say "heavy", and pilots of such aircraft are encouraged to use the term as well.

As a side note, all the "heavies" are jets; there are no active propeller-driven aircraft in this country that qualify as "heavies". One that would qualify if it were to return to the air is the Hughes H-4, better known as the Spruce Goose. The only other one that I'm aware of is the Russian Tupolev Tu-95 'Bear' bomber, but we don't get many of those at LAX!


Credits:
Opening photo: The New York Times: The Inevitability of Bumps
Dissipation graphic: FAA Airman's Information Manual

Link to next part: Boeing Heavy Jets