So, why can't aeroplanes fly right next to each other? You'd be able to get loads more in the sky and there'd be no delays and airport romances and stuff.

Well, because it's completely nuts.

Why? Speeeeed. Find some in-car footage of hurtling along the Motorway/Freeway/Autobahn and hit fast forward. That's like, slow-plane speed. If you're going really fast you need more time to react to nasty things happening up front, or even just benign stuff like stupid people that want to change lanes, turn off or get onto the motorway. Why can't people just get on the motorway and stay there.

When you're flying you need to allow lots of space for stuff like this. I could (and probably will) fill a whole other node on the rules of separation that air traffic controllers have to apply to keep aeroplanes safely apart from each other, but other things go on that you have to think about too, that affect how far apart you might put some planes.

I'm training as an air traffic controller now, so it's my job to know this; not just know it, but be able to apply it in all kinds of pinches. That last bit needs work, as do lots of other bits, needless to say. It's only been eight weeks, after all. I guess I should put a disclaimer of some sort with this information since I haven't passed or taken all of my exams yet.

Warning: do not use this information to provide an air traffic control service.


Now, Wake Vortex. Primer time (sorry). As the name might suggest, this is something that aeroplanes leave behind them. It can be quite nasty, but I'll get to that. Wake Vortex is a by-product of a wing (which all aircraft have, in one form or another) creating lift.

Wings are shaped such that when they move through air, the air moves faster over the top of the wing than the bottom. The air, being a fluid substance, tends to follow the shape of the aerofoil: Coanda Effect refers. It was discovered that the increase in speed of air flowing over the top of the wing coincided with a reduction in air pressure there. A guy called Bernoullis came up with a principle that could predict this pressure differential.

These are not the only principles that explain why wings create lift. Chief among the others is the fact that aerofoils deflect air passing over them downwards. In doing so they themselves are deflected upwards. Newton law three. However, lift is also a more complex subject than can be distilled into a few paragraphs of a node (or a Primary School class, I was once shocked to learn) and one I don't fully understand myself. However, a certain aspect of it is important to this node: the pressure differential.

A wing affects the air it passes through as it creates lift. The air below it is relatively high pressure, and flows outwards along the wing. The air above it is relatively low pressure, flowing towards the wing root. As the air below the wing flows off the end, it curls upwards into the area of low pressure left by the top of the wing, leaving a tube of rotating air behind the aircraft. A horizontal tornado, if you will.

if you think I'm ascii-ising this, forget it:

That's just a light aircraft. The bigger a wing is, the more air it's moving so the stronger this effect is going to be. So reasonably, heavy aircraft create stronger vortices than lighter aircraft because their wings are doing a lot more work. The angle of attack of the wing (the angle at which it meets the air - also called alpha) also affects the strength of the vortices - generally speaking the higher it is, the stronger the vortices. The tangential airspeed of a large aircraft's wake vortex can approach 300ft/sec. Oh, did I mention that the wake is completely invisible unless the aircraft is flying through smoke or somesuch?

An aircraft generates the strongest wake when it is flying 'clean' (no high-lift devices - flaps, slats etc - being employed) at low speed. Why when clean? You would think (well, I did) that the more crap a wing is dragging behind it, the more it's going to disturb the air it passes through. Well, high-lift devices increase the effective angle of attack of a wing meaning that its actual angle of attack can be reduced and still produce the same amount of lift. A lower angle of attack means weaker wake turbulence. On take-off however, flap reduces the length of the take-off run but makes no difference to angle of attack, so a stronger wake is left behind than on approach. In flight at cruising speed, wake is not as strong because angle of attack is less; further, the winglets seen on many jets now (the small fins that point upwards at the end of the wings) help to reduce the turbulence the wings create as well as reducing induced drag.

Getting back to the story, when these vortices are sufficiently strong they can have pretty bad effects on other aircraft. There was no real knowledge of the effects of wake turbulence until a few incidents where Light aircraft following Heavy aircraft (I'm talking jetliner-sized) too closely on takeoff or approach were completely flipped over and destroyed when they hit the ground after failing to recover.

Wake turbulence is worst for aircraft when they are landing or taking off; they are not only much closer to their stall speeds than when cruising but are close to the ground too, so do not have much room to recover in the event their flight gets disrupted. An aircraft that gets flipped over by wake turbulence at 15,000ft has a fair chance of recovery; one on a two-mile final has...less.

There are standards for air traffic controllers requiring that an aircraft following another remains separated by a minimum distance, to keep out of the area of maximum vortices generated by the leading aircraft. These standards are in effect all the time, not just on take-off or landing; they state minimum spacings for an aircraft of a particular size following another aircraft of a particular size.

Now, wake turbulence will affect aircraft of different sizes by varying degrees. A Boeing 747 is going to be less affected by the wake of another 747 than, say, a Cessna 172. A 747, at anywhere between 350 and 400 tonnes in flight has much more inertia than a C172, which weighs less than two tonnes. It's going to take an awful lot more to flip a 747 than a C172; a 747 can safely follow another 747 much closer than a C172 could. Vortex wake spacing minimums make allowances for this.

In the UK, aircraft are divided into four five weight categories for vortex wake spacing purposes:

  • Light: 17,000kg or less (e.g. C172)
  • Small: 17,001kg-40,000kg (e.g. Gulfstream 4)
  • Medium: 40,001kg-136,000kg (e.g. Airbus A320)
  • Heavy: 136,001kg or more (e.g. Boeing 747)
  • Super-Heavy: Airbus A380-800 aircraft, who amusingly have to identify themselves to air traffic control as "<Callsign> SUPER": "London, Air Singapore 123 Super"

There is also an informal fifth sixth weight category called 'Upper Medium' that is not listed because it only applies to five aircraft: the Boeing 707, Boeing 757, Vickers VC10, Ilyushin IL-62 and Douglas DC-8. Most of these aircraft are actually Heavies in pure weight terms but they produce vortex wakes closer to those of Mediums, so are classed as Mediums with slightly higher vortex wake spacing requirements.

Needless to say the largest spacings are for Light aircraft following Heavies/Supers; these spacings reduce progressively for larger following aircraft.

The spacing minimums are split up into two categories: final approach and departure. These apply distance-based and time-based spacings respectively.

Spacing requirements: Final Approach

Leading Aircraft       Following Aircraft      Spacing (nautical miles)
A380-800               A380-800                4
                       Heavy                   6
                       Medium                  8
                       Small                   8
                       Light                   10
Heavy                  Heavy                   4
                       Medium                  5
                       Small                   6
                       Light                   8
Medium                 Heavy                   N/a*
                       Medium                  3
                       Small                   4
                       Light                   6
Upper Medium           Medium                  4
                       Small                   4
                       Light                   6
Small                  Heavy                   N/a*
                       Medium                  3
                       Small                   3
                       Light                   4
Light                  all                     N/a*

* no vortex spacing required beyond standard separation.

The spacing requirements for the A380-800 only apply if the other aircraft is either following directly behind or on a track that will cross behind the A380's track, in both cases either on the same level or up to 1,000ft below the A380. These figures are provisional and it is expected they will be revised downwards in the coming months.

These spacings also apply to aircraft taking-off from runways that cross or diverge, where the projected flightpaths would cross, or where they are taking-off from parallel runways less than 760 metres apart. Vortexes tend to sink below the level of the generating aircraft and move sideways when they get near to the ground, hence the runway spacing requirement. It makes it less likely that the vortexes will reach the other runway before dissipating.

The spacings required on take-off are much less varied and to be honest, a little confusing. The standards take note of whether the trailing aircraft is taking off at, before or after the point the leading aircraft did, as well as their sizes.

Spacing Requirements: Departure

Leading Aircraft       Following Aircraft      Spacing (minutes)
A380-800               Heavy (not an A380)     2
                       Medium, Small or Light  3
Heavy                  Medium, Small or Light  2
Medium or Small        Light                   2
Heavy                  Medium, Small or Light  3
(full length take-off)
Medium or Small        Light                   3
(full length take-off)

These spacings also apply if the leading aircraft has done a go-around or a touch and go and the lighter aircraft is either:

  • Taking-off on the same runway in the opposite direction;
  • landing on the same runway in the opposite direction
  • landing in the opposite direction on a parallel runway that is less than 760 metres away from the runway the heavier aircraft has passed. If any runways are less than 760m apart, these spacing requirements consider them to be a single runway.

Interestingly enough the rules in the UK also allow some latitude in issuing take-off clearances (I imagine other countries make similar allowances). Let's take an example looking at the spacing requirements above: we've got a Boeing 777 (Heavy) taxied into position for take-off and behind we've got an Airbus A320 (Medium) at the holding point behind it, next in the queue.

We launch the triple-seven and mark the time he starts moving. Now, we could wait two minutes before launching the A320 and it would definitely get us our vortex spacing. However, it would waste quite a bit of time, particularly if the airport is busy. See, the separation standards only apply from the moment an aircraft rotates (its nosewheel leaves the ground as it starts to lift off); this is the moment its wings start generating wake turbulence.

So we wait until two minutes after the triple-seven has rotated before launching the Airbus. Again, you can probably see this isn't a very efficient use of time. A lot more than two minutes will have passed by the time the Airbus actually builds up enough speed to rotate.

The aforementioned latitude in the rules allows the tower controller to exercise some judgement in issuing the take-off clearance for the trailing aircraft, making an allowance for its take-off run. So while less than the minimum two minutes may have passed when trailing aircraft is given its take-off clearance, by the time it gets airborne that minimum period will have passed.

So now you know a fraction about how aircraft are separated.

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