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26.11.2012: Although not reflective of the experience it partly documents, this node does (now) reflect current ATC practice in the UK.

The different conditions under which an air traffic control service is provided, the different services that can be provided and the different things you can or must do under each are a bewildering mindfield of R/T phraseology and rules. You can only <give instruction #231> to this aircraft if they're receiving <service #496> except <extenuating circumstance #348>.

Hey, I'm not complaining. It's great.

This fantastic mess has all been rumbling on the horizon for some time and, aware though we all were of its approach, its final arrival has surprised and unsettled myself and many of my colleagues (I'd call us students, but the only thing we get discounted on is our pay).

Lessons and tutorials at the Air Traffic Control College I'm at have, over recent weeks, established a broad framework. What types of ATC services can be provided, what categories of flights we are required to separate, and what optional services we can provide in each class of airspace. Noone told us we'd ever have to actually do it, dammit. We were very comfortable with the hugely-limited simulated environment we were working within, thanks very much.

* * * * *

Every Air Traffic Control Officer (ATCO), whatever their particular discipline (Area Control, Approach Control, Aerodrome Control), always provides a Basic Service and an Alerting Service for their airspace. The former means giving information to aircraft about the weather, conditions at aerodromes and whether any navigation aids in the area aren't working. The latter is alerting the emergency services - airport-based and local - about 'incidents' (this might be an aircraft accident, or an emergency on the ground that has been spotted by an aircraft in flight) so they can attend quickly.

Beyond this, the three broad categories of ATC service are: Radar Control Service, Deconfliction Service and Traffic Service. They're kind of hierarchical in that order. The services I mentioned before aren't really control of any type; neither is a Traffic Service, but I include it because the involvement by the controller is a little more active.

Up until recently, our work in the ATC simulators has been solely concerned with providing a Radar Control Service (RCS) to the 'pilots' of 'aircraft' in our 'airspace' (quotation marks used because the airspace is fake, the aircraft are spectral and the pilots are all just a dude sat in front of a PC behind a divider on the other side of the room, wearing a headset and reading a script).

When you're providing a RCS, you (the controller) are solely responsible for providing standard separation between your aeroplanes. In the UK, 'standard separation' means aircraft less than five miles apart horizontally must be at least 1,000ft apart vertically, and vice versa. These standards vary a bit in some areas, but I'm trying to keep this broad brush.

Anyway, aircraft in particular classes of airspace are subject to different services. Seven classes of airspace exist in the UK, named A-G. Save your hair, that nonsense is covered elsewhere.

Airspace classed A-E is considered 'controlled' airspace (CAS) and all aeroplanes inside have to follow ATC instructions (with - *sigh* - a few exceptions). The only exception is if a pilot thinks that following a particular instruction would endanger their aircraft, in which case they're within their rights to tell ATC to get bent. Classes F and G airspace are no-mans-land, or bandit country, depending which humorous instructor you ask.

* * * * *

Now, an air traffic controller's job is of course to get rid of all of the aeroplanes in their airspace as fast as possible. We don't want to talk to an aeroplane when we've got so much more pressing issues, like... talking to every other aeroplane on frequency. To get rid of an aeroplane we make sure they get through our airspace as safely and quickly as possible.


Yes, OK, I did edit that last sentence to add the word 'safely' after writing it.

Once we've got a plane far enough through our airspace that we can see they won't need any more 'help' from us to finish the transit safely, we dump them onto the next sector and largely ignore them. It's not even like we've been that busy yet; in the worst exercises so far we've been controlling six to eight aircraft at the same time (although the amount of aircraft you have on your frequency at once ios sometimes a sort of inverse, informal barometer for your performance in that exercise), but we've usually scrabbled to get rid of planes as early as we can get away with.

I like to think I only feel busy when I'm not confident with all my phraseology. Every regular procedure in ATC has a Civil Aviation Authority-mandated phrase attached to it:

"Alitalia four-five-four, contact Medway Control on one-three-two decimal niner" I say, trying to keep the jitters out of my voice on my first-ever simulator run.

"STOP CLOCKS!" yells my instructor.

"Clocks off," the Inputter (the guy on the other side of the room, running the sim) calmly replies. The radar screen freezes.

"Okay," the instructor continues to me. "Where did you get that from?


"Where in [The Manual of Air Traffic Services ] does it say 'contact Medway control, on one-three-two decimal niner'?"

Oh my god, I think.

"Er, I d-don't remember..."

He gets up and finds a copy of MATS and brings it back, showing me the appropriate page in the phraseology index.

"You don't say 'on', just say 'contact wherever', and then the frequency. Have you ever done anything like this before, any R/T?"


"Never before, OK."

"But I used to listen to ATC quite a bit before I joined."

"Okay, where did you listen to this?"

"On the Internet."

"Yeah, but where was it from?"

"Oh, it was from U.S. airports. O'Hare, Atlanta.."

"Okay, don't listen to U.S. controllers, they're crap."

"Er, okay," I say rather uncomfortably. "I've been listening to Heathrow lately as well, though."

"Oh, well don't listen to them either, they're crap too. While you're here, MATS is your bible."

So, you have to use this phraseology to pass the course. like so many things, the real world differs somewhat from the training environment (driving lessons refer) but you have to stick to the rules to be able to move from one to the other.

I get comfortable after I've done a few exercises with no new procedures being introduced, and since the exercises tend to get busier as you progress through a phase - a 'phase' being a series of exercises that begins with you being introduced to some new procedures, and continues with you practicing them in an increasingly busy environment - I hope that confident phraseology has a lot to do with that comfort level.

Certainly the more automatic your phraseology is, the more mental capacity you have to plan how you're going to avoid a duralumin shower this time, and the less time you have to waste trying to deliver that plan to your aircraft.

* * * * *

On we go. Recent exercises have introduced the idea that as ATCOs we will deal with stuff inside and outside CAS. Up to now, we have only aircraft following airways in the sky, which are all classed as CAS. In the first few simulator phases we were constantly instructing aircraft what direction to fly and were penalised if any of them wandered outside the airway boundaries and, thus, CAS. Airways are marked on our radar screens so we can see where aircraft are in relation to them.

A couple of phases back, aircraft started navigating themselves. No more constant vectoring by us (woo!); we now only issue climb, descent or heading instructions to change an aircraft's level, to vector it for landing, to keep it safely separated (aka 'conflict resolution') or to chop a few miles off its route, if we're bored or feeling generous. Some blips occasionally wandered around outside the lines of controlled airspace but they didn't talk to you; they were just there to put you off.

This brings me nicely to a rather disturbing principle we recently learned, which is that a CAS boundary is treated like a six-foot concrete wall. If you've got aircraft on either side of it, it doesn't matter how close they are to each other because this boundary between them 'protects' one from the other. Sort of like that painted white line 'prevents' you from driving up the hard shoulder of a motorway when you're stuck in a tailback, or traffic lights 'protect' pedestrians from getting run over.

If an aircraft is flying at flight level five-zero (about 5,000ft) along an airway with a lower limit of flight level four-five (about 4,500ft) and there's an unknown aircraft underneath it, indicating flight level four-five, we don't care. Flying on the lower limit of an airway is deemed outside controlled airspace. Even though he's only five hundred feet below 'our' aircraft, he doesn't exist. We don't care, unless we have information indicating he's lost or has a radio failure.

If you're thinking this is rather unsettling, it is. But we assume the pilot of the unknown aircraft knows the boundary is there (hence the proviso for him being lost or having R/T failure) and will not encroach it. Pilots are required to have relevant information about the airspace they're flying in, before flying in it. If they do enter controlled airspace without permission and cause an AIRPROX - what used to be called an 'airmiss', and what the media universally refer to as a 'near miss' - it's their fault, not ours. Having said that, standard practice is to keep aircraft within CAS at least a couple of miles away from the boundary to help mitigate things like this.

* * * * *

So, we're now dealing with aircraft outside CAS. The so-called ATSOCAS (Air Traffic Services Outside Controlled Airspace) suite of services. Still others - unknown aircraft - wander around outside CAS too but don't talk to you, just to get in the way of the other traffic and to annoy you.

Outside CAS, an aircraft can either get a Deconfliction Service (DS), Traffic Service (TS) or Basic Service (BS) from you. When the pilot calls you up, they usually ask for the service they want and if they don't, you just ask them what they want. There are some limitations as to what types of flights can get what type of service, which I won't go into.

Any kind of radar service you may provide to aircraft outside CAS involves giving information to pilots about aircraft that potentially conflict with that of their own.:

"Golf Foxtrot Golf unknown traffic, eleven o'clock, one-five miles, opposite direction, indicating altitude 3,000 feet, unverified."

"Roger, we'll keep a lookout, Golf Foxtrot Golf.

Aircraft flying under a Traffic Service (TS) get this all the time. Aircraft under a Deconfliction Service (DS) also get heading/level instructions "aimed at achieving planned deconfliction minima":

"Golf Foxtrot Golf turn right heading two-six-zero degrees, unknown traffic, 11 o'clock, 15 miles, opposite direction, indicating altitude 3,000 feet unverified."

"Right heading two-six-zero degrees, Golf Foxtrot Golf."

If the pilot decides that they won't follow your advice they must say so; they are then responsible for their own deconfliction, although you would continue to pass information about that conflicting aircraft.

By now some of you may be wondering about the 'unknown traffic' part. How can you give information about traffic if it's unknown? Well, you know its horizontal position (you can see the blip on the radar) but you don't know what their altitude is because the pilot isn't talking to you. You don't know if they intend to fly a straight line out of your sector or if they're manoeuvring all over the place and if they are, when. A pilot receiving a DS, on the other hand, cannot alter heading or level without your approval. Even an aircraft under a TS must tell you before they alter heading or level, and get an acknowledgement from you.

"Golf Foxtrot Golf turn right heading two-six-zero degrees, traffic 11 o'clock, 10 miles, opposite direction, no level information."

"Traffic sighted, happy to take own deconfliction, Golf Foxtrot Golf."

At which point the controller will probably mutter to themselves "why did you ask for a *******g DS, then."

Deconfliction minima increase for unknown traffic, since you just don't know what their intentions are. If both aircraft are outside CAS, you must have either five miles horizontally or at least three thousand feet vertically rather than the usual one thousand. The other reason for this extra vertical separation is that since the unknown aircraft isn't talking to us, it's impossible to verify that any altitude readout we may or may not have for them is correct. If the unknown aircraft isn't displaying any level information you have to go for five miles of horizontal separation.

It's a crazy world.

To make things worse, radar coverage also limits service. There is an very rough rule of thumb that says the base of radar coverage goes up by 1,000ft for every ten nautical miles up to 60 miles, and 1,500ft for every ten nautical miles after that. In other words, at 30 miles your radar can't see anything below 3,000ft. A Traffic Service can't be provided to aircraft more than 60nm from the radar head, because the lower limit of radar coverage is too high and ATC can't guarantee safe advice for avoiding a collision with another aircraft or terrain (A Deconfliction Service can't be provided at all if an aircraft is below the terrain-safe level for the airspace they're flygin in). There are a lot more things that can limit service that I won't go into now.

* * * * *

This all has rather uncomfortable implications for trying to provide a safe service outside CAS. It should be clear now that there is a real possibility an aircraft could just appear from nowhere on your radar as it climbs into coverage, and could immediately conflict with aircraft on your frequency. This is understood and stated in law documents:

...it is recognised that in the event of the sudden appearance of unknown traffic, and when unknown aircraft make unpredictable changes in flight path, it is not always possible to achieve [minimum separation]

Let's say an unknown aircraft appears on your screen within five miles of an aircraft on your frequency, and both are outside controlled airspace. You've already not got separation and may not spot it for a few seconds. When you do, what you say depends what service 'your' aircraft is receiving. If it's a TS, you just give traffic information like normal; if it's a DS, you give avoiding action:

"Golf Foxtrot Golf avoiding action, turn left immediately heading three-two-zero degrees, traffic at two o'clock, three miles, crossing left to right, indicating same level unverified."

"Turning left three-two-zero degrees, Golf Foxtrot Golf"

Avoiding action can be either a change in level or direction:

"Golf Foxtrot Golf avoiding action, climb immediately flight level eight zero, traffic at two o'clock, three miles, crossing left to right, indicating altitude three thousand feet unverified."

"Climb flight level eight zero leaving altitude three thousand feet, Golf Foxtrot Golf."

The Manual of Air Traffic Services says this should be given using 'clear enunciation and an urgent tone'. Too right. You don't want to give perfect avoiding action then get asked to repeat it. It could be too late by then.

You would continue to give relevant traffic information until they're clear:

"Golf Foxtrot Golf, clear of traffic, resume own navigation."

"Own navigation, Golf Foxtrot Golf"

I bet you all feel safe.

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