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I am busy studying radio for my PPL in South Africa, but I'm struggling to understand the ICAO definitions of the different airspace classes, and how ATC separates traffic in them.

I understand that the difference between airspace classes is based largely on which flights are "controlled" or "separated" by ATC, and which ones are only getting "traffic information". But I don't understand exactly what the difference is.

I fly a rotor-wing at a class D airport. Under VFR, we should only receive "traffic information", correct? But in reality, we still have to request ATC's permission to take off etc. and they can instruct us to hold, give way and so on. Doesn't that mean that we are in fact being "separated" from other traffic?

And theoretically, if IFR traffic is controlled by ATC in class D but VFR is not, and IFR is not separated from VFR, how should conflicting traffic avoid each other? If an IFR flight sees a VFR one in its way, who should do what? (Practically, the pilots will obviously maneuver to avoid each other, but what does the theory say?)

Finally, since IFR flights only get "traffic information" about VFR flights but not "separation", what happens if a VFR flight gets in front of an IFR one on final approach to land? Won't ATC tell the IFR flight to go around, or will they do nothing because they don't provide "separation"?

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  • $\begingroup$ Welcome to aviation.SE! Which country are you in? This question covers the general concepts for the US, if that's where you are. $\endgroup$ – Pondlife Jan 12 '17 at 19:29
  • $\begingroup$ Thanks for the fast response! I am in South Africa, however my confusion still lies in understanding the ICAO definitions for the airspaces, specifically the practical difference between "controlled" / "separated" and "traffic information only". Thanks, I have read through that question before posting, but am looking for a more practical explanation, from a pilots perspective? $\endgroup$ – Dane Jan 12 '17 at 19:44
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    $\begingroup$ I edited your question to try to simplify it a little and include some information you put in comments. If I've overdone it or distorted your question, please just rollback or re-edit. $\endgroup$ – Pondlife Jan 12 '17 at 21:48
  • $\begingroup$ @Dane Remember to mark an answer, if you have an answer, to close the question. $\endgroup$ – Bullfrog Jan 14 '17 at 0:20
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Different countries have different meanings to the classification of Airspace. I am only looking at the controlled airspace for ICAO.

Controlled airspace is an airspace that an aircraft would need a clearance to enter, this doesn't mean all aircraft need a clearance to enter just some aircraft.

Classes of Airspace

Class A is for IFR flights only, all aircraft are separated from all aircraft and are known to Air Traffic Control (ATC).

Class B is like Class A but with VFR. IFR and VFR are cleared to enter, all aircraft are separated from all aircraft and are known to ATC.

Class C is where the major confusion on separation usually begins. IFR and VFR are cleared to enter, all flights are provided an ATC service. IFR flights are separated from IFR and VFR flights, but VFR flights are only separated from IFR flights. VFR aircraft are only passed traffic on other VFR aircraft. All aircraft are known to ATC. I will go into what they do when they get in conflict at the end.

Class D both IFR and VFR are cleared to enter, all aircraft are provided an ATC service. IFR aircraft are separated from IFR and passed Traffic on VFR flight. VFR flights are only passed traffic on other aircraft. All aircraft are known to ATC.

Class E IFR are cleared to enter and VFR are permitted to enter, only IFR aircraft are provided with an ATC service. IFR aircraft are separated from IFR aircraft and passed traffic on known VFR aircraft. VFR aircraft do not need a clearance and are not usually known to the Air Traffic Controller.

Class F No one is given a clearance. IFR aircraft are given traffic on IFR Aircraft and known VFR. But all pilots are responsible for their own separation.

Class G No one is given a clearance. Traffic is only passed where it is requested, and only on known traffic. Only those requesting traffic or are seen on a radar are known to ATC.

Answers to Specific Questions

I fly a rotor-wing at a class D airport. Under VFR, we should only receive "traffic information", correct?

Yes, that is correct.

But in reality, we still have to request ATC's permission to take off etc. and they can instruct us to hold, give way and so on. Doesn't that mean that we are in fact being "separated" from other traffic?

When talking about Class D airspace: you are only separated from other aircraft when on the runway; when you are in the air you are not being separated. You instead will be passed Traffic Information on other flights. You however are still subject to the conditions of your clearance (Route, Level, Holding Instructions, etc.).

And theoretically, if IFR traffic is controlled by ATC in class D but VFR is not, and IFR is not separated from VFR, how should conflicting traffic avoid each other? If an IFR flight sees a VFR one in its way, who should do what? (Practically, the pilots will obviously maneuver to avoid each other, but what does the theory say?)

Finally, since IFR flights only get "traffic information" about VFR flights but not "separation", what happens if a VFR flight gets in front of an IFR one on final approach to land? Won't ATC tell the IFR flight to go around, or will they do nothing because they don't provide "separation"?

If one or both aircraft are under a clearance the cleared aircraft will request an amended clearance to avoid the traffic:

ABC request 10NM right of track due traffic

or

ABC request descent 4500 due traffic

If there is not enough time for the amended clearance the Pilot in Command (PIC) can just break their clearance to avoid the traffic; either passing the aircraft on the right or changing levels. The PIC in some countries will be considered to have broken their clearance but it is usually only a slap on the wrist for letting it get that bad that you couldn't request the clearance change.

Reality can sometimes be a very different scenario. Some controller unofficially will separate VFR from IFR, but no one knows. Don't trust that this will happen.

Also controllers may give suggested amended clearances like:

ABC traffic XFV PC12 crossing track left to right at time 23,
6500 is available advise.

or

ABC traffic unknown VFR 25NM oposite direction 500 ft below unverified,
higher available advise

If the aircraft is not subject to a clearance then they can just move to avoid the traffic under the Rules of the Air.

In uncontrolled airspace it really is see and avoid. Because the Air traffic Controller may not have all the aircraft that are traffic for you. This also includes Class E airspace where the controller doesn't know about all the VFR aircraft.

Duty of care means that the controller may intervene if they think the pilots aren't separating themselves.

I work with Class D and C Towers with lots of VFR traffic. Because the traffic are aware of each other they usually request amended clearances well before the traffic is a problem.

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  • $\begingroup$ Thanks! I think that the definition for Class D confuses me the most: IFR are separated from IFR, but not from VFR (which seems silly) - IFR won't collide with IFR, which is great, but VFR could theoretically collide with IFR, since "VFR is not separated from IFR by ATC". It is easy to understand the definitions as just words, but in practicality they sometimes don't seem to make sense to me, and in "reality" (as you have mentioned) I think that they are not necessarily implemented as per the wording (?). $\endgroup$ – Dane Jan 13 '17 at 6:44
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    $\begingroup$ It almost looks like ICAO defined the airspaces according to this neat pattern of decreasing "separation", but didn't think about how that actually makes sense in practice. I still think that it would be interesting to know, from an ATC point of view, how the airspaces differ for them, in terms of job / equipment requirements. $\endgroup$ – Dane Jan 13 '17 at 6:51
  • $\begingroup$ @Dane From an ATC Point of view (Which I am), we prefer to be control of everything because we are control freaks. Just because VFR are not separated from IFR doesn't mean it is unsafe and collision could occur. Traffic statements are able to inform pilots what they need to avoid themselves, Much better then not knowing anything like Class G. $\endgroup$ – Bullfrog Jan 13 '17 at 11:02
  • $\begingroup$ In practice, that is what I would expect - you as the ATC are going to try and keep everyone separated from everyone, regardless of "airspace class". Perhaps it is best to just memorise the definitions for exam purposes, but forget about trying to understand any actual reasoning behind the definition for e.g. Class D, since in reality things will probably be implemented differently. Or maybe this will all make more sense if/when I have actual experience with something other than just VFR in Class D / G airspace. $\endgroup$ – Dane Jan 13 '17 at 11:14
  • $\begingroup$ @Dane You still need to know the books for reality, because a lot of it is run exactly as the books say, once you are doing it the books make a lot more sense. $\endgroup$ – Bullfrog Jan 14 '17 at 0:18
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You are confusing the terms controlled/uncontrolled with separation. Being controlled does not necessarily mean that separation is provided.

In a controlled airspace (for VFR, this would be class B, C, D - for IFR class A, B, C, D, E) you need a clearance to fly. That is, the controller needs to give you permission to be there. You cannot enter a controlled airspace without a clearance, and you cannot turn/change level without a clearance.

Let's take class D, since you bring it up yourself and it is actually quite an interesting example. In class D airspace, IFR and VFR is allowed, and both IFR and VFR need a clearance (meaning that both IFR and VFR are controlled). IFR flights will be separated from other IFR flights; it is the responsibility of the controller to do so. No separation is provided for VFR flights. If two flights - either two VFR flights or an IFR flight and a VFR flight - find themselves on converging courses, it is up to the pilots to avoid a collision in accordance with the rules of the air (e.g. give way on your right). Unlike uncontrolled airspace, the pilots will know of all other traffic around them, because the controller will give traffic information. The controller can do this, because every flight in the area needs a clearance to be there, and so the controller is aware of everyone. In practise, this means that pilots on an IFR flight do not need to constantly look out the window for other traffic, because the controller will inform them of it. But if they are informed of other traffic, they need to avoid it by means of the rules of the air.

Traffic information in uncontrolled airspace is different, because it cannot be guarenteed that the controller is aware of all traffic in the area, since flight do not need a clearance to be there.

With regards to runways, airspace classes do not apply. On a runway, there are only two states: controlled or uncontrolled. If an aerodrome is controlled, the tower controller will provide runway separation, regardless of flight rules (IFR/VFR).

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  • $\begingroup$ So in e.g. Class D airspace, ATC can provide separation to VFR, but they do not have to? The difference lies in who is ultimately considered responsible for the separation? Is there any particular reasoning behind this - e.g. to reduce the workload (perhaps number of personnel required) for ATC, or based on their capabilities due to equipment / systems which they have available? Could it be said, in this example, that IFR are exactly told what to do / guided / actively controlled by ATC, whereas VFR can do what they like so long as they ask/get permission first (but may be controlled)? $\endgroup$ – Dane Jan 12 '17 at 20:40
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    $\begingroup$ @fooot You appear to be talking about FAA procedures, which this question is not about. $\endgroup$ – J. Hougaard Jan 12 '17 at 20:44
  • $\begingroup$ @Dane, No, ATC will not provide separation to VFR flights in class D airspace even if they could - because ATC is not responsible for separation in class D. Different airspace classes are used for areas with different traffic levels and complexity, and the cost/benefit of the service provided. $\endgroup$ – J. Hougaard Jan 12 '17 at 20:47
  • $\begingroup$ I think it would help if you explained the other classes, which would help to clarify the difference between "traffic information" and "separation", which the question seems to focusing on. $\endgroup$ – fooot Jan 12 '17 at 20:58
  • $\begingroup$ OK so perhaps it is important to emphasise (as I understand your answer), that runways / taxiways / etc on the airport itself are not part of the airspace, and that ATC will provide control (separation?) to all flights with regards to these? So whilst I am VFR in the circuit ATC cannot tell me what to do, only advise me of other traffic (and tell me where to report?), but when I come in to land then they can tell me what to do - whether I may land (or if I should go-around / orbit), where I may land, whether / where I may take off (and in what runway direction), etc? $\endgroup$ – Dane Jan 12 '17 at 21:00

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