Aviation Topic of the Week
By Michael Oxner, July 27, 2003


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This week's topic:
Controlled vs Uncontrolled Airspace

This is an interesting and sometimes confusing subject. VatSim makes it a little harder, too, just considering one of the major differences between VatSim and the real world: ATC isn't always available where it should be. I'll discuss some ideas for real world, and then some adaptations for use with VatSim.

Basic Concepts
Classes of Airspace
    A
    B
    C
    D
    E
    F
    G
Normal Structures of Airspace
    Graphic Profile Example of Airspace Structure
    An FIR Pictorial Presentation of the Base of Controlled Airspace
VatSim Hardships with Airspace Classes

Basic Concepts

The definitions are pretty simple. In Controlled Airspace, ATC provides air traffic control services. In Uncontrolled Airspace, they don't. ATC does still provide different types of services in uncontrolled airspace, if not control services, though. All areas in Canada, including some airspace offshore, are contained with an FIR, or Flight Information Region. Flight information ranges from weather related data and pilot reports, to the one of the most important issues called alerting service. In the real world, if you don't depart within a certain amount of time from your proposed departure time, land roughly when expected to (normally considered at an airport without an ATC or FSS facility) or even report over a certain fix within a given amount of time of the estimate for that fix, ATC will alert authorities with pertinent information and they'll send someone out to look for you. This all relates to flight information services, whether in controlled or uncontrolled airspace.

In uncontrolled airspace, ATC will not, and can not, take any action to separate you from other aircraft. As part of flight information, ATC will, workload permitting, provide you with traffic information. Much of the country's uncontrolled airspace lies in low altitude or remote areas, making radar coverage sparse. If radar coverage permits, ATC may radar identify you and provide you with radar traffic information, such as "Traffic 12 o'clock 10 miles, opposite direction Navajo at 6,000 IFR". Otherwise, the traffic report will be in relation to a fix on a chart, such as, "Traffic is a Navajo, opposite direction on RR22 estimated over Charlo NDB at 1347z, 6,000 IFR". It's then your job to determine what impact this traffic may have on your flight, if any at all.

Classes of Airspace

How do you know if you're in controlled or uncontrolled airspace? You have to know what Class of airspace you're in. A few years ago, the classifications of airspace changed to more closely match ICAO's standards in use elsewhere in the world. We now have 7 classifications, lettered A-G. Where they are, what you're allowed to do in each, and what ATC services can be expected in each, vary. A little bit of basics here is that, in Canada, High Level Airspace is that airspace from 18,000 feet ASL (considered FL180) and above, and Low Level Airspace is everything below that. Note that for ATC, many FIRs are split "High" and "Low", but this has no bearing on, and no relation to, the book definitions of High and Low Level airspaces. For example, Moncton and Gander (among others) are split high-low with high level's area beginning at FL290. Low level, therefore, is below FL290, and this includes some High Level airspace, between FL180 and up to, but not including, FL290. The following definitions of the classes are taken out of the AIP Canada, RAC 2.8, Classification of Airspaces. All of this information applies to Canadian Airspace specifically, so if you're outside of Canada, the information below may not be correct.

Class A Airspace

Class A airspace is designated where it is desirable to exclude VFR completely, and all operations, regardless of weather conditions, are to be IFR. Transponders must be on board all aircraft, and they must include mode C for altitude encoding. Class A airspace includes all controlled High Level Airspace up to and including FL600. Note that there is some High Level Airspace in the Northern and Arctic Control Areas that is not controlled. Also, all the North Atlantic Airspace is Class A airspace at and above FL55 (approx. 5,500 feet) up to and including FL600.

Class B Airspace

Airspace where IFR control service is desired, and a need to control VFR aircraft exists, is designated Class B. All Low Level controlled airspace above 12,500 (at or above the Minimum Enroute Altitude for an airway) is classified as Class B. Since it's Low Level Airspace, it goes up to, but doesn't include, 18,000 (FL180). Control Zones around airports and Terminal Control Areas may also be designated as Class B. VFR aircraft must talk to ATC and obtain clearance from ATC prior to entering Class B airspace. A listening watch must be maintained on the appropriate frequency, and the aircraft must remain in Visual Meteorological Conditions (VMC). If VMC cannot be maintained, the VFR aircraft must request an amended clearance to allow the flight to continue VMC. If this is not practicable, leave Class B airspace via the shortest, safest route, by either descending out, or leaving laterally if practicable, and report actions taken to ATC as soon as possible.

Class C Airspace

Both IFR and VFR flight is permitted in Class C airspace. As with Class A and B, all IFR aircraft are provided with control service, and therefore separation from other aircraft at all times. VFR flights must request and receive a clearance to operate in Class C airspace. As necessary to resolve conflicts between IFR and VFR aircraft, ATC has authority to direct VFR aircraft and provide IFR separation minima between IFR and VFR aircraft. For VFR vs. VFR,  aircraft are provided with traffic information, and "conflict resolution" will be provided upon request. This means that aircraft will be given instructions to avoid each other, rather than a hard and fast separation standard.

Terminal Control Areas and associated Control Zones at airports may be classified as Class C airspace, and these areas revert to Class E airspace when the associated ATC units are not in operation. All flights must be equipped with Mode C transponders in Class C airspace.

Class D Airspace

As with Class C, both IFR and VFR are permitted in Class D airspace. VFR flights wishing to operate with Class D airspace need only establish two-way communication with the appropriate ATC unit to enter, so a clearance is not required. IFR aircraft are provided with separation between other IFR aircraft. Equipment and workload permitting, conflict resolution will be provided between VFR and IFR aircraft, and upon request only between VFR aircraft and other VFR aircraft.

Terminal Control Areas and associated Control Zones may be designated as Class D. These airspaces revert to Class E airspace when the associated ATC unit is not in operation. Many Class D areas are designated as Transponder Airspace. If so, a Mode C transponder is required for operation within unless otherwise authorized by ATC.

Class E Airspace

Now the services start to thin out a bit. Much of the controlled airspace in Canada is Class E. This includes all of the controlled airspace that is not already designated Class A, B, C or D airspace. Sounds vague, right? All Low Level airways, control area extensions, Control Zones around uncontrolled airports (yes, this happens in many areas) and other areas are designated Class E. IFR flights still require a clearance to operate within Class E and they are provided with separation only from other IFR aircraft, while VFR flights need nothing more than VMC. No communication is required with ATC, nor is a clearance. VFR aircraft are not entitled to ATC separation in Class E airspace, though traffic information can be provided, workload and equipment permitting.

How could you have a control zone without a control tower at an airport? They have established these to ensure controlled airspace exists right to the ground. The two effects this has, even without a tower, is to allow ATC the authority to apply IFR separation right to the ground at such an airport, and to raise the weather minima around such an airport for operating VFR to increase the safety margin in a mix of IFR and VFR traffic. As mentioned above, Class C and D airspace reverts to this class of airspace when the associated ATC unit is not in operation. This includes terminal control areas and control zones that normally have towers in operation.

Class F Airspace

Class F airspace is designated to confine activities that are potentially hazardous to aircraft not involved in the activity. Hence, Class F is also called "Special Use Airspace". There are two categories, Advisory, and Restricted. Class F airspace assumes the rules of the airspace surrounding it when it becomes in active.

The airspace within an Advisory area may be controlled or uncontrolled, or even a combination. Typically Class F Advisory airspace is designated where activities such as gliding, parachuting, high traffic training areas, and military operations take place and it would be of benefit to aircraft operators to be aware that such activities are taking place there. While there are no specific restrictions, VFR aircraft should avoid flight in these areas if practicable. IFR aircraft will not be permitted to enter Class F Advisory airspace unless the pilot has stated he has obtained permission to enter the area, he is operating on an Altitude Reservation, or he has been cleared for a Contact or Visual Approach. Otherwise, he'll be kept 500 feet clear vertically, unless wake turbulence separation is needed (at which time it becomes 1,000 feet), and he will not be permitted to penetrate the lateral boundaries. Pilots entering an Advisory area should monitor the published frequency, if one is designated, to gain a better knowledge of activities within the area. 126.7 is used if none is designated.

Restricted areas are a little more dangerous. These are generally established when safety or security is an issue, such as when military operations are heavier, including such things as live firing or high speed training, or around high importance locations. One example of the latter occurs with the G8 Summits involving heads of states. Other aircraft are prohibited from entering these areas. While the agency who controls the area may occasionally permit ATC to clear aircraft through a Class F Restricted area, normally, IFR aircraft are kept clear of these areas by the appropriate vertical minimum (1,000 feet below FL290, and 2,000 feet above FL290, even for RVSM aircraft) or by an appropriate lateral minimum. Where radar separation can be used, this is expected.

Additionally, airspace may be classed as Class F Restricted if air traffic would benefit from certain restrictions. Reducing airport operations to only traffic departing and arriving, avoiding the congestion of VFR overflights is one example of such a practice, though there aren't many of these in Canada.

Class F airspace is designated in the following fashion: For example, the military training area for CFB Gagetown in New Brunswick is CYR 724. Also, the Advisory area used for soaring out of Stanley, Nova Scotia, is CYA 753 (S).

Class G Airspace

Uncontrolled airspace. That's about it, in general terms. All other airspace in Canada, whether high or low level, not designated as anything else, is considered Class G. Both IFR and VFR flights may operate without a clearance, and neither are entitled to ATC separation. Both can get flight information from ATC, equipment and workload permitting, but essentially, you're on your own. Weather minima for flying VFR are lower in uncontrolled airspace, reducing the safety margin of IFR operations down low, such as on approach. This is the reason for establishing controlled airspace in the form of transition areas close to airports. The higher weather minima force a better chance of "see and be seen" for both aircraft.

Normal Structures of Airspace

The major airports in Canada are surrounded by controlled airspace, mostly Class D control zones. These control zones are built from the surface up to a specified altitude, typically around 3,000 feet above aerodrome elevation (AAE). These can be other classifications as well. Outside of most control zones are transition areas, typically based at 700 feet AGL. Then there are control area extensions outside of these areas, and they are normally based at 2,200 AGL. Depending on the area involved, many areas go to uncontrolled airspace between airways, so the base of controlled airspace might then become "above 12,500 ASL", meaning that 12,500 itself is uncontrolled. Above 12,500, if it's controlled airspace, it becomes Class B airspace. Some outlying areas actually have the base of controlled airspace up at 18,000 feet ASL, or FL180 as we know it.

Whatever the base, controlled airspace is designated where it is determined that safety will benefit from the establishment of such additional structure. As alluded to earlier, controlled airspace increases the weather minima for VFR flight, in theory giving pilots of both IFR and VFR aircraft a better chance of seeing each other. The control area extensions and, in particular, the transition areas established near airports are place there to allow IFR aircraft to conduct IFR approaches with that very restriction: to force VFR operations out in lower weather to increase the margin of safety for IFR flights. Commonly used tracks, mostly based on NAVAIDs, are typically located within controlled airspace as well, since historically they are frequented by IFR aircraft. Outside of airspace commonly used for IFR flights, you'll often find uncontrolled, or Class G, airspace. Given the way the airspace is actually designated, the common description is often considered to be an upside-down wedding cake. Have a look at the following diagram.

Controlled Airspace Profile

To give an idea of what an FIR looks like when the base of controlled airspace is plotted out, I've included the following diagram to demonstrate.

QMFIR

You can see the designations in place, and just how many there are. The red areas are part of the "Maritime Control Area Extension", a region described within the Designated Airspace Handbook, a Transport Canada publication. All of the dark green areas are control zones, whether they be Class D or E in this case, where controlled airspace extends to the surface of the earth. The light green are the control area extensions, as is the yellow area surrounding Halifax in the middle of the FIR. The blue area in the bottom left as also part of a transition area surrounding Yarmouth, NS, CYQI. The orange area to the left hand side (west of YQI) and the purple areas extending southward are actually oceanic routes to get out to the oceanic airspace in New York's FIR. The purple bar at the bottom is actually oceanic airspace. Looking north, the blue area at the top of the FIR is also based above 12,500 ASL. You can also see an area outlined in black between YSJ and YFC on the left-hand side of the picture between two areas of green. This is actually CYR724, Class F airspace capped at FL250. At the time I completed the diagram, the airspace was actually contained within the Maritime CAE, and therefore would be considered to have a base of 2,200 AGL if it were released to Moncton ACC for control, as is occasionally the case.

VatSim Hardships with Airspace Classes

VatSim has it's own ups and downs in dealing with controlled airspace. Not all controllers are aware of the boundaries, or even the differences, between controlled airspace and uncontrolled airspace procedures. Not all pilots are, either. Different levels of experience and knowledge often clash and provide pilots with confusing notions of what he's supposed to be doing, and what he's required to do. Different pilot experience and knowledge levels present controllers with frustrating times at the "sector", too. Also, the fact that a pilot is often in an area he was treating as uncontrolled due to a lack of ATC, and suddenly ATC joins and takes the helm plays a part. Then, the pilot who was flying along, thinking he's alone, suddenly gets an unfriendly blast from a controller, warning him to check for ATC. Here are some things we should be doing.

Pilots, always be aware of where you are. Whose FIR you're in can be confusing sometimes, but no ATC should ever get impatient with you for asking. I know, they do sometimes. They shouldn't. This is not a big deal. Personally, as a real world ATC, I'd rather a pilot ask me in advance to clear things up rather than do something wrong and have both of us suffering the paperwork (or just me in the most unfortunate of circumstances). Keeping an eye on what FIR you're in will let you know who you should be looking for online. This also means you should be familiar with some FIR idents in the VatSim world, so you know whether your current FIR is online. Any controller *should* be familiar with his surrounding FIR names, so if you call in, he should at least be able to tell you if the appropriate region is online and perhaps help you learn their name.

If ATC is not online for the area you're flying in, treat it as uncontrolled airspace. Make altitude changes considering traffic you may know about (by TCAS, by reports, etc). If ATC is online, make sure you know if you need a clearance to do what you want in the airspace you're in. As you can see above, most IFR operations require a clearance to do just about anything. Only in Class G are you safe to operate without clearances, and sometimes controllers may not be aware, depending on their background and experience, just where Class G airspace lies in his region. For example, last night you flew from an airport that the controller online as CTR told you was uncontrolled, so taxi and take-off were at pilot's discretion. Tonight, the controller gets mad when you taxi out on your own and insists you require a take-off clearance. Take it easy and don't get too mad about it. Perhaps asking for clarification and informing ATC of the discrepancy might just lead a controller to ask someone else about it and learn something in the process. If you're online ATC reading this, are you aware of the various classes of airspace in your FIR? If not, check some charts and consult with others, especially your FIR instructors, to see if they know. Also, check to see what has become standard for the FIR, since local practices often evolve.

Pilots should be checking periodically while flying if they're not in communication with ATC already. Any time you are about to enter a new FIR, have a look. This includes oceanic FIRs, places with known terminal areas, and while on approach to ensure a tower hasn't popped up since you started out. Periodic checks while flying in an area previously thought to be dormant are also a good idea, even when flying 4x.

Another thing to do is keep the flight plan up to date. Especially with ASRC in controller's hands now. If you change altitudes while en route, like most do when they plan step-climbs over the ocean or other long-haul flights, update the flight plan when you get a spare minute. This way, if you encounter ATC, he has the current information. If ATC is online and he clears you to a new altitude, he should be updating that for you, so don't worry about it then.

Controllers could be a little more forgiving of folks zipping through, too. A friendly message reminding a pilot to check for ATC, rather than an ALL-CAPS FLAME stating how stupid he is for not checking, is much better received. Remember, guys, we're here for entertainment and learning, so let's keep it civil. Any real pilot knows that a friendly ATC is better than a grumpy one, and the same goes for VatSim.

Another thing controllers could do is answer pilot's questions politely, rather than get mad at them. The pilot who is smart enough to ask deserves a good answer. Whether it is regarding the class of airspace he's in, what he's expected to do, or even who he should be talking to, he deserves a reasonable tone of voice (or text) when the answer is delivered.




More will follow in coming weeks as we delve further into the concepts of VFR and IFR flight within the various classes of airspace. I'm always open to feedback, positive or negative, so e-mail me at moxner@nbnet.nb.ca. Once again, thanks for reading!