Aviation In Canada

Saturday, June 18, 2005

IFR Non-Radar Control

A comment posted on the article a couple of days ago about IFR in uncontrolled airspace had a question about ATC in the non-radar sense. I'll try to detail it a little, and see what questions pop up.

First and foremost, most ATC systems currently rely, at least to some extent, on paper strips to keep control information on each aircraft up to date. The strips used in Canada are 1" high by 8" wide, and contain details like aircraft ident, type, speed, point of departure, destination and so on. Flight plan route, as well as estimates for various fixes along the way, and altitude information are also kept for reference.

We use a "data board" in the Canadian system, as do a number of other control agencies. Each board will contain at least one "fix posting", which means a geographical location in relation to which our control data is organized. The larger the piece of airspace, the more of these fix postings there are in a data board. For example, a terminal control unit will often have only one "header" in their board, since they're often only concerned with one place, the major airport they're meant to serve. If there are larger satellite airports in the area that they are responsible for as well, they may have a posting for that airport, too. Larger areas, especially those without radar coverage, will often have a number of fix postings. These fixes can be airports, commonly used fixes with significance, such as those where two commonly used airways cross, or they can be a single header that represents a number of fixes that, say, line a boundary area where there isn't much traffic overall. The fixes are generally posted depending on the amount of traffic, and are typically posted if an aircraft over one fix posting is geographically separated from an aircraft over another posted fix.

If an aircraft is crossing a large area, the controller may use several strips to represent the aircraft at individual fixes along the route of flight. As the controller removes the strips from the printer, the flight plan is examined and now a concrete representation of that aircraft is held by the controller, and as it is placed in the board, the controller gets an image of where that flight is going, depending on which fixes have to be posted for that flight. This is also the time for a controller to ensure no other flights are already posted for the same fix at the same altitude at the same time. If one is found, then control action may be required to ensure separation exists prior to that fix (actually, prior to the point where separation is lost). As an aircraft progresses past a fix, the time is noted on the strip, and the altitude and routing is confirmed with the aircraft to ensure the pilot is doing what is expected. If a pilot is rerouted (pilot request or for separation), the strips may be removed from under one or more headers and placed under others to maintain the data board.

In a radar environment, ATC can see Mode C readouts and aircraft positions, so the strips are generally kept more as a backup in case of a radar failure. Altitude changes are noted as pilots are cleared to change altitude, and rough times (perhaps only the sequence of aircraft) are kept as well, just in case. This way, the controller has a place to record information for use with or without radar as well.

Those are the basics of non-radar control. The ways we determine if separation exists between aircraft vary depending on the facilities available in the area (VOR with or without DME, for example), the capabilities of an aircraft (RNAV or GPS, or just the old-fashioned, steam-driven VOR and ADF), and the nature of the airspace involved (controlled vs. uncontrolled, and special considerations like oceanic vs. domestic airspace). If there is interest, perhaps I'll talk a little about some of the ways we look at non-radar separation...