Aviation In Canada

Monday, October 10, 2005

ATC and Non-Radar Separation, Part 2

Continuing from yesterday, we'll talk a little about lateral separation, looking at how much airspace must be protected for an aircraft on any given track. How much ATC must reserve for an aircraft depends on how he's navigating. Aircraft operating in domestic airspace with no navigation guidance from NAVAIDs or on-board systems (basically dead reckoning his way from point A to B) are provided with 45NM either side of track in a non-radar environment. Quite a chunk of airspace. It means that the pilot may, as far as ATC is concerned, be anywhere within a 90NM swath of airspace centered on the flight planned track. This is hardly efficient if you have more than one aircraft who want to operate in the area. So we need some navigation guidance, something to narrow down the amount of airspace to protect.

If he's using a VHF NAVAID like a VOR, then the basic airway width is 4NM either side of track until it meets lines that splay at 4.5° from the airway centerline, at a distance of about 50NM. An airway based on two VORs less than 100NM apart is, therefore, only 4NM wide throughout. It's only when you exceed the 100NM distance between VORs that the 4.5° splay comes into place. NDB airways are similar, though slightly wider in basic width and splay. Technically, parallel airways based on VORs need only be a little over 8NM apart to say that the tracks are laterally separated. I don't know if there is anywhere in Canada that two airways run on parallel tracks that could be used practically in this way, or at least for any long distance.

If an aircraft is using RNAV, also known as Area Navigation, then ATC has other options. For aircraft that are operating in RNPC airspace (which stands for Required Navigation Performance Capabilities) and are RNPC certified, ATC can use 10NM either side of track. It's a bigger cut than VORs, but not everywhere has NAVAIDs to be used on the tracks desired. For aircraft that are on crossing tracks but not on airways, RNAV distances from a common point on both tracks may be used to determine longitudinal separation. Ok, so who is RNPC certified? Anyone with a "G" in their flight plan counts in this category. This is why in a recent post about the non-IFR certified GPS I made the point about not filing G in the flight plan. If that's what ATC sees, he may apply an RPNC minimum between you and another airplane. If your receiver isn't certified, then you can't be 100% certain you are where your receiver says you are for a variety of reasons, and less than ideal circumstances may exist, possibly leaving less than the required separation. But look at it this way: You may get to enjoy the in-flight movie on the other airplane.

It is more often that tracks converge than run parallel. For example, most VORs in the country are "tie downs" for more than one airway, which means that the airspace from one airway will overlap the airspace to be protected for another. If ATC were to have to aircraft converging at a common VOR at the same altitude, he'll look first at the times for the two aircraft, and see if longitudinal separation exists. The aircraft must be considered in this, too, since a faster behind a slower won't necessarily keep enough separation after passing the NAVAID. If there isn't enough time between them, ATC must act to ensure another form of separation exists before the second aircraft enters the area of overlap, and this is most often done by forcing an altitude change to one aircraft. ATC must also provide a buffer case the estimates are off, and this "fudge factor" is equal to half of the appropriate longitudinal separation minimum. If time is being applied and 10 minutes is the minimum, then the second aircraft must be at the new altitude 5 minutes before before he is estimated to enter the area of overlap. If distance is the tool, then half that minimum is what's applied. DME can be useful for determining the separation to be applied, as can RNAV (including GPS), as mentioned earlier.

So now you know a little about how ATC controls in a non-radar environment. ATC has many other tools in the toolbox, but this was a basic look at how ATC must visualize the protected airspace concept.