For many pilots, GPS somehow translates to fat, dumb and happy— and maybe in hot water! When you’re flying along using your GPS moving map, you can be fairly sure that those airports are where it says they are, give or take a hundredth of a minute of longitude—maybe. They’re the same locations as those given in the Airport Facilities Directory (AFD), which are for the so-called ‘center of mass’ of all usable runways, given in degrees, minutes, and hundredths of a minute. (A hundredth of a minute in terms of latitude, if you’re interested, is about 61 feet.)
Now, here’s the problem: The GPS databases use that AFD location to determine how far you are from the airport, but if you’re in a busy terminal area you could be in trouble. The Class B rings are drawn about a different reference point on the airport: the VORTAC. After all, ‘TCAs’ (as they were known) existed long before GPS or even LORAN were in popular use, and DME was the logical choice for determining where the rings were located. If you’re close to the walls of one of those upside-down wedding cake ‘rings,’ or you’re navigating near the sides of a flyway or VFR corridor, you could actually be inside that airspace.
Inside Information: According to information received from a technical specialist at Jeppesen (who supplies databases to many GPS manufacturers), the potential for significant differences between these two ‘reference points’ exists, nationwide! Translation: The GPS-indicated distance from the airport is not a good representation for the location of Class B rings. Having the GPS use the wrong reference point is the same as setting the wrong frequency in your NAV radio!
The margin of error: Take a look at this diagram. It’s a close-up of the Class B airspace in the Washington DC area. It shows Dulles Airport, Washington National, and some of the surrounding airspace, as well as one of the two VFR flyways — in this case, a North-South corridor. You can eyeball this thing and guess which of the two black crosses is at the center of Dulles’ inner ring (the airport on the left), correct? Now guess how far the airport reference point is from that. C’mon now, no peeking!
…And where that puts you. Did you come up with … 500 yards? No? Well, how about 1.2 miles? If you were heading North along the Eastern side of the flyway, at 2500 feet (the recommended altitude, shown on the back of the Baltimore-Washington Terminal Area Chart) and you were using your handheld GPS more than your pilotage — many new pilots do — you could be intruding inside the Class B. Worse yet, you could be busting the Class B by one mile — and doing it just to the West of National Airport! Holy Hannah! Could this wreck your day, or what?
The offsets at National (shown to the right) and BWI to the North (not shown) are less–about a half mile. But it could still bite you. If two airports in another Class B beehive happen to have radar antenna ‘offsets’ in opposite directions, any VFR airway or corridor might be a lot narrower than you think!
No matter how you slice it. In case you were wondering: The Class B rings programmed into the moving map are not drawn from a bitmap. Translation: The error ‘propagates’ from that reference point. So you could also get bitten if you are using the ‘distance from airport’ function, because you’re getting distance from the airport ‘center’ and not the center of Class B rings. (Have I said that enough?) Of course, if you’re just following what that graphic display tells you, well, you’re out of luck then, too.
SELF DEFENSE
- Use darned good pilotage (read: ground references), whenever possible!
- Don’t hug those Class B walls!
- Use the VFR waypoints that have recently been added to some terminal area charts. They’re denoted by small white circles inscribed within four-pointed black cycloids, and have each been given names (some of which are amusing — like the one at the East end of the NW-SE flyway between National and BWI: VPOOP). They all begin with the letters ‘VP’, which makes them easier to spot on the chart and their exact locations are listed in the AFD. When they coincide with a Visual Check Point (VCP), they’ll be portrayed with a VCP flag. By the way, they’re not intended to be pronounceable, nor used in ATC communication … regardless of how much you might want to call in ‘over VPOOP.’
Final Caution: If you use DME off of an ILS (admittedly not something I do regularly, myself) as an indicator of how far you are from the airport, you also get an error — and I don’t mean slant range, either! Here there’s less mystery, of course. You did know that the localizers are usually installed at the far end of the runway, right? So if the DME is collocated, you can be a mile off — more, at a ‘big’ airport.
Ahh, flying! You did get in it for the fun of it, and for the challenge, right?
