Runway direction and designation

Doper pilots, I saw something on Google maps that confuses me. Looking at Phoenix Sky Harbor airport the runways seem to run perfectly east-west, that is completely parallel to the edges of the map image and all the other east west streets in Phoenix. Only problem is they are labelled 7/25 which is 20º anti-clockwise of perfectly east-west. I thought runways were designated from true north but if they used magnetic north that would only account for about 12º. Is the problem with my understanding of runway designations, the google maps or a little of both.

Whoops, this may help.

Professional pilot here …

Runway designations are generally based on magnetic direction, rounded to the nearest 10 degrees. But, the local magnetic variation varies over time, so a runway that was initially aligned at 255 degrees magnetic and hence labeled Rwy 26 may, 15 years later, really be at 254 degrees mag. Runways will not be renumbered when that happens.

The basic rule mentioned above also applies only in the case of 1, 2, or 3 adjacent parellel runways. Once we get to 4 runways, the standard is to number one set based on the mag variation as above, and number the other set 1 number/10 degrees off.
In PHX, the so-called “south complex”, the runways south of the central terminal, are designated 7/25. The north complex is designated 8/26. All the runways are abolutely parellel at 077.7/257.7 degrees. This 1-page pdf http://www.naco.faa.gov/content/naco/online/airportdiagrams//00322AD.pdf is the official runway diagram for PHX.

I haven’t flown into PHX in some months, but my recollection is the runways are slightly offset from the street grid. It’s subtle enough you need to be in an airplane to see it; I doubt the precision of a typical road atlas would show it up.

The same situation with parrellel but differently numbered runway sets exists in LAX, DEN, ATL and DFW just to name the ones that instantly come to mind.
As an aside, I wouldn’t put too much stock in the idea that roads are laid out absolutel true East/West, nor that road map sheets are cut on true East/West either. Even in a totally artificial grid place like PHX, there’s no guarantee the original street layout was done perfectly.

Heck, as large as greater Phoenix is now, the earth’s curvature would mean a truly straight street would have a detectable azimuth shift from the far east to far west edge of the conurbation. Alternatively, keeping the street to a constant true course would require a very gentle curve, or more likely, a series of slight kinks every 5 miles.

New controller: Cessna 12345, cleared to land runway two-four. Beechcraft 98765, cleared to land runway six.

N98765: Tower, you just cleared the Cessna onto two-four!

New controller: Y’all be careful, now, y’heah!

I don’t get it.

Took me a minute… :smiley:

Subtract 60° from 240°. :wink:

Runways are renumbered all the time. The government does semi-regular surveys and if the magnetic pole shifts enough, the runway will be renumbered.

Here in Edmonton we have a bar off the approach end of runway 30 called “runway 29”. When the bar opened, the runway was numbered 29. The bar opened and put up its big neon sign and all that, and then a couple of years later the runway was re-designated ‘30’.

Thanks. I had become a bit distrustful of the accuracy of the google images when I noticed some pretty glaring mismatches at photo edges and oval shaped crop circles. Irrigation circles that is, the alien kind are hidden from satellite view. Google’s map makes the runways look absolutely aligned with 9/27 but the Topo maps I got from Garmin show them both slightly out of parallel with each other. I had not noticed the 8/26 designation on the north runway until you pointed it out.