[HN Gopher] Airport runway names shift with magnetic field ___________________________________________________________________ Airport runway names shift with magnetic field Author : olddustytrail Score : 65 points Date : 2022-12-29 20:06 UTC (2 hours ago) (HTM) web link (www.ncei.noaa.gov) (TXT) w3m dump (www.ncei.noaa.gov) | simplyinfinity wrote: | CGP Grey did a couple of episodes on runways[0] and airport | codes[1] that are just fascinating and worth the watch | | [0] https://www.youtube.com/watch?v=qD6bPNZRRbQ | | [1] https://www.youtube.com/watch?v=jfOUVYQnuhw | carl_dr wrote: | Magnetic north moves in a fairly predictable way. | | If you are in the UK you might be interested in Standup Maths' | video https://youtu.be/HcFvegnQpPo. | | Currently true north and magnetic north are broadly the same in | the UK - and over the next year or so, there is a point where | grid north (Ordnance Survey maps) also coincides with both other | "norths". That point will track north over the UK. | | The video shows Matt visiting Cornwall at a place where that | point was. | _s wrote: | This is only one half of what actually happens while navigating; | you have true north and magnetic north - see | https://theprepared.com/wp-content/uploads/2017/08/Magnetic-... | to visualise. | | The difference between them depends on where you are on the globe | - it could even be 180 if you happen to stand in the middle of | the two! | | Magnetic compasses always point to magnetic north, but with earth | being molten metals - magnetic north likes to move around a bit, | so we work out the difference between the two and apply it to the | directions we track. | | For example, where I am - the difference is around 11 degrees. In | order to head directly west, I can't just let my compass point | north and go directly perpendicular port - I have to point north | plus 11 degrees, and then go. My actual track will be 281 degrees | magnetic, not 270. | | What's more interesting is that most navigation charts have all | their directions and bearings in "true" north, with the variance | also charted, whereas when you actually fly you track the | magnetic headings plus/minus the variance. If you go long enough | - the variance also changes so you have to take that into account | when planning your route. | | When you take off / land, runway headings directly corresponding | to your heading are a good sanity check - alarm bells go off if | you're meant to be flying west but come across Runway 09 in front | of you (the opposite!), not to mention we still rely on magnetic | compasses - you don't want to be coming into land at a track for | 281 degrees and see 27 painted on it; chances are something went | wrong somewhere. | adolph wrote: | At what distance a flight would you use a great circle route? | How is a continuous compass heading derived for a GC? | | Example, San Francisco US, SFO (37deg37'08"N 122deg22'31"W), to | Granada Spain, GRX (37deg11'19"N 3deg46'38"W) would go north of | Newfoundland Canada: | | http://www.gcmap.com/mapui?P=SFO-GRX%0D%0A&MS=wls&DU=mi | metadat wrote: | Why is it desirable to correlate runway names with the Earth's | magnetic fields? Is it because the navigational equipment breaks | / becomes inaccurate when there's a field shift? | | It seems like it could be confusing over time to keep updating | runway names as flight paths are repeatedly flown by generally | the same sets of pilots, and as paths becomes familiar. For fancy | autopilot setups most of the in-the-moment thinking is avoided | and this may become a very minor detail. | | Edit: Thanks for the quick info everyone! Now this makes sense. | Happy holidays :) | SOTGO wrote: | It's so that in the event of mechanical failure a pilot can | always rely on a magnetic compass for navigation, without | having to know or look up the magnetic variation of the | airport. | TylerE wrote: | And also so when they get cleared for "runway 15" they know | which side to come on without having to consult a chart. | | Also so there's no confusion, each direction is named, so a | runway that runs due North-South will be Runway 36 in the | northern direction, and 18 to the south. | | In practice it is rare for all but the largest airports to | have more than 3 or 4 runways so it isn't like there will be | a Runway 2 and a Runway 3 at the same airport. If they need | more capacity they'll build parallel runways which will be | named with a suffix, like 9L and 9R. | | (Also, magnetic compasses aren't that accurate in the first | place, since there's iron in the engines if nothing else. In | theory there should be a compensation/calibration done with | the results on a card, but, no guarantees that it's done or | up to date). | s1dev wrote: | The correction card is legally required equipment and there | are a few maintenance items that trigger a requirement to | update it | OliverJones wrote: | With respect, calibrating compass compensations is in fact | very important in FAA approved aircraft. It's part of the | inspection checklist signed off by the licensed mechanic. | | My flight instructor once switched off the electricity to | the airplane's controls and radios and told me to land. At | an airport with a control tower. Using the magnetic | compass, airspeed, barometric altimeter, RPMs, that's it. | Nothing else works without electric power. There's a whole | procedure for doing all that. Being able to navigate safely | with an old-school magnetic compass is life-critical. | zzless wrote: | Not to doubt your story but being a flight instructor | myself, I cannot imagine intentionally turning off power | to airplane radios at an airport with a control tower | (this is also illegal). As far as the pure mechanics of | landing go, one does not need any instruments whatsoever: | one can judge the airspeed by the sound and the wing | angle. I routinely make my students do this when I teach | tailwheel flying. | TylerE wrote: | Ok, that's totally fair. It's been 25 years since I took | a few lessons, purely dim since then. | pdonis wrote: | _> If they need more capacity they'll build parallel | runways which will be named with a suffix, like 9L and 9R._ | | There are even some airports that have three runways with | the same heading, so they have "L", "C", and "R". (Two | examples that I've personally took off or landed on all | three of are 17/35 at DFW and 01/19 at IAD.) | sokoloff wrote: | Or 4 parallel runways like Detroit Metro (KDTW): 4R, 4L, | 3R, and 3L. | | https://aeronav.faa.gov/d-tpp/2213/00119AD.PDF | Mountain_Skies wrote: | Atlanta has five parallel runways: 27R, 27L, 9R, 9L, and | 28/10. It appears the main four that are close to each | other are set alternating east landing and west landing | as they're not marked on both ends like 28/10 is. | sokoloff wrote: | They are numbered on both ends on both the satellite view | (real-world) and on the FAA diagram: | http://aeronav.faa.gov/d-tpp/2213/00026AD.PDF | | There would be no (or extremely rare, at a minimum) | operational reason to alternate directions for takeoff | and landings on parallel runways. (There are rules for | simultaneous operations to parallel runways and having | them be in opposite directions would be problematic as | you have to assume some aircraft will execute a missed | approach and would then be climbing into opposite | direction landing traffic with only a small lateral | offset.) | TylerE wrote: | About the closest I can think of to that is some mountain | airports that do all takeoffs one way and all landings | the other. I'm not aware of any of those with more than | one runway and traffic is usually extremely limited. | addaon wrote: | 27L and 9R are the same runway, always. 27R and 9L are | the same runway. In Atlanta, besides 27L/9R and 27R/9L, | there's also 26L/8R and 26R/8L, as well as the 28/10 you | mention. See | https://aeronav.faa.gov/d-tpp/2213/00026AD.PDF | pdonis wrote: | This is the same dodge that ORD uses, as DocTomoe pointed | out: pretending some of the runways are a different | heading when they're actually not. I'm not sure how that | can be avoided with four runways, though, since I don't | see how to fit a fourth option in with "L", "C", and "R". | | In fact, taking this dodge into account, the DFW example | I gave actually has _five_ parallel runways; the two they | label 18 /36 are actually the same heading as the three | they label 17/35. I wonder if they'll ever need to go to | six and add an 18/36C. | TylerE wrote: | Could just go A B C D E left to right I guess. If only 3 | use B C D so the middle is always C regardless of system. | pishpash wrote: | What's L C R and why can't there be more letters? | sokoloff wrote: | Left, Center, and Right. | asciimike wrote: | KDEN also has four parallels (16-34L/R and 17-35L/R). All | are 172.6o or 352.6o. | DocTomoe wrote: | Also, EDDF/FRA, in which one runway got renamed to C when | they opened a smaller one in the 2010s. | | Then you have airports like ORD which have more than | three parallel runways - but pretend some of them are ten | degrees off to get more naming space. [1] | | [1] https://upload.wikimedia.org/wikipedia/commons/a/aa/F | AA_O%27... | Jalad wrote: | I agree with the other commenter, it's also explained in this | CGP Grey video https://youtu.be/qD6bPNZRRbQ | mr_woozy wrote: | [dead] | Rebelgecko wrote: | So that they line up with the magnetic compasses inside of the | planes | akira2501 wrote: | One other thing not covered, on any airport chart, the exact | alignment of the runway will be noted, usually down a tenth of | degree. | | So the numbering is a useful backup and discriminator, but the | charts have much more detail, and modern navigational and | instrument systems on runways obviate much of this for most | flights anyways. | nilsbunger wrote: | It's a convention for A LOT of aspects of flying: air traffic | control instructions ("turn left heading 090"), charts ("fly | heading 070 after takeoff"), etc are all in magnetic | coordinates. | | Planes also a normal compass as their ultimate backup | instrument, which of course is also magnetic. | | Finally, while modern systems could operate off true north | instead, not ALL planes have that. | inamberclad wrote: | Basic safety check. If I'm on RWY 31 at PAO, then my compass | better read 310. | | Also, winds (and just about everything else) are in magnetic | degrees. If I have wind from 280 while I'm landing, then I | immediately know that I have a left quartering headwind and | what to expect. | quest88 wrote: | Haha of course someone on HN would use 31 at PAO as their | example :). | fragmede wrote: | For the uninitiated, PAO is a small general aviation | airport in the city of Palo Alto aka the heart of Silicon | Valley. Nearby is also SQL, adjacent to Oracle's (former) | HQ. That name is purely coincidence! ___________________________________________________________________ (page generated 2022-12-29 23:00 UTC)