9 ms·

Airport runway names shift with magnetic field

198 points by olddustytrail 4y ago

metadat 4y ago
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 4y ago
  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 4y ago
    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).
    - pdonis 4y ago
      > 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 4y ago
        Or 4 parallel runways like Detroit Metro (KDTW): 4R, 4L, 3R, and 3L. https://aeronav.faa.gov/d-tpp/2213/00119AD.PDF https://aeronav.faa.gov/d-tpp/2213/00119AD.PDF
        asciimike 4y ago
        KDEN also has four parallels (16-34L/R and 17-35L/R). All are 172.6º or 352.6º.
        stevehawk 4y ago
        KDEN also put their runways so close together that it's not uncommon to for aircraft on approach on the parallel to set off another airplane's TCAS. I think most airline pilots consider that airport ripe for a midair.
        Mountain_Skies 4y ago
        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 4y ago
        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 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.)
    - s1dev 4y ago
      The correction card is legally required equipment and there are a few maintenance items that trigger a requirement to update it
    - OliverJones 4y ago
      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.
      - TylerE 4y ago
        Ok, that’s totally fair. It’s been 25 years since I took a few lessons, purely dim since then.
      - zzless 4y ago
        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.
        TheRealPomax 4y ago
        Not to mention that no one would ever be insane enough to turn off electronics and then say "right, now IFR your way in". You're flying VFR, you're going to have eyes on the runway.
        stevehawk 4y ago
        > 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). You just pre-arrange it with the tower and it's no big deal. Probably exceptions for busy class Cs or Ds, but it's no big deal to any airport near me.
        grepfru_it 4y ago
    - V99 4y ago
      There are only 3 suffixes, so if there are 4 or more parallel runways one will get the "wrong" number, like 2/3L/3C/3R.
- Jalad 4y ago
  I agree with the other commenter, it's also explained in this CGP Grey video https://youtu.be/qD6bPNZRRbQ https://youtu.be/qD6bPNZRRbQ
  - mr_woozy 4y ago
    [dead]
- Rebelgecko 4y ago
  So that they line up with the magnetic compasses inside of the planes
- nilsbunger 4y ago
  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 4y ago
  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 4y ago
    Haha of course someone on HN would use 31 at PAO as their example :).
    - fragmede 4y ago
      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!
      - inamberclad 4y ago
        Hah, I never actually made the SQL <-> Oracle connection.
  - deepsun 4y ago
    > Also, winds (and just about everything else) are in magnetic degrees. Not always: "If you read it, it's true. If you hear it, it's magnetic." https://aviation.stackexchange.com/a/62/51758 https://aviation.stackexchange.com/a/62/51758
  - shostack 4y ago
    Having just flown out of there yesterday, on the return it is also helpful, it's also helpful to know without having to look it up what I should set my OBS to in order to help align for practicing my pattern and approach.
  - DocTomoe 4y ago
    > Basic safety check. If I'm on RWY 31 at PAO, then my compass better read 310. Actually, it's more like "310ish". Both 305 and 314 are acceptable readings for a runway 31.
- akira2501 4y ago
  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.
- mmaunder 4y ago
  It’s super helpful and intuitive when you’re flying. Took me a while to get the compass visualized in my head but now if I’m making a visual approach and they give me a runway I can look at my current magnetic heading on my heading indicator - which you periodically sync to your magnetic compass - and know exactly what direction relative to my current heading I’ll be landing. So that’s the fundamental reason why most things flight related are magnetic except rare items like winds aloft reports. Wind direction reports at the field? Magnetic, so that you know how many degrees off the wind is from your landing direction and can do a bit of mental math to get the crosswind component. If you’ve just taken off ATC may say fly runway heading - again an easy one using your heading indicator which is synced to your magnetic compass. As a side note, modern avionics like G1000 auto sync your heading indicator to magnetic. So if we started flying with electronic avionics that are auto synced and have a database of magnetic variation around the world, then it’s feasible to always show true on the heading indicator and have runways and everything else be true bearings. But that’s not the case and I still fly some of the many steam dial planes out there as do others, so we’re stuck with magnetic runways and magnetic everything else used in real-time flight and situational awareness for pilots.
simplyinfinity 4y ago
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 https://www.youtube.com/watch?v=qD6bPNZRRbQ [1] https://www.youtube.com/watch?v=jfOUVYQnuhw https://www.youtube.com/watch?v=jfOUVYQnuhw
_s 4y ago
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-North-Pole.png 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 4y ago
  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 (37°37'08"N 122°22'31"W), to Granada Spain, GRX (37°11'19"N 3°46'38"W) would go north of Newfoundland Canada: http://www.gcmap.com/mapui?P=SFO-GRX%0D%0A&MS=wls&DU=mi http://www.gcmap.com/mapui?P=SFO-GRX%0D%0A&MS=wls&DU=mi
  - kortilla 4y ago
    The first non-stop flights to Hawaii from mainland US were part of a competition put on by Dole leaving from Oakland. The 1st place plane took off after the 2nd place plane. This happened because the navigator of the 2nd place flight didn’t take into account the great circle distance and flew straight headings.
carl_dr 4y ago
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 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.
- jameshart 4y ago
  * Dorset
- stygiansonic 4y ago
  My understanding is that the change in magnetic declination (caused by the change in magnetic north) is predictable in the short term (this is why maps state the rate of change but only for a certain time period) but is not predictable over the long term. Ref: https://www.sco.wisc.edu/learning-center/magnetic-declination/ https://www.sco.wisc.edu/learning-center/magnetic-declinatio...
- paganel 4y ago
  Got to love the Brits and their love for physical maps.
  - Symbiote 4y ago
    You can see the map he's looking at here: https://maps.walkingclub.org.uk/os.html?center=50.59818,-2.03948,16 https://maps.walkingclub.org.uk/os.html?center=50.59818,-2.0... -- centred on the village containing the pub, not the true north place. Note it doesn't work with an adblocker enabled. (I don't think the API to show the maps is free.)/
    - paganel 4y ago
      Thanks for the actual link, I see lots of physical details included in there, which I pretty much dig. Related, during the last two weeks or so I've been immersing myself in exploring the topographical maps of Bulgaria the Soviets drew towards the end of the '80s (it was for a personal project of mine). For those interested the map can be found in here [1], by scrolling further down the page, a typical map-sheet looks something like this [2] [1] http://web.uni-plovdiv.bg/vedrin/index_en.html http://web.uni-plovdiv.bg/vedrin/index_en.html [2] http://web.uni-plovdiv.bg/vedrin/details/large/K-35-016-3.jpg http://web.uni-plovdiv.bg/vedrin/details/large/K-35-016-3.jp...
mfkp 4y ago
Interesting fact: the magnetic variation in Canada changes at a more frequent rate, and some northern airports got tired of updating their runway numbers every year or two so they now use true instead of magnetic headings (I believe indicated on the runway numbers like 17T to indicate true instead of 17 which would indicate magnetic).
- bpye 4y ago
  NOAA has a neat tool to determine the difference (declination) [0]. Looks like it's about 16° for me in Vancouver, BC. [0] - https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#declination https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#d...
- colechristensen 4y ago
  Additionally, the magnetic pole is actually in northern Canada
  - bruce511 4y ago
    Technically there's a place (well, line) where North is one way, but the compass points south. Flying a great-circle route that intersects the North Pole, and magnetic North Pole, would mean travelling both north and south at the same time for a period. It's perhaps unfortunate that we live in an era where they are "close enough" that we have one word (North) for two different directions.
    - josephcsible 4y ago
      > we live in an era where they are "close enough" that we have one word (North) for two different directions. Is it possible for the magnetic poles to ever exist far from the geographic poles? (I know the magnetic field could have been the opposite polarity, or not existed at all, but in either of those cases, we still wouldn't have gotten another word.)
      - wolrah 4y ago
sandworm101 4y ago
Simple question: is magnetic north actually a "north" or "south" pole? Which part of a magnetic compass is attracted to it?
- TheRealPomax 4y ago
  The Earth's "magnetic North" is a magnetic south pole. Any magnet's "north" pole will point in/be attracted to the Earth's magnetic North.
  - kqr 4y ago
    How is magnetic north defined? In relation to negative electric charge, or just by reference to a standard compass? (I.e. "whatever is attracted to this side of my magnet is magnetic north" )
    - TheRealPomax 4y ago
      Those are the same thing. The Earth is a planet sized (highly irregular) magnet, and compasses are very simple measuring devices for finding the alignment of the (local) magnetic field. Magnetic North is the "point" (it's not actually a point at all) on the surface of the planet where all magnetic field lines that you can trace by walking towards compass-incidated North converge. And those lines won't be even remotely straight =)
- judge2020 4y ago
  While the other comment is true, technically it has and might once again completely flip https://ec.europa.eu/research-and-innovation/en/horizon-magazine/earths-magnetic-poles-could-start-flip-what-happens-then https://ec.europa.eu/research-and-innovation/en/horizon-maga...
quickthrower2 4y ago
Wow I never would have guessed they use the magnetic compass for those 18L etc. markers. I guess that is to match up with the plane’s instruments.
swader999 4y ago
I got criticized for not just hard coding the runway names in a billing app. I said don't y'all know these are the end times and we are at the early stages of a pole shift. I had the last laugh a few years later when the runways changed.
- deleted 4y ago
  [deleted]
luxuryballs 4y ago
I always wondered how sure they were that the core was iron and nickel, they say it so confidently like we just know.
- sbaiddn 4y ago
  When I was a kid I wondered how we knew space was a vacuum. We have a good (excellent) idea of the density of the core. We also have an excellent idea of how density changes with pressure. Add in that the core must be magnetic and a fairly common isotope in the universe, and you're pretty much left with Fe.
  - luxuryballs 4y ago
    So basically it’s process of elimination in a sense, we don’t know what we don’t know so until we learn more it’s Fe.
- perilunar 4y ago
  I still wonder how spinning molten iron can generate a magnetic field. Spinning a piece of solid iron doesn't produce a magnetic field, so why does molten iron?
  - kube-system 4y ago
    Because it’s an electromagnet!
perilunar 4y ago
FWIW, there was a discussion a year ago on HN about the aviation industry converting from magnetic to true: "Why aviation’s compass is shifting towards True navigation" https://news.ycombinator.com/item?id=28985745 https://news.ycombinator.com/item?id=28985745 Summary: "The debate about changing from Magnetic to True navigation is now moving towards how to change, and when, with March 2030 as the proposed date."
jjwiseman 4y ago
There was an interesting event a few y̶e̶a̶r̶s̶ months ago around Long Island, where it appeared that the Air Traffic Control software got confused by the magnetic declination. Basically it was generating false radar targets in the wrong place: They were rotated 13 degrees, the magnetic declination in the area, from the actual aircraft. Thread with details here: https://twitter.com/lemonodor/status/1508505542423064578 https://twitter.com/lemonodor/status/1508505542423064578 A short summary is that sometimes when ATC radars see an aircraft, if it doesn't seem to have ADS-B (is in a position where no known ADS-B-equipped aircraft is), it will create a synthetic target, broadcasting it out so that aircraft with ADS-B In can see it on their traffic displays. That system is called TIS-B. Somehow the radar in Islip got confused by the magnetic declination: it was generating TIS-B targets that were rotated 13 degrees from where the aircraft really were. And it was doing it for pretty much every aircraft, whether or not it had ADS-B. I assume it wasn't able to correlate the known ADS-B position with the secondary radar returns because they were offset by 13 degrees. Very weird way for a pretty important piece of ATC infrastructure to fail.
- defrost 4y ago
  "A few years ago" would have made perfect sense .. the tweet, however, is dated Mar 29, 2022 and states "Seems like something weird was going on with ATC systems around New York a few days ago." None the less I'd (semi educated) guess that someone at the rader site in question "updated" | "improved" the implementation of The World Magnetic Model [1] in use there and effectively dropped the correct corrections from the end result (for a few days until it was noticed and fixed). Normally coefficient sets are updated every five years .. and frequently sites and software that use the WMM find themselves with a current IT support person that has not previously encountered a WMM update and then backtrace their way into an "understanding". This can cause errors until noticed. Critical infrastructure places (Military, Air Traffic Control, etc) tend to fare better and have longer periods of staff retention, etc. but it can still happen that "we no longer have anyone that was here the last time (five years ago) this happened" However; given the last epoch update (WMM2020) was released 12/10/2019 (in 'merc date speak MM/DD/YYY (barbarians)) and remains current until expiring on December 31, 2024 we can rule out a simple epoch update FUBAR. This leaves the other semi regular source of WMM errors - programmers that don't like FORTRAN (esp. super opaque single letter variable 25th degree spherical harmonic functions that date from the 1960s) and feel the need to "do it better". Having barely survived the prior epoch update and knowing that another will happen again, they carve out some space to build a better mousetrap so that next time will be easier. This is often a, uhh, interesting intersection of ego and hubris. [1] https://www.ngdc.noaa.gov/geomag/WMM/ https://www.ngdc.noaa.gov/geomag/WMM/
helsinkiandrew 4y ago
It does seem archaic that true north isn't used. This NavCanada presentation - gives a good overview of the issues in changing. https://www.navcanada.ca/en/magnetic-north-vs-true-north.pdf https://www.navcanada.ca/en/magnetic-north-vs-true-north.pdf
- deleted 4y ago
  [deleted]
dataflow 4y ago
Since everyone is talking about how there are 2 north poles (geographic and magnetic), I'll add onto that that we have a 3rd north pole, the geomagnetic: https://www.earthdate.org/episodes/triple-north-pole https://www.earthdate.org/episodes/triple-north-pole
bketelsen 4y ago
This happened here in Tampa (TPA) just a few years ago. They shut down the runways to rename them from 18 to 19 to account for the magnetic shift. Must be a lot of repainting, it took a few days.

198 points by olddustytrail 4y ago

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 4y ago
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 4y ago
  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).
  - pdonis 4y ago
    > 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 4y ago
      Or 4 parallel runways like Detroit Metro (KDTW): 4R, 4L, 3R, and 3L. https://aeronav.faa.gov/d-tpp/2213/00119AD.PDF https://aeronav.faa.gov/d-tpp/2213/00119AD.PDF
      - asciimike 4y ago
        KDEN also has four parallels (16-34L/R and 17-35L/R). All are 172.6º or 352.6º.
        stevehawk 4y ago
        KDEN also put their runways so close together that it's not uncommon to for aircraft on approach on the parallel to set off another airplane's TCAS. I think most airline pilots consider that airport ripe for a midair.
      - Mountain_Skies 4y ago
        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 4y ago
        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 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.)
  - s1dev 4y ago
    The correction card is legally required equipment and there are a few maintenance items that trigger a requirement to update it
  - OliverJones 4y ago
    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.
    - TylerE 4y ago
      Ok, that’s totally fair. It’s been 25 years since I took a few lessons, purely dim since then.
    - zzless 4y ago
      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.
      - TheRealPomax 4y ago
        Not to mention that no one would ever be insane enough to turn off electronics and then say "right, now IFR your way in". You're flying VFR, you're going to have eyes on the runway.
      - stevehawk 4y ago
        > 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). You just pre-arrange it with the tower and it's no big deal. Probably exceptions for busy class Cs or Ds, but it's no big deal to any airport near me.
      - grepfru_it 4y ago
  - V99 4y ago
    There are only 3 suffixes, so if there are 4 or more parallel runways one will get the "wrong" number, like 2/3L/3C/3R.
Jalad 4y ago
I agree with the other commenter, it's also explained in this CGP Grey video https://youtu.be/qD6bPNZRRbQ https://youtu.be/qD6bPNZRRbQ
- mr_woozy 4y ago
  [dead]
Rebelgecko 4y ago
So that they line up with the magnetic compasses inside of the planes
nilsbunger 4y ago
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 4y ago
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 4y ago
  Haha of course someone on HN would use 31 at PAO as their example :).
  - fragmede 4y ago
    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!
    - inamberclad 4y ago
      Hah, I never actually made the SQL <-> Oracle connection.
- deepsun 4y ago
  > Also, winds (and just about everything else) are in magnetic degrees. Not always: "If you read it, it's true. If you hear it, it's magnetic." https://aviation.stackexchange.com/a/62/51758 https://aviation.stackexchange.com/a/62/51758
- shostack 4y ago
  Having just flown out of there yesterday, on the return it is also helpful, it's also helpful to know without having to look it up what I should set my OBS to in order to help align for practicing my pattern and approach.
- DocTomoe 4y ago
  > Basic safety check. If I'm on RWY 31 at PAO, then my compass better read 310. Actually, it's more like "310ish". Both 305 and 314 are acceptable readings for a runway 31.
akira2501 4y ago
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.
mmaunder 4y ago
It’s super helpful and intuitive when you’re flying. Took me a while to get the compass visualized in my head but now if I’m making a visual approach and they give me a runway I can look at my current magnetic heading on my heading indicator - which you periodically sync to your magnetic compass - and know exactly what direction relative to my current heading I’ll be landing. So that’s the fundamental reason why most things flight related are magnetic except rare items like winds aloft reports. Wind direction reports at the field? Magnetic, so that you know how many degrees off the wind is from your landing direction and can do a bit of mental math to get the crosswind component. If you’ve just taken off ATC may say fly runway heading - again an easy one using your heading indicator which is synced to your magnetic compass. As a side note, modern avionics like G1000 auto sync your heading indicator to magnetic. So if we started flying with electronic avionics that are auto synced and have a database of magnetic variation around the world, then it’s feasible to always show true on the heading indicator and have runways and everything else be true bearings. But that’s not the case and I still fly some of the many steam dial planes out there as do others, so we’re stuck with magnetic runways and magnetic everything else used in real-time flight and situational awareness for pilots.

simplyinfinity 4y ago

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 https://www.youtube.com/watch?v=qD6bPNZRRbQ [1] https://www.youtube.com/watch?v=jfOUVYQnuhw https://www.youtube.com/watch?v=jfOUVYQnuhw

_s 4y ago

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-North-Pole.png 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 4y ago
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 (37°37'08"N 122°22'31"W), to Granada Spain, GRX (37°11'19"N 3°46'38"W) would go north of Newfoundland Canada: http://www.gcmap.com/mapui?P=SFO-GRX%0D%0A&MS=wls&DU=mi http://www.gcmap.com/mapui?P=SFO-GRX%0D%0A&MS=wls&DU=mi
- kortilla 4y ago
  The first non-stop flights to Hawaii from mainland US were part of a competition put on by Dole leaving from Oakland. The 1st place plane took off after the 2nd place plane. This happened because the navigator of the 2nd place flight didn’t take into account the great circle distance and flew straight headings.

carl_dr 4y ago

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 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.

jameshart 4y ago
* Dorset
stygiansonic 4y ago
My understanding is that the change in magnetic declination (caused by the change in magnetic north) is predictable in the short term (this is why maps state the rate of change but only for a certain time period) but is not predictable over the long term. Ref: https://www.sco.wisc.edu/learning-center/magnetic-declination/ https://www.sco.wisc.edu/learning-center/magnetic-declinatio...
paganel 4y ago
Got to love the Brits and their love for physical maps.
- Symbiote 4y ago
  You can see the map he's looking at here: https://maps.walkingclub.org.uk/os.html?center=50.59818,-2.03948,16 https://maps.walkingclub.org.uk/os.html?center=50.59818,-2.0... -- centred on the village containing the pub, not the true north place. Note it doesn't work with an adblocker enabled. (I don't think the API to show the maps is free.)/
  - paganel 4y ago
    Thanks for the actual link, I see lots of physical details included in there, which I pretty much dig. Related, during the last two weeks or so I've been immersing myself in exploring the topographical maps of Bulgaria the Soviets drew towards the end of the '80s (it was for a personal project of mine). For those interested the map can be found in here [1], by scrolling further down the page, a typical map-sheet looks something like this [2] [1] http://web.uni-plovdiv.bg/vedrin/index_en.html http://web.uni-plovdiv.bg/vedrin/index_en.html [2] http://web.uni-plovdiv.bg/vedrin/details/large/K-35-016-3.jpg http://web.uni-plovdiv.bg/vedrin/details/large/K-35-016-3.jp...

mfkp 4y ago

Interesting fact: the magnetic variation in Canada changes at a more frequent rate, and some northern airports got tired of updating their runway numbers every year or two so they now use true instead of magnetic headings (I believe indicated on the runway numbers like 17T to indicate true instead of 17 which would indicate magnetic).

bpye 4y ago
NOAA has a neat tool to determine the difference (declination) [0]. Looks like it's about 16° for me in Vancouver, BC. [0] - https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#declination https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#d...
colechristensen 4y ago
Additionally, the magnetic pole is actually in northern Canada
- bruce511 4y ago
  Technically there's a place (well, line) where North is one way, but the compass points south. Flying a great-circle route that intersects the North Pole, and magnetic North Pole, would mean travelling both north and south at the same time for a period. It's perhaps unfortunate that we live in an era where they are "close enough" that we have one word (North) for two different directions.
  - josephcsible 4y ago
    > we live in an era where they are "close enough" that we have one word (North) for two different directions. Is it possible for the magnetic poles to ever exist far from the geographic poles? (I know the magnetic field could have been the opposite polarity, or not existed at all, but in either of those cases, we still wouldn't have gotten another word.)
    - wolrah 4y ago

sandworm101 4y ago

Simple question: is magnetic north actually a "north" or "south" pole? Which part of a magnetic compass is attracted to it?

TheRealPomax 4y ago
The Earth's "magnetic North" is a magnetic south pole. Any magnet's "north" pole will point in/be attracted to the Earth's magnetic North.
- kqr 4y ago
  How is magnetic north defined? In relation to negative electric charge, or just by reference to a standard compass? (I.e. "whatever is attracted to this side of my magnet is magnetic north" )
  - TheRealPomax 4y ago
    Those are the same thing. The Earth is a planet sized (highly irregular) magnet, and compasses are very simple measuring devices for finding the alignment of the (local) magnetic field. Magnetic North is the "point" (it's not actually a point at all) on the surface of the planet where all magnetic field lines that you can trace by walking towards compass-incidated North converge. And those lines won't be even remotely straight =)
judge2020 4y ago
While the other comment is true, technically it has and might once again completely flip https://ec.europa.eu/research-and-innovation/en/horizon-magazine/earths-magnetic-poles-could-start-flip-what-happens-then https://ec.europa.eu/research-and-innovation/en/horizon-maga...

quickthrower2 4y ago

Wow I never would have guessed they use the magnetic compass for those 18L etc. markers. I guess that is to match up with the plane’s instruments.

swader999 4y ago

I got criticized for not just hard coding the runway names in a billing app. I said don't y'all know these are the end times and we are at the early stages of a pole shift. I had the last laugh a few years later when the runways changed.

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luxuryballs 4y ago

I always wondered how sure they were that the core was iron and nickel, they say it so confidently like we just know.

sbaiddn 4y ago
When I was a kid I wondered how we knew space was a vacuum. We have a good (excellent) idea of the density of the core. We also have an excellent idea of how density changes with pressure. Add in that the core must be magnetic and a fairly common isotope in the universe, and you're pretty much left with Fe.
- luxuryballs 4y ago
  So basically it’s process of elimination in a sense, we don’t know what we don’t know so until we learn more it’s Fe.
perilunar 4y ago
I still wonder how spinning molten iron can generate a magnetic field. Spinning a piece of solid iron doesn't produce a magnetic field, so why does molten iron?
- kube-system 4y ago
  Because it’s an electromagnet!

perilunar 4y ago

FWIW, there was a discussion a year ago on HN about the aviation industry converting from magnetic to true: "Why aviation’s compass is shifting towards True navigation" https://news.ycombinator.com/item?id=28985745 https://news.ycombinator.com/item?id=28985745 Summary: "The debate about changing from Magnetic to True navigation is now moving towards how to change, and when, with March 2030 as the proposed date."

jjwiseman 4y ago

There was an interesting event a few y̶e̶a̶r̶s̶ months ago around Long Island, where it appeared that the Air Traffic Control software got confused by the magnetic declination. Basically it was generating false radar targets in the wrong place: They were rotated 13 degrees, the magnetic declination in the area, from the actual aircraft. Thread with details here: https://twitter.com/lemonodor/status/1508505542423064578 https://twitter.com/lemonodor/status/1508505542423064578 A short summary is that sometimes when ATC radars see an aircraft, if it doesn't seem to have ADS-B (is in a position where no known ADS-B-equipped aircraft is), it will create a synthetic target, broadcasting it out so that aircraft with ADS-B In can see it on their traffic displays. That system is called TIS-B. Somehow the radar in Islip got confused by the magnetic declination: it was generating TIS-B targets that were rotated 13 degrees from where the aircraft really were. And it was doing it for pretty much every aircraft, whether or not it had ADS-B. I assume it wasn't able to correlate the known ADS-B position with the secondary radar returns because they were offset by 13 degrees. Very weird way for a pretty important piece of ATC infrastructure to fail.

defrost 4y ago
"A few years ago" would have made perfect sense .. the tweet, however, is dated Mar 29, 2022 and states "Seems like something weird was going on with ATC systems around New York a few days ago." None the less I'd (semi educated) guess that someone at the rader site in question "updated" | "improved" the implementation of The World Magnetic Model [1] in use there and effectively dropped the correct corrections from the end result (for a few days until it was noticed and fixed). Normally coefficient sets are updated every five years .. and frequently sites and software that use the WMM find themselves with a current IT support person that has not previously encountered a WMM update and then backtrace their way into an "understanding". This can cause errors until noticed. Critical infrastructure places (Military, Air Traffic Control, etc) tend to fare better and have longer periods of staff retention, etc. but it can still happen that "we no longer have anyone that was here the last time (five years ago) this happened" However; given the last epoch update (WMM2020) was released 12/10/2019 (in 'merc date speak MM/DD/YYY (barbarians)) and remains current until expiring on December 31, 2024 we can rule out a simple epoch update FUBAR. This leaves the other semi regular source of WMM errors - programmers that don't like FORTRAN (esp. super opaque single letter variable 25th degree spherical harmonic functions that date from the 1960s) and feel the need to "do it better". Having barely survived the prior epoch update and knowing that another will happen again, they carve out some space to build a better mousetrap so that next time will be easier. This is often a, uhh, interesting intersection of ego and hubris. [1] https://www.ngdc.noaa.gov/geomag/WMM/ https://www.ngdc.noaa.gov/geomag/WMM/

helsinkiandrew 4y ago

It does seem archaic that true north isn't used. This NavCanada presentation - gives a good overview of the issues in changing. https://www.navcanada.ca/en/magnetic-north-vs-true-north.pdf https://www.navcanada.ca/en/magnetic-north-vs-true-north.pdf

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dataflow 4y ago

Since everyone is talking about how there are 2 north poles (geographic and magnetic), I'll add onto that that we have a 3rd north pole, the geomagnetic: https://www.earthdate.org/episodes/triple-north-pole https://www.earthdate.org/episodes/triple-north-pole

bketelsen 4y ago

This happened here in Tampa (TPA) just a few years ago. They shut down the runways to rename them from 18 to 19 to account for the magnetic shift. Must be a lot of repainting, it took a few days.