[HN Gopher] Airport runway names shift with magnetic field
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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!
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(page generated 2022-12-29 23:00 UTC)