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Comment by metadat | Hacker News Reader

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 :)

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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.

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).

> 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.)

Or 4 parallel runways like Detroit Metro (KDTW): 4R, 4L, 3R, and 3L.

https://aeronav.faa.gov/d-tpp/2213/00119AD.PDF

KDEN also has four parallels (16-34L/R and 17-35L/R). All are 172.6º or 352.6º.

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.

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.

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.)

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.

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

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.

What's L C R and why can't there be more letters?

Left, Center, and Right.

There is also the following suffixes for runway identification: S = STOL runway, G = glider runway, W = water sealane or waterway, and U = ultralight runway. For example runway 8G / 26G in KBDU (Boulder) and 3W / 21W in KSFF (Spokane).

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.

You want to have a consistent designation for opposite direction runways. (9R is always the same piece of pavement as 27L)

Doing that for 4 would need one direction to be A B C D and the other to be B C D E (which is workable, but doesn't seem particularly more elegant than just using left, right, and center [and in fact is worse in some ways, since from one direction, B is the left-most and from the other, it's A])

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/FAA_O%27...

The correction card is legally required equipment and there are a few maintenance items that trigger a requirement to update it

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.

Ok, that’s totally fair. It’s been 25 years since I took a few lessons, purely dim since then.

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.

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.

> 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.

I once took off with my pitot tube covered. I didn’t even notice while taking off it wasn’t until I was 100ft agl when I scanned my instruments and realized my airspeed indicator was dead. After a split second freak out I realized I had gotten here based on external indicators. So I calmly went right back into the pattern and landed using purely sound, wing angle, visual reference, and muscle memory.

I had a scuba instructor who used to sneek up behind and turn off our tank valves... in the pool. You dont do such things in the real world.

Also, in the real world, i would probably break out my smartphone. Short of an emp strike, it will still run on its own battery and can act either as a rough set of instruments or to phone the tower and declare an emergency.

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.

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.

Haha of course someone on HN would use 31 at PAO as their example :).

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!

Hah, I never actually made the SQL <-> Oracle connection.

> 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

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.

> 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.

In order to need compass, I will need to lose 5 GPS devices in the plane, 2 portable GPS (iPad and iPhone), 2 VORs, ADF, radar (yes it can be used for navigation), and 3 radios (ask for vectors). No I don’t care about compass or it’s accuracy.

Edit: forgot 2 HSIs ;)

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.

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.

I agree with the other commenter, it's also explained in this CGP Grey video https://youtu.be/qD6bPNZRRbQ

So that they line up with the magnetic compasses inside of the planes

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.

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