Let me add that, on a much broader scale, the flyways (skyways?) themselves – at least the transcontinental and transoceanic ones – were already designed to roughly approximate a great circle route…this is why you have several waypoints between, say, New York and London (yes, there are other factors as well, like minimizing times spent very far from land).
But the pilots are expected to fly rhumb lines between each waypoint. (Chords of an arc).
No different than what Titanic did.
(I recall once flying from NY to Paris, and seeing both sunrise AND sunset from my seat on one side of the plane…and being confused until I remembered we flew close to north, and later close to south.)
We fly great circle tracks actually. If using a GPS it happens automatically. If flying between ground based NavAids the outbound track from point A is not the same as the inbound track to point B, ie a great circle track is approximated with one track change at the halfway point.
Thanks for the correction. So, are the compass bearings appearing on aero charts obsolete, then? And, are pilots now used to not worrying if they notice that their compass is showing them a slightly different bearing than what they know to be the number on the chart?
I assume they switch to pure compass-bearing mindset when approaching for a landing (autopilot off), and still get instructions from ATC like “hold your course at 270”. Right?
One last question…since you now start the flight’s computed path by entering final destination coordinates (or beacon name, or airport code, or whatever), without having to enter compass bearings, would the famous Varig disaster of several decades ago (mixing up “27 degrees” with “270 degrees”, resulting in a crash in the Amazon jungle) be impossible to occur today?
Even at the beginning/lower levels of aviation, even those of us still doing map-and-compass navigation, are used to seeing some degree of mis-match between compass bearings and the numbers on the chart. Crosswinds, magnetic declination, and probably other things I can’t recall before my second cup of caffeine this morning can all result in pointing your airplane somewhat differently from the chart’s indication, even for the pilot of the smallest Cessna. By the time the pilot gets to the big leagues such corrections/deviations are no big deal, you just watch to make you “deviate” by the correct amount for current conditions.
Except, of course, when the autopilot and GPS are doing it for you, but it’s probably a good idea if the guys in the seats way up front retain some awareness of where they are, where they are going, and how they’re supposed to get there.
Thanks, that makes sense. The aforementioned Varig pilots were faulted for having taken longer than they should have to notice that the terrain below them was quite different (in topography, vegetation, land use patterns…) from what it should have been. Yet another example of humans sometimes discounting the most direct means to situational awareness, in favor of technology.
We never have worried about having a slightly different number on the compass because the instruments aren’t that accurate. The compass itself is an emergency instrument that is never used other than to check heading accuracy on the primary heading instruments prior to flight. When it is used the compass is subject to some big errors from aircraft manoeuvring or turbulence and can be several 10s of degrees off what it should be until you fly straight and level for a bit so it can stabilise. There are typically two independent primary heading sources and they supply information to the captain and copilot’s heading indicators. There can be a difference of several degrees between the sources and so the heading indicators for each pilot can be a few degrees out. Although it is rare to see errors this large, the allowable tolerance is 6º between the primary instruments and 10º between the standby compass and the primary instruments.
Unless the distance is quite short, a flight route has waypoints between the departure and destination airfield, even if it is a straight line, there are still intermediate waypoints normally not much more than 200nm apart. The heading difference due to flying a great circle track is only a couple of degrees. There can also be heading changes due to the change in magnetic variation, the difference between magnetic north and true north.
As an example, on a route we fly, the longest leg between waypoints is 213 Nm. The track at the start of that leg is 077º and the track at the end is 073º. The magnetic variation at the start is about 10ºE and 12ºE at the end, so there is only a change of 2º due to the great circle track, significantly less than the allowable instrument error. Those numbers are the track you fly, the heading itself could be something quite different to allow for drift due to the wind.
The tracks printed on the charts definitely aren’t obsolete. For one, there are still aircraft that don’t have a GPS* that need to fly those tracks directly, but also you need to have a way of checking the GPS tracking. No single piece of information can be trusted on the aeroplane, you always cross check it against something else. Approaching every waypoint the next track and distance on the GPS is checked against the track and distance on the chart for accuracy.
It’s not so much switching to a compass-bearing mindset as switching to a heading mindset. You don’t get told to fly a course, you get told to fly a heading. It is only a small part of the flight though, and many flights don’t have radar vectors like that at all. You take off on a standard departure that is a series of tracks that transition you from the runway to the enroute track then you have a STAR at the end that takes you to the runway, never a direct instruction from ATC other than a clearance for the route to fly, altitude clearances, and take-off/landing clearances.
I wasn’t aware of that accident and have only just read the wiki entry on it. 1989 wasn’t all that long ago and I’d have to read the full report to get a complete understanding of why the FMS was programmed the way it was. The FMS or FMC (flight management system/computer) normally has a database supplied by the FMS manufacturer that contains all airports, waypoints, beacons, etc that you could possibly need. Today you would enter the code for the airport you are at, type in the waypoints for the route, finish off with the destination airfield and check the resulting tracks and distances off the chart. Because the information in the FMS is derived from the database you can’t really make errors with tracks unless you have typed in the wrong waypoint. It sounds like they programmed the FMS directly with a pilot entered track and distance rather than using a database.
To answer your question, the error could probably still happen, but a lot of individual errors would have to line up. Our company is a bit behind the times (until recently we operated Boeing 727s) and still takes off and departs with reference to the aerodrome navigation beacon if it has one and we still manually set the course on the ND for the initial track, but if we made the same error they did, our FMS would be telling us conflicting information and we wouldn’t depart until that had been resolved. That’s what is supposed to happen, but you learn to never be surprised by the ability of people to make errors in new ways!
*GPS vs FMS. I say GPS but airliners have an FMS or flight management system that takes inputs from all of the navigation sources, GPS, VOR, DME, IRS and comes up with a best computed position based on those inputs. In reality the GPS sensor is normally given a very high priority and the FMS position is typically the same as the GPS position. If the GPS was to drop out though, and there were ground based navaids in range, the FMS would still be able to navigate using great circle tracks. In that sense, great circle tracking is made possible by the use of the flight computer rather than GPS.
I just did Las Vegas to Orlando (Southwest Airlines), and the return 5 days later…both times I was using my cell phone to track our flight and both times we were cruising at 39,000 feet, with about 480-540 mph ground speed. The first flight was on one of the newer jets (could not remember what model was said), but we still did the same altitude and speed on an older jet on the return flight. Can’t remember when we started descending though. Both times, we did some minor course adjustments due to weather…on approach to Orlando to avoid moderate-heavy rain, and to avoid some large thunderheads (and turbulence) on the return trip over North-Central Texas.
More or less, although once you get to Class A airspace (essentially, what you’d have above FL280 although the exact altitude varies somewhat) there is, in fact, no speed limit.
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FL180. 18,000 ft MSL indicated.
I explain the same phenomenon three ways, and at least one of them sticks for each person:
Imagine you’re standing on a map, facing north. Point to China. Are you pointing right? Good. Now stand up in your room and point to China. Are you pointing right? Bad. China isn’t to the right. China is under your feet…but not exactly! If you went straight under your feet, you’d be in the Indian ocean. China is in the northern hemisphere, so you actually need to go a little more north and “straight down”. So guess where the airplane heads? North and to the left/right, depending on if you’re in NYC or LA. And once you go “up” the globe, just like a hill, guess what’s on the other side - downhill, aka South. Same thing on the original map you stood on.
Imagine metal bars of equal size. These are lines of longitude. In your head, tape two pieces of string across the bars. One string should be straight across and one should be curved above that, just like airplane routes. Once you have that in your head, imagine you took all those bars and touched their “north poles” together, just like on the real Earth. What does the “straight” string look like now? And the curved one?
Look at this basketball. In the image, there are two arcs and a straight line. But if you rotate the ball down fifteen degrees, all of a sudden, the top line is the straight one and the middle and bottom ones are now curved. This is why a plane at the equator would travel a rhumb line, but a plane in the northern hemisphere travels in a northerly arc and an Aussie pilot goes in a southerly arc. It’s just perspective.
There’s not much difference. To put it in perspective, the magnetic deviation in Washington, DC is 10 degrees. Ohio is about 8. Even in fast jets going E/W, you’re talking 1 degree of change every 15 minutes.
Heading change due to flying a great circle is not the same thing as heading change due to magnetic variation though (deviation is change in compass heading due to magnetic interference from the aircraft and is a different thing again.)
Good exercises, Chessic (though I had trouble doing the second one).
To continue your basketball analogy… So, what’s the only view where all this lines appear as similar concentric circles at the same time? (and, all lines of longitude as straight lines?) From above (or below). Hence, the polar projection DOES show great circles as straight lines.
Richard Pearse, your long post made me realize that I was confusing the general concept of “heading” (rhumb line, which actually traces a sort of spiral on the earth, if you keep going far enough) with the more specific “compass bearing”, which is just one of several methods to help approximate one’s heading.
I blame too much time spent on small sailboats, pre-1990s.
Correction: My comments extending the basketball analogy don’t work the way I write them when the basketball is turned 90 degrees (from the usual orientation of a globe) as it is in Chessic Sense’s excellent exercise. Make it “from the left” and “from the right” instead of “bottom” and “top”.
But even that idea of “usual orientation of a globe” can be misleading. There is nothing sacred about our putting north up. Indeed, most mounted globes show the 23-degree angle between axis of orbit and axis of rotation. If that angle happened to be more than 45 degrees (as it is in one of the planets – Uranus, maybe?), then we would indeed conceptualize the Earth’s poles as “to the left” and “to the right” (probably). It’s all just perspective, like Chessic said.
One can always go back the the fact that the plane traced by a great circle route will intersect the center of the Earth. As Chessic mentioned, the equator does this…and I would add, so does any line of longitude. (As we can see by their being straight lines in a polar view*, where the only straight line that ISN’T a great circle route is the equator.)
*I say “polar view” rather than “polar projection”, because the entire Earth is visible in a polar view of a transparent globe, whereas in certain polar projections the equator and anything beyond it is not visible, being infinitely outside the frame (much like the poles in a Mercator projection).
Dunno if there was in 2012, but in 2025 you can get lots of details from flightaware.com. If you ask them about Jetblue flight 734, flying SFO to BOS as I type, you look under Aircraft Details to find Flight Data, and under that is Route – the flight plan.
It starts with TRUKN2 (several Grateful Dead names in the Bay Area). The TRUKN departure (version 2) is what they do immediately after takeoff; you can look it up at airnav.com or at some FAA website. Next point on the route is SYRAH – that’s probably the end of the TRUKN departure. From SYRAH they follow route Q128 to JSICA, and from there fly direct to ILC.
All those five-letter names are just spots on a map; you can get their lat-lons from airnav.com or some FAA site. Q128 is a route, not a point; the high-altitude chart on skyvector.com will show it.
Apparently it goes to JSICA; next point after that is ILC, which, being three letters, is not just a point on a map – it’s a radiobeacon that you can see on aerial photos. Hill City is the full name, as I recall. Airnav.com gives you its lat-lon too, if you want to hunt it up.
More points in the flightplan, then the flightplan ends with JFUND2 – that’s the arrival route into Boston. Another thing you can look up on airnav.com.
The flight-plan route is what Jetblue is supposed to follow if ATC doesn’t tell him anything different. Which they may well do. If he’s headed east at FL370, someone flying Seattle to Dallas at the same altitude might come along, and it’s up to ATC to tell one or the other to deviate for a few miles, then resume the planned route.
If you look up the JFUND arrival route into Boston, you’ll notice it ends with the arriving aircraft at 5000 or 6000 ft altitude. By then ATC will have given them further instructions on what to do next.
