AirCalSNA
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How Do Modern Airliners Navigate?

Fri Mar 25, 2011 7:09 pm

I've been reading about flight navigation online, but haven't found an article says if there is a standard way that airliners navigate within the US. For example, do most airliners use GPS to navigate, or do they use VOR? At what point do these navigation aids kick in, so to speak--from listening to ATC it sounds like most planes first fly runway heading, are then given various heading instructions, and then told to proceed on course. Can someone provide a brief synopsis of take-off to landing navigation for the layperson? Thanks!
 
Mir
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RE: How Do Modern Airliners Navigate?

Fri Mar 25, 2011 7:22 pm

Most modern airliners use a Flight Management System (FMS) to navigate. The FMS isn't really a navigation system on its own - it gets input from various other navigation sources (such as VOR, DME, IRS if installed, GPS if installed), calculates a position, and then calculates which way to go in order to fly the route. Take away all those sources, and the FMS is useless. The capability exists to navigate solely using VORs, but there's no real reason to unless the FMS starts acting up - the only time you'll be navigating by something other than the FMS in normal operations is when you're flying an ILS or LOC approach.

Quoting AirCalSNA (Thread starter):
At what point do these navigation aids kick in, so to speak--from listening to ATC it sounds like most planes first fly runway heading, are then given various heading instructions, and then told to proceed on course. Can someone provide a brief synopsis of take-off to landing navigation for the layperson?

It depends. Some airports have charted departures that will involve using the FMS to navigate right after takeoff. Other airports will vector their departures toward their route of flight, and in this case the pilots will just fly the headings while the FMS sits in the background waiting to take over when ATC gets the aircraft on its route.

-Mir
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bio15
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RE: How Do Modern Airliners Navigate?

Fri Mar 25, 2011 11:48 pm

Quoting AirCalSNA (Thread starter):
For example, do most airliners use GPS to navigate, or do they use VOR?

Mir's reply should give you a good idea of how it works.

It's important to note that it's not a matter of "VOR or GPS" usage. If you have them available, both inputs should work together, whether you're flying a big airliner or a small piston-engine plane. The important thing is that you can accurately determine your position, regardless of the means you possess on the flight deck to do so.

All the airspaces in the world are changing and will move towards airspaces that rely less on ground based stations, as GNSS grows to be a more reliable and robust worldwide system.

----------

Quoting AirCalSNA (Thread starter):
Can someone provide a brief synopsis of take-off to landing navigation for the layperson?

- Usually you depart following a navigation procedure which is created - in most cases - by the local aviation regulation authorities. You may find many different departure procedures published for a single airport, and you use the one that suits you better depending on the route you're flying and the runway you're using. These procedures may use altitude turns, headings, radar vector (ATC instructions) segments, VORs and distance/radials, waypoints, or a combination of all. Waypoints are imaginary points in space created to design more efficient air routes that don't necessarily depend on ground stations, and GPS is precise enough to help you fly accurately between waypoints --> Depending on your aircraft navigation certified capabilities, you may or may not be able to navigate between waypoints. If you can't, you can always go back to the good old VOR navigation.

- In general terms, en-route you may navigate inside VOR airways or inside Waypoint airways (RNAV airways). Again, waypoints are carefully positioned to create straighter and therefore more efficient routes, so if you're certified to fly them, waypoint airways are the better choice.

- For approach you have the same methods available as for departure.

- For landing you may use several navigation means. These are just some of the commonly used, the most precise on top:
  • -ILS systems which give you precise vertical and lateral guidance towards the runway (ground based)
  • -Localizers (LOC) which is an ILS without the vertical guidance (ground based)
  • -GPS based approaches (satellite based)
  • -VORs (ground based)


Depending on the airport you may have various approaches available for each runway, and ATC can assign you the approach that best suits the current conditions. You may also request a different approach to that runway for whatever reason, and ATC generally has no problem accommodating that for you.


Alfredo
 
Mir
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 12:07 am

Quoting bio15 (Reply 2):
It's important to note that it's not a matter of "VOR or GPS" usage. If you have them available, both inputs should work together, whether you're flying a big airliner or a small piston-engine plane.

In my experience, in small piston aircraft, you're generally only using one or the other - the two don't work together the same way they would in an airliner.

-Mir
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bio15
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 12:54 am

Quoting Mir (Reply 3):
In my experience, in small piston aircraft, you're generally only using one or the other - the two don't work together the same way they would in an airliner.


--

If you have them available both inputs should work together.

Things clearly work different in smaller GA airplanes.


Alfredo
 
Mir
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 1:13 am

Quoting bio15 (Reply 4):
If you have them available both inputs should work together.

That's my point - they don't. I can drive a CDI off of VOR or GPS, but not both at the same time, even if both are available (and that includes the G1000). One can (and I often do) use one CDI on GPS for navigation and another CDI off of a VOR for crosschecking, but that's still a case where only GPS is actually running the navigation.

That's in contrast to a situation where an FMS is running the navigation and being fed by both VOR and GPS simultaneously. I don't know of any light aircraft avionics system that can do that.

-Mir
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Embraer195
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 1:41 am

Quoting Mir (Reply 5):
That's in contrast to a situation where an FMS is running the navigation and being fed by both VOR and GPS simultaneously

And so if the inputs from the two sources are significantly different, what happens? Is the FMS programmed to give each source a certain 'weighting' over the other to determine the position that it deems to be correct and to show on the ND?
 
Mir
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 2:43 am

Quoting Embraer195 (Reply 6):
And so if the inputs from the two sources are significantly different, what happens? Is the FMS programmed to give each source a certain 'weighting' over the other to determine the position that it deems to be correct and to show on the ND?

That's a function of the FMS software, and will vary from unit to unit. In the unit I'm familiar with, there is a hierarchy of what the FMS looks at, with GPS first. If there's no GPS, then it goes to DMEs, and finally to AHRS and ADC data (basically a poor man's IRS). It will alert the pilot to these happenings, of course, and the pilot can take action to disable inputs that are clearly defective, or update the FMS position with known good data. Even if the FMS craps out entirely, the pilot can still revert to old-fashioned VOR-to-VOR navigation if need be.

-Mir
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dxing
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 3:59 am

Quoting AirCalSNA (Thread starter):
At what point do these navigation aids kick in, so to speak--from listening to ATC it sounds like most planes first fly runway heading, are then given various heading instructions, and then told to proceed on course. Can someone provide a brief synopsis of take-off to landing navigation for the layperson? Thanks!

Most major airports in the U.S. (and the world for that matter) use what are called SIDS which are standard instrument departure routes and listed in the Jepp manuals. Some are RNAV some are not. On a normal day once the wheels are up and the plane is into the second climb phase on goes the autopilot and the box flies the route unless the departure controller instructs otherwise. On bad weather days near the field an airliner can expect tactical reroutes which will lead them to either a reroute or to a common point at which they are hopefully past the weather and handed off to fly as filed. The box goes on and flies the route. At the other end it is pretty much exactly the reverse. Major airports have what are called STARS or standard arrival routes also published in Jepps. The crew is cleared from point to point and expected to make speed and altitude points along the way. Once down close its more vectoring than flying the box although if it is a slow day the box can be flown right up to final.

[Edited 2011-03-25 21:02:20]

[Edited 2011-03-25 21:02:52]
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bio15
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 5:48 am

Quoting Mir (Reply 5):
That's my point - they don't. I can drive a CDI off of VOR or GPS, but not both at the same time, even if both are available (and that includes the G1000). One can (and I often do) use one CDI on GPS for navigation and another CDI off of a VOR for crosschecking, but that's still a case where only GPS is actually running the navigation.


-
Again: If you have them available both inputs should work together.

Navigation is NOT defined as "what you couple with the autopilot", but as the process of determining your position and controlling the movement of the aircraft. If your autopilot is coupled with the GPS and you have VOR information displayed at the same time, then you're navigating with both: Only one is controlling movement of the airplane via the autopilot (on small GA airplanes), but the other part of monitoring and determining your position doesn't cease to exist. When you use both inputs to crosscheck, then both inputs are working together to give you position information.

If you have them available GPS and VOR are not exclusive, even if your airplane's autopilot can only fly coupled to one of them at a time, and that's my point. I don't plan on discussing GA Avionics vs. Airliner Avionics, that was not my point at all.


Alfredo
 
Mir
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 7:15 am

Quoting bio15 (Reply 9):
If you have them available GPS and VOR are not exclusive, even if your airplane's autopilot can only fly coupled to one of them at a time, and that's my point. I don't plan on discussing GA Avionics vs. Airliner Avionics, that was not my point at all.

It's okay, we both mean the same thing - we're just using different ways of saying it.

-Mir
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Embraer195
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 12:47 pm

Quoting Mir (Reply 7):

Thanks Mir. Plenty of redundancy built into the system then, as per usual with aircraft design.
 
AirCalSNA
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 9:16 pm

Thanks for the very interesting information.

So I gather from the above discussion that the RNAV and other routes that airliners generally follow do not consist of a strict point-to-point flying from one VOR to another, for example. Rather, it sounds like the FMS uses the various data and then plots a course between and among the various reference points, such as VOR transmitters. Is that right?

[Edited 2011-03-26 14:17:21]
 
dxing
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RE: How Do Modern Airliners Navigate?

Sat Mar 26, 2011 9:33 pm

Quoting AirCalSNA (Reply 12):
Is that right?

Sort of but with INS it's more internal navigation augmented by outside references like GPS and VOR's. The pilots are not tuning into a new VOR signal every couple of hundred miles.

[Edited 2011-03-26 14:34:31]
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tdscanuck
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RE: How Do Modern Airliners Navigate?

Sun Mar 27, 2011 4:32 am

Quoting AirCalSNA (Reply 12):
So I gather from the above discussion that the RNAV and other routes that airliners generally follow do not consist of a strict point-to-point flying from one VOR to another, for example. Rather, it sounds like the FMS uses the various data and then plots a course between and among the various reference points, such as VOR transmitters. Is that right?

Sort of. There are really two separate functions here...how does the airplane know where it is, and how does it define the route. A modern airliner knows where it is by filtering (combining) all the various position references (GPS, inertial, radio navaids) into a single "the position".

Once you know where you are, the FMS compares that to where you want to be (the flight plan) and directs the flight crew (flight director) or autopilot appropriately.

The flight plan route is defined by points in space...these can be VORs (many airways use VORs as endpoints), waypoints (defined by lat/long but referred to by names), lat/long positions, various abeam and intercept functions, or points defined relative navaids (e.g. 25 miles from the xxx VOR on the 125 radial), etc. A modern FMS can take input in all these forms and plays connect the dots to build the flight plan.

In autoflight, some airliners will also "lead the turn" to avoid overshooting the flight plan path. I.e. they start to turn before actually reaching the waypoint, so you may not actually overfly the waypoint.

Quoting dxing (Reply 13):
Sort of but with INS it's more internal navigation augmented by outside references like GPS and VOR's. The pilots are not tuning into a new VOR signal every couple of hundred miles.

No, but a modern navigation radio will constantly autotune and may be switching VORs (invisible to the flight crew) even more often than every couple of hundred miles.

Tom.
 
Confuscius
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RE: How Do Modern Airliners Navigate?

Sun Mar 27, 2011 5:00 am

Quoting AirCalSNA (Reply 12):
So I gather from the above discussion that the RNAV and other routes that airliners generally follow do not consist of a strict point-to-point flying from one VOR to another,

Isn't the FAA just doing an overlay of current approaches to major airports which is counter to the RNAV RNP concept?
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tdscanuck
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RE: How Do Modern Airliners Navigate?

Sun Mar 27, 2011 5:08 am

Quoting Confuscius (Reply 15):
Isn't the FAA just doing an overlay of current approaches to major airports which is counter to the RNAV RNP concept?

Do you mean LAAS and WAAS approaches? They're not counter to the RNAV/RNP concept, they're just an alternative technology to achieving the same goal as ILS.

Tom.
 
Confuscius
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RE: How Do Modern Airliners Navigate?

Sun Mar 27, 2011 5:22 am

Quoting tdscanuck (Reply 16):
Do you mean LAAS and WAAS approaches?

I'm not sure. Here's the article...

http://www.aviationweek.com/aw/gener...hannel=awst&id=news/aw081009p3.xml
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MSJYOP28Apilot
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RE: How Do Modern Airliners Navigate?

Sun Mar 27, 2011 7:18 am

At my airline, I dont normally file flights on RNAV SIDs and STARS. This makes it a whole lot easier when we have an FMS inop mel, which is a fairly common mel item. High VORs have a range of 130 miles. Low VORs have a range of 40 miles and terminal VORs have a range of 25 miles. Most of the non RNAV SID and STARS use waypoints that are on the published plates for that airport and our also often waypoints on jet routes. For arrival and departure routes, they are built to route you to transition VORs along published jet routes. Some airports have departures and arrival routes that do radar vectoring to transition VORs.

Overall, the only real difference between RNAV routes and non RNAV is that the RNAV routes cannot be used without an FMS. There really isnt a huge difference.

With that said, Q routes do offer an advantage over J routes but in normal operation domestically I rarely ever file flights on Q routes. We only have a few routes where GPS and RNAV are required.

GPS approaches are still in their growing stage in aviation. At my airline, we cannot use GPS or RNAV approaches for planning alternates. If an alternate airport is not in VFR conditions, it better have a working ILS, LOC, VOR or NDB approach otherwise we cant use it as an alternate. It might have a GPS approach but we cannot derive our alternate minimums based on it.

One of the things I dread is a long flight with an FMS inop. I have to check the whole route to make sure we are always in adequate coverage for VORs. When some VORs are OTS, it makes things kind of interesting. The FMS will pick up on VOR locations even when the VOR is OTS. But with no FMS, you need the radio frequency to pick up the radials. Surpisingly, major transitions VORs out of places such as ATL like Vulcan Vortac are OTS rather frequently.

The one thing that wont change for a while is ATC. When vectoring, ATC uses radar and compass headings for many of their instructions enroute. I am not sure how ATC could vector and do holding patterns using only GPS and waypoints. So much of holding and vectoring is not planned in advance so it is nearly impossible to have vector points everywhere for flow control purposes.

Legally, my airline can operate into areas of forecast convective activity without RVSM, autopilot and FMS. In the pre-planning, it makes it very tricky routing around the highest echo tops and tstorms because you need the VOR coverage and sometimes the re-routes can be pretty far out. In the CRJ we only have one FMS so if we dont have one then we dont have a second one as a backup like we do in the CR9.

I really wish that the weather radars we had were better. It would make navigation in summer weather much more easier. They are limited in range and accuracy. Plus, none of the weather products can see cumulus phase CBs. If the CB doesnt have moisture falling to the ground, none of our radars can see it but these CBs are a real hazard to flying even if they arent yet producing visible moisture.
 
dxing
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RE: How Do Modern Airliners Navigate?

Sun Mar 27, 2011 3:49 pm

Quoting MSJYOP28Apilot (Reply 18):
Overall, the only real difference between RNAV routes and non RNAV is that the RNAV routes cannot be used without an FMS. There really isnt a huge difference.

I'd disagree based on the individual origin and destination. There can be quite a difference in milage alone.

Quoting MSJYOP28Apilot (Reply 18):
With that said, Q routes do offer an advantage over J routes but in normal operation domestically I rarely ever file flights on Q routes. We only have a few routes where GPS and RNAV are required.

Shame, I plan on the Q routes all the time and it can save 10-20 minutes depending on destination and winds. That said, there are still days when running the coastline is the better alternative.

Quoting MSJYOP28Apilot (Reply 18):
GPS approaches are still in their growing stage in aviation. At my airline, we cannot use GPS or RNAV approaches for planning alternates

Given the minimums for a fair amount of GPS approaches, it might as well be VFR. However it is nice to be able to fly the RNAV/RNP approach if the localizer fails. That happened a few weeks ago at DCA and if it wasn't for the fact we can fly the RNAV/RNP we wouldn't have gotten in.
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tdscanuck
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RE: How Do Modern Airliners Navigate?

Mon Mar 28, 2011 5:21 pm

Quoting MSJYOP28Apilot (Reply 18):
The one thing that wont change for a while is ATC. When vectoring, ATC uses radar and compass headings for many of their instructions enroute. I am not sure how ATC could vector and do holding patterns using only GPS and waypoints.

Why would they need to? I don't think anyone is proposing doing away with heading hold and altitude hold autoflight/flight director modes, and that's all you need to follow vectors.

Tom.
 
Mir
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RE: How Do Modern Airliners Navigate?

Mon Mar 28, 2011 6:48 pm

Quoting tdscanuck (Reply 16):
Quoting Confuscius (Reply 15):
Isn't the FAA just doing an overlay of current approaches to major airports which is counter to the RNAV RNP concept?

Do you mean LAAS and WAAS approaches?

What he means is the practice of developing new RNAV procedures that are pretty much the same as current procedures, particularly in the area of SIDs and STARs. For example: take a look at these two STARs into ATL:

http://tiles.skyvector.com/sky/files/tpp/1103/pdf/00026SINCA.PDF

http://tiles.skyvector.com/sky/files/tpp/1103/pdf/00026CANUK.PDF

You'll notice that the two are EXACTLY the same until you get close to the airport, at which point you're probably getting radar vectors from ATC anyway. So there's not a whole lot of advantage of the RNAV procedure - I guess having one charted turn allows ATC to give one less vector, but it's still not the capacity brought on by additional routes that RNP has the potential for.

Ironically, the RNAV SIDs out of ATL are very diverse, and are more in line with the RNP concept. It's the FAA - who really knows why they do what they do?

-Mir
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Magcheck
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RE: How Do Modern Airliners Navigate?

Mon Mar 28, 2011 9:27 pm

Quoting tdscanuck (Reply 16):
they're just an alternative technology to achieving the same goal as ILS.

Same goal ("principle") as ILS, but not the same degree of accuracy, correct? Even with LAAS, it's still not possible to get Cat III or even CAT II precision (minima), is it?
 
Mir
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RE: How Do Modern Airliners Navigate?

Mon Mar 28, 2011 10:51 pm

Quoting Magcheck (Reply 22):
Even with LAAS, it's still not possible to get Cat III or even CAT II precision (minima), is it?

It should be with LAAS - at least CatII. But LAAS kind of got pushed to the back burner because of cost issues for now, so WAAS (which in theory allows CatI minimums, but I've always seen them just a bit higher than that) is what we've got. Still a significant improvement from regular GPS, though.

-Mir
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flybaurlax
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RE: How Do Modern Airliners Navigate?

Tue Mar 29, 2011 12:26 am

Quoting Mir (Reply 21):
You'll notice that the two are EXACTLY the same until you get close to the airport, at which point you're probably getting radar vectors from ATC anyway. So there's not a whole lot of advantage of the RNAV procedure - I guess having one charted turn allows ATC to give one less vector, but it's still not the capacity brought on by additional routes that RNP has the potential for.

Most RNP procedures in and out of bigger terminal areas are the same as RNAV and other procedures, but it's in the smaller, less traveled airports where RNP really saves you. If you look at Alaska, there are tons of RNP procedures that save miles, thus save fuel. They can do direct descent and direct to closer waypoints, than ILS or NDB approaches. Instead of the dive and drive, they can basically cut to idle 100 nm out and only use power a few miles away as they configure for landing.

Here's a brief list of airports in Alaska that AS utilizes RNP procedures: ADK, OME, OTZ, SCC, JNU, and a few others. They definitely save fuel.
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B727LVR
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RE: How Do Modern Airliners Navigate?

Tue Mar 29, 2011 1:52 am

Quoting tdscanuck (Reply 16):
Do you mean LAAS and WAAS approaches?

Speaking of WAAS... I read an AEA article on it and was rather impressed, as an avionics tech, by what it will allow.

The article:
http://www.aea.net/AvionicsNews/ANAr...hives/MainFeatureJun10_WAASGPS.pdf


How many operators use WAAS now, or is it still new to the game? I know the development has been around for a while, I I am wondering how wide its usage is.

The acft I maintain, one of which turned 44 yesterday, either use a GPS system, or a UNS type FMS. They are pretty reliable in their own right, I am anxious to work on something a little newer...
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tdscanuck
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RE: How Do Modern Airliners Navigate?

Tue Mar 29, 2011 4:32 am

Quoting Magcheck (Reply 22):
Same goal ("principle") as ILS, but not the same degree of accuracy, correct? Even with LAAS, it's still not possible to get Cat III or even CAT II precision (minima), is it?

LAAS can get CatIII equivalent precision, and without interference from stuff in the ILS critical area...I don't think LAAS approaches are certified that low right now but that's a regulatory problem rather than a technical one.

Quoting Mir (Reply 21):
What he means is the practice of developing new RNAV procedures that are pretty much the same as current procedures, particularly in the area of SIDs and STARs.

I always assumed that was a precursor to getting rid of the navaids that support the non-RNAV procedures...by having them overlay, the traffic patterns all stay the same so ATC is happy, but they can swap out the technology underneath. I assume the same thing will eventually happen with ILS and LAAS.

Tom.
 
dxing
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RE: How Do Modern Airliners Navigate?

Tue Mar 29, 2011 1:59 pm

Quoting tdscanuck (Reply 26):
LAAS can get CatIII equivalent precision, and without interference from stuff in the ILS critical area...I don't think LAAS approaches are certified that low right now but that's a regulatory problem rather than a technical one.



Both, technical as in they have to get the equipment sited and running and cost is holding it back, regulatory in that the airlines will still have to train and get ops specs authorization to fly CATIII based on LAAS technology.

Quoting tdscanuck (Reply 26):
I always assumed that was a precursor to getting rid of the navaids that support the non-RNAV procedures...by having them overlay, the traffic patterns all stay the same so ATC is happy, but they can swap out the technology underneath. I assume the same thing will eventually happen with ILS and LAAS.



Ding. At an airport like ATL it's much easier to just overlay the existing approaches until everyone is on the same page then they can start issuing new ones. Plus there might be some underlying problems such as noise they have to be concerned about. When IAH opened up 8L/26R a few years back there were a whole bunch of people that not only got surprised by a low flying airplane coming over their house, they got very vocal about it as well. So a curving, descending approach to the airport that might save miles, might also create a firestorm of controversy.
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tdscanuck
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 4:58 am

Quoting dxing (Reply 27):
Both, technical as in they have to get the equipment sited and running and cost is holding it back, regulatory in that the airlines will still have to train and get ops specs authorization to fly CATIII based on LAAS technology.

By "technical" I meant there's no reason you can't get CatIII accuracy out of a LAAS...but you're absolutely right that it doesn't do any good until you install it, get the approval, and get the procedures.

Tom.
 
Quokka
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 1:10 pm

So the FMS determines current position (location) relative to destination and calculates a flight path. If on route another aircraft is encountered, does the FMS detect this other presence and automatically adjust the flight path by changing direction and/ or altitude, or is direct input by the pilot required?
 
Fly2HMO
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 2:32 pm

Quoting Quokka (Reply 29):
So the FMS determines current position (location) relative to destination and calculates a flight path.

Sort of. It knows it's position relative to any point on the earth. And you as a pilot have to tell the FMS how you want to get to your destination, if you let it choose on it's own most systems will just draw a straight line (direct to). Some routes are already preloaded on the system. Others you may have to build manually. And even with those preloaded ones chances are you will have to modify them significantly as well to accommodate any changes mandated by ATC.

Quoting Quokka (Reply 29):
or is direct input by the pilot required?

No system out there currently provides automatic route deviations do to traffic. But such systems are in development.
 
Flighty
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 5:48 pm

I guess this thread is really asking the question, what happens when/if GPS has a virus and goes down? How big of a safety problem is that, with thousands of airliners flying? I would expect there is backup (and the best backup would be inertial navigation, but is that still used?)
 
Fly2HMO
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 6:36 pm

Quoting Flighty (Reply 31):
what happens when/if GPS has a virus and goes down?

That's a Hollywood scenario.

A slightly more realistic scenario is a massive solar flare wiping out the satellites.

Quoting Flighty (Reply 31):
How big of a safety problem is that, with thousands of airliners flying?

Currently? Shouldn't be too big of a deal. It's not like there will be planes falling off the skies. Planes would just revert to inertial or land based navaids.

However, a decade or two from now once Nextgen comes online, which will rely exclusively on GPS, things may not be so pretty.

Quoting Flighty (Reply 31):
I would expect there is backup

Idiotically the GPS satellite system in itself has no backup. eLORAN would have been a perfect backup but no, it was shut down i the name of penny-pinching and last time I checked there has been no progress made in finding a backup that will work for everyone.

[Edited 2011-03-30 12:07:19]
 
Flighty
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 8:48 pm

Quoting Fly2HMO (Reply 32):
However, a decade or two from now once Nextgen comes online, which will rely exclusively on GPS, things may not be so pretty.

Cool thanks for your reply.

Yes it sounded like LORAN had a theoretical need. I think China has the capacity to disable sats if they really want to.
 
Confuscius
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 9:18 pm

Quoting Flighty (Reply 31):
I guess this thread is really asking the question, what happens when/if GPS has a virus and goes down?

There's a compass in front of the overhead panel.



[Edited 2011-03-30 14:20:01]
Ain't I a stinker?
 
Tristarsteve
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 9:19 pm

All this talk about GPS going down being the end of navigation, it wasn't so long ago that we didn't have it.
I've been working here in ARN since 1987. When I started we used the B737-200 across the North Sea to LHR.
Navigation was by VOR/DME. No Inertial Nav, No GPS, No FMS.
Between Denmark and the UK there is a navpoint called Dunker where you had to turn. This was out of range of Nav aids and the turn was procedural. You were navigating with no navaids for about 20 mins.
I am sure that todays pilots could do this. They all learned it at Flight School, and anyway all large airliners now have at least two IRS to find the way.
I remember once sending a B732 off back to LHR with both autopilots inop. Crew were a bit concerned, but the MEL said OK, and off they went.
 
bio15
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 9:56 pm

Quoting tristarsteve (Reply 35):
I remember once sending a B732 off back to LHR with both autopilots inop. Crew were a bit concerned, but the MEL said OK, and off they went.

It's good to have some hand-flying fun every once in a while    Gets you tired real quick though!

Alfredo
 
B727LVR
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 10:00 pm

Quoting tristarsteve (Reply 35):
I would expect there is backup

As stated above, the only back-ups currently are INS/IRS, VOR, ADF, and your Standby Compas but if you need to use this there are more than likely some serious problems happening. Reverting back to INS/IRS is no big deal, its basic dead-recogning navigation, that advantage you have with it is that the plane knows where it is already, by way of manual input.

The best use for ADF for me is listening to the raido staions when I am missing the game.   
I'm like a kid in a candy store when it comes to planes!
 
Fly2HMO
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RE: How Do Modern Airliners Navigate?

Wed Mar 30, 2011 10:06 pm

Quoting tristarsteve (Reply 35):
All this talk about GPS going down being the end of navigation, it wasn't so long ago that we didn't have it.

The problem is in the near future we won't have much to fall back on. Dead reckoning may be fine crossing an ocean or some other barren place, but good enough to squeeze you into your FAF coming into ORD at rush hour while in an emergency? Hell no.

Quoting tristarsteve (Reply 35):
I am sure that todays pilots could do this. They all learned it at Flight School

I'm not to sure I'd count on that. I'm of the last generation of pilots to learn to navigate the good old way, and actually put it to practice. Nowadays kids are getting tossed with zero time into glittery glass cockpits and while they may be taught dead reckoning and how to use the whiz wheel, I bet that in all their solo flights they're just hitting the direct-to button  
 
tdscanuck
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RE: How Do Modern Airliners Navigate?

Thu Mar 31, 2011 5:29 am

Quoting Quokka (Reply 29):
If on route another aircraft is encountered, does the FMS detect this other presence and automatically adjust the flight path by changing direction and/ or altitude, or is direct input by the pilot required?

Nope. TCAS will warn you of impending conflicts and provide guidance to the flight crew as to which way to go (if the other aircraft also has TCAS), but it's up to the flight crew to actually alter the flight path.

Quoting Flighty (Reply 31):
I guess this thread is really asking the question, what happens when/if GPS has a virus and goes down?

Not much...older airliners that didn't have GPS won't notice. Newer ones that have combined navigation systems (INS+VOR/DME+GPS) will just drop GPS from the solution and use VOR/DME/INS, which can be just as good, and is almost always good enough.

Quoting Flighty (Reply 31):
How big of a safety problem is that, with thousands of airliners flying?

Not much.

Quoting Flighty (Reply 31):
I would expect there is backup (and the best backup would be inertial navigation, but is that still used?)

Inertial is absolutely used...it's used more now than it used to be, although that's a side effect of FBW flight control systems needing really good inertial data (the FBW cares about rates and accelerations, not positions, but it's just one more integration). Good INS takes 15 minutes or more to drift out of even tight RNP requirements when it has nothing else to work with, and if it's got VOR/DME available it will happily keep updating itself and work just fine.

Quoting Fly2HMO (Reply 38):
The problem is in the near future we won't have much to fall back on. Dead reckoning may be fine crossing an ocean or some other barren place, but good enough to squeeze you into your FAF coming into ORD at rush hour while in an emergency? Hell no.

You've still got all the navaids out there + inertial + magnetic compass...

Tom.
 
Tristarsteve
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RE: How Do Modern Airliners Navigate?

Thu Mar 31, 2011 9:01 am

Quoting tdscanuck (Reply 39):
Good INS takes 15 minutes or more to drift out of even tight RNP requirements when it has nothing else to work with, and if it's got VOR/DME available it will happily keep updating itself and work just fine.

On a newish airliner the INS is really an IRS. a Sensor not a nav system.
Back on the B747-200 the INS actually navigated the aircrat, but nowadays the IRS gives an input
to the FMC.
It is the FMC that corrects the IRS position with DME readings.
When you shut down at the end of the flight, look at the IRS page in the FMC (before you turn them off)
and you can see the accumulated IRS errors since they were last turned on.
 
Pihero
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RE: How Do Modern Airliners Navigate?

Thu Mar 31, 2011 11:04 am

Quoting tristarsteve (Reply 40):

On a newish airliner the INS is really an IRS. a Sensor not a nav system.

The raw data do exist, are available if needed as a means of *inertial based* dead-reckoning.

Quoting tristarsteve (Reply 40):
It is the FMC that corrects the IRS position with DME readings.

The navigation system of a modern airliner is a lot more integrated than the average person would think.
1.- to start with, there is the FMS as said above : It collects all the information from the various boxes around and malaxes them before presenting you with a posqition and a route.
2/- There are now units that collect all the aerodynamic data : altitude / speed / temperature... with the Inertial reference data : position / altitude / ground speed / acceleration / heading (true as well as magnetic);
These units are referred to as ADIRUs for air data and inertial reference units.
3/- ...and then we still have the radio-nav boxes : VOR / DMEs and ADFs

So how does this work during a flight ?
First of all, during cockpit preparation, thre gate coordinates are inputted as accurately as possible into the FMS for the benefit of the IRUs, then the flight plan, including the SID is entered.
The system starts then by giving a mix of INS and GPS positions, called GPIRS, which would later in the flight be the main reference.
At takeoff, as the coordinates of the runway (and the intersection you could be departing from ) are known very accurately, the FMS will take that position as the start of the trip.
Later , that original FMS position updated with the inertial acceleration, speed... data will drift towards the GPIRS
If everything goes normally, the aircraft will stay on that refernce as it is by far the most accurate position.
In the case of lost GPS information, the system will revert to a less accurate - but still very accurate - DME/DME/IRS mode, then if further degraded, to a VOR/DME/IRS mode.
The total loss of inertial and radio references will cause the FMS to use the position called *mix IRS*, a combination of the three IRUs outputs.... BUT not immediately : it will start with the latest precise position it had and very slowly drift tiward the pure mix IRS .
In the real world, I have rarely lost GPS data for more than ten minutes on oceanic trips.
Contrail designer
 
travelavnut
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RE: How Do Modern Airliners Navigate?

Thu Mar 31, 2011 11:12 am

Quoting Pihero (Reply 41):
DME/DME/IRS



So this would be automatic "triangulation" with 2 DME beacons? And data from this will compensate the IRS drift?
Live From Amsterdam!
 
Fly2HMO
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RE: How Do Modern Airliners Navigate?

Thu Mar 31, 2011 12:23 pm

Quoting tdscanuck (Reply 39):

You've still got all the navaids out there

Again, when Nextgen comes online this will not be the case. If anything a couple of VORs will remain in essential areas. The decommissioning of NDBs is well underway as of a couple of years ago and the FAA just started shutting down some VORs last year. Granted this is in the US only but the rest of the world will follow suit soon enough.
 
Pihero
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RE: How Do Modern Airliners Navigate?

Thu Mar 31, 2011 1:07 pm

Quoting travelavnut (Reply 42):

So this would be automatic "triangulation" with 2 DME beacons?

Yes

Quoting travelavnut (Reply 42):
And data from this will compensate the IRS drift?

No ; It will become the basis for the FMS position.
The system will never compensate for IRS drift :
1/- The IRSs are left on their own. What the FMS does is only keep a continuous record of the mean position derived from the three IRUs;
2/- Thne FMS at all times defines a "bias", which is the vector between the FMS position and the Mix IRS position;
3/- When GPS or Rdionav is lost, the FMS position will be kept in memory and left to gradually drift toward the mix IRS position, therefore progressively cancelling the bias over time.
Contrail designer
 
travelavnut
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RE: How Do Modern Airliners Navigate?

Thu Mar 31, 2011 1:48 pm

Quoting Pihero (Reply 44):
No ; It will become the basis for the FMS position.
The system will never compensate for IRS drift :
1/- The IRSs are left on their own. What the FMS does is only keep a continuous record of the mean position derived from the three IRUs;
2/- Thne FMS at all times defines a "bias", which is the vector between the FMS position and the Mix IRS position;
3/- When GPS or Rdionav is lost, the FMS position will be kept in memory and left to gradually drift toward the mix IRS position, therefore progressively cancelling the bias over time.

As always thank you Pihero!

Quoting Pihero (Reply 44):
Quoting travelavnut (Reply 42):

So this would be automatic "triangulation" with 2 DME beacons?

Yes

Trianugation between 2 points also results in IIRC 2 possible locations, how is this handled/solved?
Live From Amsterdam!
 
Pihero
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RE: How Do Modern Airliners Navigate?

Thu Mar 31, 2011 2:04 pm

Quoting travelavnut (Reply 45):
how is this handled/solved?

by eliminating the intersection that's the most unlikely - either through the mix IRS position or the use of a third DME, very often available in continental airspaces.
Contrail designer
 
Tristarsteve
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RE: How Do Modern Airliners Navigate?

Thu Mar 31, 2011 3:12 pm

And remember that DME/DME does not mean two DME beacons, it means two DME boxes.
The DME constantly retunes looking for new DME, so could be using many DME transmissions
to find its position.

And IRS is not that inaccurate nowadays.
I've just met an A330 off a 6 hr flight.
The IRS data on the grond shows average drift of 0.2 NM/HR on all three ADIRUs.
So just over 1 NM inaccuracy after 6 hours.
 
Pihero
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RE: How Do Modern Airliners Navigate?

Thu Mar 31, 2011 3:18 pm

Quoting tristarsteve (Reply 47):
The DME constantly retunes looking for new DME, so could be using many DME transmissions
to find its position.

Five stations in all on the FMs I'm using the most

Quoting tristarsteve (Reply 47):
The IRS data on the grond shows average drift of 0.2 NM/HR on all three ADIRUs.
So just over 1 NM inaccuracy after 6 hours.

On each ADIRU or on the mix IRS position ?
Contrail designer
 
tdscanuck
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RE: How Do Modern Airliners Navigate?

Fri Apr 01, 2011 5:30 am

Quoting Pihero (Reply 44):
The system will never compensate for IRS drift :

That's not universally true...some IRS's can do an in-flight realignment, which zeros out their bias.

Tom.

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