futureualpilot
Posts: 2402
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In-flight Naviagtion On Airliners

Mon Jan 05, 2004 12:41 pm

Im sorry if this has been asked before(my search isnt working right now) or if it is an ignorant question, but anyway

When in flight, how do airliners navigate? I understand they have way points and such, but what instruments track them, and on the flight displays, what shows the crew they are on the correct route if there is no GPS display? Does the magenta line (or whatever color it may be) on the nav display show the route as planned with waypoints and such?

(in this case the line is green Smile


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thanks for your help!
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jhooper
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RE: In-flight Naviagtion On Airliners

Mon Jan 05, 2004 2:45 pm

I love seeing the old school cockpits with the round dials and VOR/HSI gauges you'd find in general aviation aircraft. Back in the day, they'd use those!

Now, most modern airliners have Flight Management Systems (FMS) which they just input the route into a computer and the route is displayed on the Primary Flight Display (PFD). ...and yes, that magenta line shows the route with the progammed waypoints. When you load an approach into the FMS, the route depicting the missed approach procedure is also displayed. Really cool stuff, actually.

BTW, congrats on your checkride....

[Edited 2004-01-05 06:46:06]
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FredT
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RE: In-flight Naviagtion On Airliners

Mon Jan 05, 2004 10:44 pm

In the old days, it was on radio navigation and dead reckoning/star navigation when no radio navaids were available.

Today, it is all handled by the inertial navigation system. The position of the INS is updated through radio beacons, when available, and sometimes in some airlines through GPS.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
KLM777
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RE: In-flight Navigation On Airliners

Mon Jan 05, 2004 11:09 pm

The INS determines it's position on radio beacons as Fred said before. After this starting point is determined it calculates the current position based on the movements of the aircraft.

After a while this position will become somewhat inaccurate, so it needs to be updated and checked via radio beacons again. On long stretches of open water this cannot be done so you are not 100% sure of your position.

GPS is always very precise so this is of course preferred.

Kind regards,

Jeroen

[Edited 2004-01-05 15:11:46]
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goboeing
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RE: In-flight Naviagtion On Airliners

Tue Jan 06, 2004 12:01 am

When flying across the United States, for example, does the INS automatically update the position to more accuracy with NAVAIDs around it, or does the pilot do this once in a while? Also, if it does it automatically, is it a periodic update or is it more of a continuous checking of position sort of like RNAV using vectors from two VORs.

Nick
 
FredT
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RE: In-flight Naviagtion On Airliners

Tue Jan 06, 2004 12:06 am

It is more or less continuous. Modern INS select two beacons at as close to a right angle to each other to get the best possible precision.

The INS is what gives the aircraft RNAV capability, and it is (typically, might be exceptions?) updated using DME distances rather than bearings ("vectors") but otherwise, you've got it.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
SlamClick
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RE: In-flight Naviagtion On Airliners

Tue Jan 06, 2004 1:36 am

While GPS is the current state-of-the-art for airliners, it is not required for ETOPS. Some aircraft are doing transoceanic with FMS, IRS equipment to RVSM standards.

The airplane does not get "off course" because it is being flown by the autopilot which steers it to track the intended route. If you depart a track to avoid a thunderstorm, for example, it continues to plot the track's actual location and will give you a re-intercept. You can also offset a selected number of miles Left or Right of track and fly parallel to it. This is standard procedure on some overwater routes.

If the airplane knows where it is, and it knows what heading it must steer to make good a given course over the ground, then it can determine for itself what the local winds are. It gets true airspeed reported to it by an air data computer. It knows the rate-of-change of its present position, in other words how fast it is actually moving over the surface. The difference between the reported TAS and the observed GS is the wind factor and helps to confirm the wind speed and direction. (The forecast winds aloft are also loaded in before takeoff.)

It displays the planned ground track in green (Airbus) or magenta (Boeing) (typically) and it shows airplane heading at (or near) the top of the nav display. It shows a wind arrow with calculated values nearby. It displays TAS and GS.

It depicts upcoming nav waypoints on a selectable scale. It may show navaids being autotuned. It can show nearby airports. Some can show "abeam" points. They can show VOR and NDB's along the way. Weather radar and enhanced GPWS maps can be displayed along with the navigation information, as are TCAS targets.

I too like the look of old iron gauges but I sure do love flying this stuff. It is a gigantic improvement over the stuff in older/smaller airplanes.

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tom775257
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RE: In-flight Naviagtion On Airliners

Tue Jan 06, 2004 2:19 am

FredT, KLM 777:
INS is not really used these days. INS used a gyro stabilised platform sometimes alligned with north with acceleromers on it. Modern IRS units use laser ring gyros and accelerometers attached to the airframe in a 'strap down system.' These are entirely different systems.
The IRS/INS position is not updated. Once alligned the INS/IRS can't be re-alligned while moving, if you take the INS/IRS off nav mode all you can get inflight is attitude and if you help it a bit heading information.
The position produced by the FMS displayed on the EFIS uses multiple inputs into the flight management computer. Say triple IRS, twin VOR/DME etc. The computer then uses a 'Kalman' filter to decide best position. On land I think the order of RNAV accuracy is something like: best to worst - DME/DME fix, VOR/DME, VOR/VOR finally worst is IRS (assuming no GPS). Even though error may found when you coast in, the IRS position will not be updated, however the filter in the FMC will use the more accurate fixes to produce the new position shown on the EFIS.
Cheers, Tom.

[Edited 2004-01-05 18:23:12]
 
tom775257
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RE: In-flight Naviagtion On Airliners

Tue Jan 06, 2004 2:32 am

A quick description from 737-400 op manual:

FMC navigational computations are based upon am FMC position which is established using radio inputs and/or IRS position. The FMC position may be based only on IRS data only, however, available DME inputs are used to refine and update the FMC position......

Radio tuning...The dual freq-scanning DME radios are auto tuned by the FMC. Radio position is determined by the intersection of 2 DME arcs.
 
AAR90
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RE: In-flight Naviagtion On Airliners

Tue Jan 06, 2004 2:57 am

The INS determines it's position on radio beacons as Fred said before.

INS is Inertial Navigation System and once aligned it will determine its position based solely upon perceived aircraft movement.

After a while this position will become somewhat inaccurate, so it needs to be updated and checked via radio beacons again.

The INS position itself is never actually updated --although, depending upon display option installed may appear to be updated. What actually happens is that the INS position becomes the starting point with outside sources [GPS, VOR, DME, ILS, etc.] providing a refined position. That refined position [called "FMS position" on FMS systems] is what the system will use for navigation and display. The INS position will remain [not updated] but not normally visible to user. The INS "drift" is normally "zeroed" out by the updating, but the INS position itself never is.

GPS is always very precise so this is of course preferred.

Not "always" but rather almost always... GPS is an outside transmitter and anything that transmits can become unreliable for a variety of reasons.

When flying across the United States, for example, does the INS automatically update the position to more accuracy with NAVAIDs around it, or does the pilot do this once in a while?

The INS does no updating [see above]. The FMS automatically updates its position... continously. Most non-GPS systems begin automatic updates once getting airborne. GPS systems normally begin updating upon completion of INS alignment but can be limited to airborne updates [software programming] if so desired.

Also, if it does it automatically, is it a periodic update or is it more of a continuous checking of position sort of like RNAV using vectors from two VORs.

Continuously updated. After GPS, the preferential order [for accuracy] is normally DME/DME, DME/radial, radial/radial.

I too like the look of old iron gauges but I sure do love flying this stuff.

Push a button or rotate a knob to get "steam guage" displays. I regularly do this to keep myself proficient.... just in case. Same with flying the standby instruments.... just in case. Would hate to be forced to fly them for real without at least some practice.  Big grin
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futureualpilot
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RE: In-flight Naviagtion On Airliners

Tue Jan 06, 2004 2:58 pm

thanks all, I appriciate it very much! Smile/happy/getting dizzy

Clear skies

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DeltaGuy
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RE: In-flight Naviagtion On Airliners

Tue Jan 06, 2004 3:00 pm

AAR90,

Flying on standby gages must remind you of your Hawkeye days  Big thumbs up

Anyways.......to answer the question at hand...

On that INS, there's usually 2 of em on, say, a 737/MD-88, but 3 on a 757/777 type jet. System 1 to AFCS 1, and so on....On Capt's leg, they usually use AFCS 1, F/O's, it's AFCS 2...so the INS units are both aligned, but one autopilot is used for flight at least...I know on most newer jets, excluding the MD-80 series, all autopilots have to be engaged for APP mode.

If you move those selector knobs out of "NAV" while in the air, forget it, you've lost your data. You can't fully realign in the air really, so it's back to charts and VOR's for you  Big grin Thankfull,y the knobs have to be pulled OUT of their detents before you can move them from NAV...so the crew's noggins won't kill the system when they get up for a lav break  Big grin

At the gate, when you're doing your POS INIT page on the FMS, you enter the airport and gate, but before you're allowed to enter that as the IRS position, you have to go to a quick realign on the overhead, rotating the knobs to ALN, then back to NAV..this elminates "drift errors" and any drift from the last flight. Then you're allowed to tell the FMS where you are. On the first flight of the day, it takes considerably longer to align them from OFF....10 mins IIRC? Remember this has to be done at the gate, as you can't move the aircraft at all during the alignment. That's why airborne align doesn't work.

I'm speaking from the Delta MD-88 PRM on most of this, I know things may be a little diff on the newer generation systems, ex 757->738. I'll crack it open if I mucked it up and somehow didn't tell you correctly the first time  Smile

DeltaGuy
"The cockpit, what is it?" "It's the little room in the front of the plane where the pilot sits, but that's not importan
 
AAR90
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RE: In-flight Naviagtion On Airliners

Wed Jan 07, 2004 8:19 am

Flying on standby gages must remind you of your Hawkeye days

Not really, the Hummer cockpit was never [heck, it still isn't] that advanced. 1950's with a couple of add-ons to make things confusing --and very difficult to keep lit properly at night.  Pissed
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DeltaGuy
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RE: In-flight Naviagtion On Airliners

Wed Jan 07, 2004 3:05 pm

AAR90, I meant, steam gages like the standby's on the airliners..nevermind  Big grin lol

Go Hornets  Big thumbs up

DeltaGuy  Smile
"The cockpit, what is it?" "It's the little room in the front of the plane where the pilot sits, but that's not importan
 
AAR90
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RE: In-flight Naviagtion On Airliners

Wed Jan 07, 2004 8:39 pm

DeltaGuy, the Hawkeye's primary guages (there are no standby guages) are less advanced than the basic airliner standby guages. Sorry, but USN spent all the money on the classified stuff in "the hole."  Sad
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