FMC track bearings versus Flight Plan track bearings

[JammedStab]

This quote is from a recent oceanic ops article which seems to confirm the conflicting information I have noticed when comparing flightplan(LIDO) to FMC(Boeing). Is this typical in airline operations?

"...it is also vital that flight crews understand how the FMS computes flight plans and that their flight planners are using a compatible method.

“It’s really important to understand the information on the flight plan and how it integrates with the FMS,” he told BCA. “You need to know how your FMS determines magnetic track. For example, does it determine it using the magnetic variation at the outbound waypoint location or at the midpoint between two waypoints? Your computerized flight plan should use the same data; otherwise you can wind up with conflicting data between the FMS flight plan tracks and the computerized ones.""


Operations: Building an Oceanic Flight Plan | Business Aviation content from Aviation Week (http://aviationweek.com/business-aviation/building-oceanic-flight-plan)

[wiggy]

Well in the context of oceanic flight plans we get round the problem by simply not using magnetic info for track checking on oceanic sectors.

We select True track for our nav info whenever oceanic/for track checking prior to going oceanic, that gets checked against the Initial True track between waypoints that are printed on the flight plan.

We do also get mean magnetic track between waypoints on the flight plan but that is not used for oceanic track checking.

[eckhard]

We select True track for our nav info whenever oceanic/for track checking prior to going oceanic, that gets checked against the Initial True track between waypoints that are printed on the flight plan.


I thought that was the way we used to do it? Nowadays, don't we just check and re-check the entered waypoint lat/longs before reading them out and checking them again?

RLATS on the NAT and use of 1/2 degree waypoints have made the ITT 'track and distance' deemed to be not reliable enough.

That's what it says in my manual anyway!

[wiggy]

You're right, times have moved on...just making badly the point that if a track comparison is what you are after the the simple answer might be not to use mag. at all.

[JammedStab]

OK, so here is the question....is it really that vital that we know how the FMS "computes flight plans" and that our flight planners such as our dispatch "are using a compatible method" or not really. I have done the true track selection thing for checking bearings in the past but it seems to be frowned upon and is not done on the new airplane type. And I think I was the only one checking on the previous airplane type.

Is it possible that our experts in the article are exaggerating when they say "vital".

[JammedStab]

To be honest, I am not sure which method the Boeing FMC uses and I don't see anything in the manuals.

[Spooky 2]

I have asked that exact question from a Boeing Technical pilot and all I got was some goblet gook of answer say I really would not understand how the FMC works with regards to that question. WTF? At any rate in my past life we did all oceanic crossings in True which made everything much easier when working the flight plan x checking. I understand that is not the policy in effect today a the same airline.


The problem of course is the MagVar tables used in the flight planning engines are probably much more current than most aircraft. There are Boeings running around with the same MagVar tables that were installed when the airframe was first delivered 15 or more years ago.. I guess if your on that magenta line nothing could go wrong?

[JammedStab]

Good point. We had a temporary restriction for low vis approaches at a couple of northern airports a few years back, apparently based on an FAA notam although it was quickly rescinded. Based on older magvar tables for our aircraft.

[oceancrosser]

In the fleet I fly, old magvar tables popped up as an issue when we started flying RNP approaches. Turns out, as someone said above, that these had not been updated since the a/c were built.

[JammedStab]

From Bill Bulfer's FMC guide.....on Boeing aircraft.

"Magnetic course will normally agree with the flightplan within approximately two degrees. On longer legs in areas of large magnetic variation changes, the difference may be greater.

It should be noted that the magnetic(or true) course on most flight plans is determined at the mid-point of the leg, wheras the FMC magnetic(or true) course is measured at waypoint outbound. The difference in the point of measurement can account for a difference of several degrees in areas of large magnetic variation changes. If in doubt, compare true courses.

It has been suggested that the paper flight plan display courses measured at the same point as the FMC, but to do so would be contrary to the long-established procedures taught to all pilots."

I am not quite sure what is meant by the last paragraph.

[Sidestick_n_Rudder]

When I first started flying Oceanic flights, I have found it rather disconcerting the OFP course and FMS be significantly different. How to check one against the other to confirm you are flying in the correct direction?

I started digging into manuals, found nothing there. Then I trawled through an old copy of EASA ATPL Navigation handbook and I think I figured it out. FWIW, here's what I found:

OFP course is Rhumb Line, i.e. constant course between waypoints,
So that you can hold it using your compass. It is also TRUE referenced

FMS course is Great Circle course from your PPOS to the waypoint. It is Magnetic referenced.

The difference between these two is normally on order of several degrees.

To convert the OFP course into FMS course, you need to do the following:

1) calculate and apply Conversion Angle (0.5*diff in longitude*sine of mid latitude)

2) apply MagVar from the chart.

I started doing it on my ocean crossings (out of boredom, if nothing else) and normally my calculations and FMS match to within 1 degree.

[JammedStab]

Thanks for the info,

I suspect that there may be different methods of calculating the magnetic track between waypoints for different providers which is what was alluded to in the first post.

[foggygyro]

I would appreciate a practical answer to the following:
In a 777 at CRZ on a long-haul flight:
When a flight plan has a final waypoint that does not have any further destination and in LNAV, what happens?
I have been told several things...
If the hdg/trk switch is in hdg mode, the flight continues on the existing heading (as or as converted into a magnetic heading. The hdg (presumably the equivalent true heading) changes with time in accordance with the change in magnetic deviation along the actual flight path and the ground course is deviated from a no wind situation due to cross winds.
On the other hand, if the switch is in the trk mode, the flight continues on the existing true heading indefinitely (rhumb line) and the cross winds are compensated so that the rhumb line course is not deviated in the cross track direction.

Reading a thread from a number of years ago, I understood that it was not uncommon for a pilot to choose trk in order to determine the heading to a proposed new waypoint or other navigation planning purposes. In such an instance, does the pilot always return the switch to the hdg position.

I am interested in actual practice rather than a training manual answer, if they are different. My impression is that it is not possible to determine the position of the switch unless you are looking at it.
Thanks,...

[flapsupdown]

I would appreciate a practical answer to the following:
In a 777 at CRZ on a long-haul flight:
When a flight plan has a final waypoint that does not have any further destination and in LNAV, what happens?
I have been told several things...
If the hdg/trk switch is in hdg mode, the flight continues on the existing heading (as or as converted into a magnetic heading. The hdg (presumably the equivalent true heading) changes with time in accordance with the change in magnetic deviation along the actual flight path and the ground course is deviated from a no wind situation due to cross winds.
On the other hand, if the switch is in the trk mode, the flight continues on the existing true heading indefinitely (rhumb line) and the cross winds are compensated so that the rhumb line course is not deviated in the cross track direction.

Reading a thread from a number of years ago, I understood that it was not uncommon for a pilot to choose trk in order to determine the heading to a proposed new waypoint or other navigation planning purposes. In such an instance, does the pilot always return the switch to the hdg position.

I am interested in actual practice rather than a training manual answer, if they are different. My impression is that it is not possible to determine the position of the switch unless you are looking at it.
Thanks,...

I don't fully understand the second part of your question, but in the B777, when passing the last waypoint the lateral mode remains in LNAV and maintains present heading

[foggygyro]

Does the "heading" depend on the trk/hdg switch position? Does the heading depend on the Mag/true switch position?

[Airmann]

Having flown a crossing with an old school Captain I was taught the method for calculating the overall track change for a great circle route. I did a few calculations and worked out that for most of the latitudes that one would fly over the Atlantic for a 10 degree change in longitude your starting track will differ from your final track by around 8 degrees.

Checking FMS vs OFP headings (mag or true) differences were rarely more than one degree. And always were the initial track at the beginning of the leg, never the track at the midpoint or the end of the leg. In fact there are tables published stating the initial true tracks made good between any two waypoints over a 10 degree change, and it's left to the pilots to workout changes in track required to fly a great circle.

As for comparing FMS vs OFP we are told that the check must be done using true tracks not using magnetic presumably because of the differences in variation databases. In any case I have never seen a difference in true tracks of more than one degree and that's the tolerance limit.

I do have a hunch that the differences may be down to the fact that either the FMS or OFP actually calculates it's courses down to some decimal places and the method for rounding may be the reason for the difference in the tracks.

[foggygyro]

That was very helpful. I would guess that on an oceanic track route, a pilot might leave the switch in the True position.

You have been very kind in answering my questions. I might explain my motivation. I am an original member of the Independent Group (IG) that has been contributing to the continued search for MH370. Several of us have sophisticated path and fuel simulators that have been collaboratively developed. But there is yet no agreement after all this time on a specific navigation scenario. What I have been exploring recently is a particularly waypoint scenario which is interrupted by a selection of trk hold True. If known waypoints are used, the last waypoint ends in a discontinuity. I won't waste bandwidth here on all of the details unless there is interest.

One other question relates to the use of a path offset. The Inmarsat data, which forms the underlying basis for the study, is consistent with a offset from a programmed track. There is some uncertainty regarding how and when the offset was cancelled. I have seen statements that the offset is cancelled if a waypoint turn of 90deg is made, but other documents state 135deg. Alternatively, if a pilot were to cancel the offset, how close to the waypoint is this possible? Are there any other automatic situations for cancelling the offset?

If there is a better forum for discussing these technical points, please let me know.