Switching to a true north reference outside polar regions

[sacul12]

I understand the reason for switching to a true north reference (my A/C actually does it automatically) when flying inside the polar regions, but why so many pilots switch their ADIRU reference to true north when far away from 82°N on the North Atlantic Track System, is it recomended on the NATS only? or is there any other explanation or recommendation? please enlighten me...

[Spooky 2]

Don't know what aircraft you are flying but lets pretend it's a B777. The FMC data base recognizes when your above N82 and also recognizes when your in the Keyhole region of Northern Canada, better known as the AMU or NCA and in turn auto switches there as well. Remember that if your north of 82 degrees and your not in LNAV, you must manually select True for the sytem logic to work correctly. Now for switching earlier than crossing N82, recall that this is not a Boeing recomended procedure, however you can operate in True in Class ll airspace without any real issues as long as you have True data on your flight plan for checking your track changes as they occur. True is the purest form of navigation and all those magvar tables that get factored into the building of the flight plan or are used to generate FMC nav data do nothing but mess up what was once True nav data. Don't know if I answered your question and I would certainly not go against your company policy so take what I have written with a grain of salt and enjoy.;)

[wiggy]

One explanation.. it's in our Ops Manual...because:

Our paper flight plans show Initial True Track between waypoint pairs but only gives Mean Magnetic Track between the two. Sticking the Hdg ref into True on the NATs means immediately after waypoint crossing you have an a indication that you have turned onto the correct track to the next waypoint...something Spooky 2 mentioned. What does your Ops Manual say?

[sacul12]

My company does not mention anything about switching to TRU N once you are on the tracks, only policy is to switch on the ground once the FMS is loaded with the track coordinates and verify initial true track and distance with the OFP and the plotting chart...but it does make sense and it is easier to check in flight the True Track change with that of the OFP which in our case would be initial true track...oh by the way it is a B777
Thanks!

[Spooky 2]

Your right as there is no requirement to operate in True while in the tracks. The company I flew for operated in True at anytime we were in Class ll airspace...regardless of latitude. Since I have left I understand that they switched back to Mag in Class ll airspace. Go figure. I agree that True is a more honest navigation reference but policy is policy and it's hatf to convince many of the younger crowd what works best. Obviously switch back to Mag after leaving Class ll airspace in a signifcant event after a crossing and coasting in to the radar enviornment.

[eckhard]

sacul12,

I agree that it's a good idea to switch to TRUE on the ground, to check that the ITT in the FMS matches the OFP.

In the case of a change to the flight plan after take-off, you would have to enter the new waypoints into the FMS and cross-check the ITT and distance against a reference publication, such as a table of tracks and distances.

These new waypoints, ITTs and distances would then be written down, perhaps on the OFP, 'overwriting' the original plan.

Setting the HDG REF to TRUE enables an extra check that the above process has been correctly accomplished. As each waypoint is crossed, the actual track and distance to the next waypoint can be checked against the revised plan.

As the OTS Track Message says, "85% of gross navigational errors occur after a change in routing" (or words to that effect).

To keep the SOP simple, it may be a good idea to always select TRUE when navigating in NAT airspace, even if not required by regulations.

Just my 2-cents' worth.

[Spooky 2]

Eckhard,

Agree with you comments although I would caution anyone regardless of flight plan construction, to strickly observe the procedures set don in your ops manuals. When one crewmember is off on his own doing his own nav techniques is when one is most likely to encounter an error that leads to something bigger. :eek: Having said that, I also prefere True over Mag for all the reasons stated.

Sacul12,

Not a big deal as it relates to this dicussion but the FAA defines the "Polar Region" as anything above N78. There is another undefined Area of Magnetic Unreliability, (AMU) in Russian airspace and crews should be aware that when operating in these regions, True headings are applicable and shown as such on the Jepp charts. These areas are in Maggadan airspace as I recall.

Google Advisory Circular AC120-42B, Chapt. 6 along with MNPS manual Edition 8, for more information on both polar and MNPS flight operations. Good stuff if not a little dry.:ok:

[FE Hoppy]

I don't know why we don't dump mag altogether these days. It's pretty irrelevant isn't it? How many modern aircraft have mag compasses?

[Spooky 2]

Well all the ones I have flown have a mag whiskey compass installed. The airline I use to fly for always included a total loss of nav standby flight plan that used the whiskey compass for the entire crossing. Never tried it but others that I know said it worked amazingly well....considering.:}

I do agree that True is simply a better nav solution for most oceanic flight plans IMHO.

[sacul12]

Thx Spooky & Eckard, many things are coming together now... and yes, just good judgement an airmaship to switch to TRU inside class II navigation airspace :ok:

Sacul12

[Spooky 2]

I would stick with what ever your company procedures as either way will work and maintaining company prescribed procedures will work to everyone's benefit in the end.