Crossing the Pond and the FMC

[Guest iaflyer]

Hello everyone - I've got a question for the folks out there. I have made many Atlantic crossing using 767PIC (great isn't it?). Under the PIC menu, Realism and Carrier Options I have selected: IRU Will Drift (On) I understand the basics behind the IRUs, drift, and updating via DME/DME fixes. After I leave ground based navaids and get the "IRS NAV ONLY" message on the FMC, the FMC reverts to the IRS fix. Now, after I cross the Atlanic, the IRS navigation fix has drifted (as I imagine it does in real life). When I come up to the coast of Ireland, I reenter the range of the DME stations. The Radio fix comes alive, and the FMC takes this to be the better fix. Of course, since the IRUs drifted, the two fixes don't agree. No problem there - but if I am in LNAV mode, 767PIC immediately makes a large left or right turn (20 degrees) to get "back" on course. Usually at this point, the aircraft is around 1.5 miles left or right of course. So I guess my real question is - what does the actual aircraft do when it gets ground based update? I don't remember such a large turn in real life when we got close to land when I am a passenger. Thanks - Alex

[Guest ACA856_at_work3]

I think this question relates to Map Shift...and a good question it is. Does the A/P fly towards the new path when back in Radio range without warning (like the PIC does), or should the FMC simply notify/MSG saying that a map shift has occured and fly on HDG HOLD until a decision is made by the PF to intercept the new corrected path?Interesting subtlety I have wondered about as well.Rob.

[Peter Clark]

I would think that the aircraft probably just turns toward re-establishing LNAV. Since the turn is coordinated, you likely wouldn't notice it unless you were looking for it at the right time. I know that I've been pax in enough BOS->LAX/SFO and BOS-EGLL flights that obviously have to execute turns enroute and have been surprised to look up from a book or something to see that we were in a bank and the only reason I did notice the turn is because I saw it out the window..

[Guest gremel]

Or....would the aircraft switch to Hdg Hold mode if outside the 2.5 nautical mile course and give you the following FMC message: Not on intercept heading...LNAV is selected and the airplance is not within the capture criteria of the active leg, or the current heading will not intercept the active leg.....Indeed, a very good question..Regards,jack

[Guest HPSOV]

This is indeed known as a 'map-shift'. In the real aircraft when you come back into range of navaids and the FMC position is updated the map can appear to 'jump' sideways, so you are no longer on the LNAV track. The worst I have seen is around 1nm, but this is very rare. The aircraft will set up an intercept to regain the track, but generally uses very small angles of bank, so from a passengers point of view it wouldn't be noticeable. I doubt the intercept angle would ever exceed 10 degrees. In reality it would appear the update is blended in slowly sometimes, often you will be 0.3nm left of track, with around 4 or 5 degrees intercept angle, and this will remain for a minute or so. It seems the track keeps getting further away from you.IRS NAV only will appear if there has been no radio update for 15 mins in cruise (I think) or at 15000ft on descent if it has no recieved an update (only place I routinely see this is going into Darwin from the north, where the map can remain a mile or so out while turning final... always makes it fun).The FMC position will update from DME/DME, LOC/DME and VOR/DME (within 40nm) fixes. For aircraft fitted with GPS the GPS signal is taken as another form of radio update, with priority over all other forms.

[Guest ACA856_at_work3]

Great answer!Should there be a message appear on the FMC when radio range is re-entered and the Map Shift occurs? I see no message in the pic767 FMC, just the path shift and the turn executed to intercept the LNAV path.Thanks,Rob.

[Guest iaflyer]

I did think that perhaps the new position was blended in slowly, to avoid such a problem. But it sounds like a map shift is immediate. I guess a multi-million dollar airplane would be a little smoother about getting on course than a inexpensive PC sim... but 767PIC continues to amaze me. Really, it has spoiled me - I was flying the PSS A330 the other day, and it's just not the same. I have FS2004, but I rarely fly using it - no PIC767! (yet)Alex

[Guest HPSOV]

There is no message in the real aircraft just like in PIC to alert you too the recommencement of radio updating. The only way to see this is on Prog 1/2 (Although in PIC's Peagasus FMC this may be on the Nav Rad page?) Where it shows what station each of the VOR's is tuned too, above this is will display DME, VOR-DME etc to indicate what form of update is taking place.

[Guest MB327]

Great stuff Chris!How about that Rob, three years and we are still learning something new about every day.