IRS Problem On long haul???

[Guest]

Hey Guys, I just landed in London from San Francisco on my first Long Haul flight in PIC. Now my problem is somewhere in the route I got a message saying "IRS Nav Only" in the FMC. I got it about twice and im not sure what this means, the aircraft still navigated properly and I got to the runway but I dont know why its giving me this message. Can anyone tell me what this is???Thanks, Tom

[Guest ilh]

It means you were not getting any VOR/DME updates so you were running on IRS only. The IRS position will drift over time so this is warning you that your position is less reliable. Once you are back in range of land VOR/DME stations, your position should become more reliable again.Lee Hetherington (KBOS)

[Guest WorkingStiff]

The IRS Nav Only message is generated when the NAV1 receiver does not obtain a radio signal for 12 minutes or longer. It means that the aircraft is now navigating using an average of the three IRS positions.Please see page 10 of the 767 PIC FMC manual.

[Guest KenG]

To build on what Lee said, once you are back in contact with VORs and off IRS-Nav (you won't be told your off IRS-Nav and back on normal radio navigation), you may see a "map shift". That is, you will all of a sudden find yourself a mile or two off course. Again, due to less reliable navigation by IRS. That is because once your NAV radio's pick up a VOR signal again, the FMC will triangulate your position based on those radios and compare it to what the IRU's are telling it. As radio navigation is more reliable (theoretically) than IRS-based navigation (they drift over time), the FMC will adjust the position of the plane on the EHSI based on the newly identified position by the NAVs. The A/P will then get you back on course.All of this is described to some degree in the manual...I think.

[Guest WorkingStiff]

Hi Ken,Nice to see you participating here again.

[Guest MB327]

I can hardly believe you only got the message twice. What nav data are you running? I want it!

[marcom 139]

Hi Mike!I think you only get the IRS Nav only message once after 12 min's of IRS navigation. A quick glance at the POS page will tell you what source the FMS is using for its navigation. If you are flying USA->Europe, you should get the IRS Nav only message twice:Once when you are enroute to BANCS/COLOR/RAFIN, you won't be near any VOR's on that leg. This is where you get the first message.Then as you near the last true waypoint before going transatlantic, you'll have a short time where you can recieve Torba VOR (YYT). That will update the FMS position and 12 mins later, you'll recieve the IRS Nav only message for the second time.Mark

[Guest]

oops - sorry Mike - I forget to send you my Navaid.txt file ...will do so tonight.Rob.

[Guest MB327]

Thanks Rob.The odd thing here is that we should be able to find a Navaid.txt file newer than last summer (the one you are using) that would consistently locate radio referrences. As you know, I am using the files released with the 2002 Pic Patch (Jason's.) I am scratching my head as to why I have such spotty coverage over the continental US as to evoke the IRS messages every couple hundred miles.

[Guest MB327]

Mark,Thanks for the breakdown on the IRS Nav message. I could not fly that route, however, without several additional appearances of the IRS message. See my remarks under Rob's reply below.

[Peter Clark 21]

Greetings.May I ask if you're leaving your EHSI selectors set to MAP mode and 320NM range? For some reason my autotuner seems to be intermittant (at best) (and has been since I installed the aircraft, 2K or 2K2, whichever navdata I'm using) at MAP/320NM setting, but if I drop it back down to 160NM I get autotune fine (and thus no IRS NAV ONLY messages while flying over CONUS)...So, I leave it at 160 and switch to 320 when I know I'm out of range, or want to see a little more over the horizon.Hey, every aircraft has its quirks, I guess mine has a tempromental autotune :)Luck!P

[Guest MB327]

I have never tried to tie the IRS Nav message appearance to the range on the EHSI MAP. This is an interesting suggestion though. I use the MAP view exclusively on 99.9% of my flights. On nearly all my flights, the range starts out at 10 on departure and is stepped up to 320 at some point and then steps back to 10 on final.For my next few flights, I will keep the range within 160 and see what happens. Hmmm!