IRS-alignment: Warning lights?

[Guest Susan27]

Hi guys,I loaded the "Cold & dark" scenario from the FSX-PMDG panel-state menu:After I switched battery on and started the APU I would like to align the IRS. Therefore I switched all 3 IRS-switches from "off" to "Nav" position (before I changed the "IRS-alignment time" in the PMDG-option menu to 15 sec.):The result: The three orange IRS-warning lights are constantly flashing...I really dont know what I did wrong here...Could someone please tell me what to do so that those warning lights will go out/stop to flash...? Here is the situation (overhead):Thanks for your kind help! :)

[3-2-1-Now 126]

Hi,>> The three orange IRS-warning lights are constantly flashing...I >> really dont know what I did wrong here...You didn't do anything wrong. :)What you need to do is go to the INIT page of the FMS, and tell the system where it is. The lights flash once the units are warmed up and ready for alignment. It means they've no valid position.Once you've given it a valid position, you need to wait the alignment time before the units are fully aligned, and you can move the aircraft/go fly. :)Best regards,Robin.

[xkoote 57]

sorry, eating and typing. if you have gns option enabled, just hit align irs since the gns is already sending pos info to the irs'sxander

[Guest Susan27]

Thanks for the quick response, that worked...Robin! :)Followup:Well, when I am looking at the Init page of the FMC there is allready a valid position for Lon/Lat at LSK 3L & 3R. At LSK 4R there is "Initialize IRS":When I am pressing LSK 4R only an error message appears at the CDU: "Not allowed". After entering an origin and destination Airport (From/To) at LSK 1R and pressing now "Initialize IRS" the Warning lights go out indeed...So my question is: Why does the IRS-alignment process needs a valid origin and destination Airport (LSK 1R) in order to align properly (lights go out)? I thought during the IRS-alignment phase the system "calculates" the *current* (present) position of the airplane...no matter where I want to fly to...For that purpose the "From/To" information at the Init page (LSK 1R) is NOT needed...,or?

[3-2-1-Now 126]

Hi,It should have worked.....? Did you check the actual position equaled the GNS position first (it should, but worth checking just in case)?Just pressing INITIALIZE IRS* should be sufficient. You could also try cycling the IRS and after switching all the units on, going directly to INITIALIZE IRS* before the lights start flashing?Best regards,Robin.

[Guest rellehenk]

Flashes amber if:

[Mace 1,896]

>I thought during the IRS-alignment phase the>system "calculates" the *current* (present) position of the>airplane...no matter where I want to fly to...I'm not sure if the MD-11's inertial navigation system is the same as traditional INS's, but my understanding of inertial navigation systems is that they do not know where they are like a GPS system does. For an INS you have to tell it where you are...which is why you can't be moving to set it up.Then a complex series of gyros and accelerometers tell the INS where you've moved relative to your starting point.Now as to whether or not the MD-11's INS/IRS is the same, I do not know. :-boom Sounds the same though--since it has the gyro alignment period too.RhettFS box: E8500 (@ 3.80 ghz), AC Freezer 7 Pro, ASUS P5E3 Premium, BFG 8800GTX 756 (nVidia 169 WHQL), 4gb DDR3 1600 Patriot Cas7 7-7-7-20 (2T), PC Power 750, WD 150gb 10000rpm Raptor, Seagate 500gb, Silverstone TJ09 case, Vista Ultimate 64ASX Client: AMD 3700+ (@ 2.6 ghz), 7800GT

[3-2-1-Now 126]

Hi,Same principle. Does the MD-11 use RLGs?Best regards,Robin.

[Guest David Ketchum]

Yes it does. I don't think anything built within the last 30 years still uses the old mechanical-type gyros. :)

[Guest David Ketchum]

The most modern laser-driven INS/IRS's still have to be aligned in the same fashion as has always been the case with the old mechanical systems. The INS first needs to know where it is on the surface of a known sphere (the earth), then needs to 'align' by actually sensing the motion of the earth's rotation by use of super-sensitive accelerometers. Once the INS knows where it is and analyzes the angular momentum of the earth's rotation at a known geographical point (the lat/long you enter during alignment), then the system can very accurately calculate true north. This is why you never have to enter a heading during alignment. After wards, the INS/IRS measures deviation from this initial point with accelerometer readings to calculate current position as you mention.

[Guest Susan27]

Thanks guys, its working now with the IRS...! :)Follow-Up:As I learned the IRS needs the present position in order to align the gyros...So why they dont just use a GPS which tells the IRS the present position at once...so the crew doesnt have to enter the present gate position (with the possibility of faulty inputs) and as a "sideeffect" the airplane would have another navigational aid (for emergency use only...)?Thanks again! :)

[basa 0]

>>So why they dont just use a GPS which tells the IRS the>present position at once...so the crew doesnt have to enter>the present gate position (with the possibility of faulty>inputs) and as a "sideeffect" the airplane would have another>navigational aid (for emergency use only...)?>Hi Susan, nice to have a girl in this forum. There was a topic, some weeks ago, wonderig the percentage of female presence in FS world and in the RW. Well, in the PMDG 744 you are twice prompted: "enter IRS position", and, (following the manual), you normally take the GPS position as a good one to be entered. That is the way I always do it.Happy flights.

[Guest Susan27]

Thanks for your kind words...after some time here I feel like E.T. lol...!A nice sunday for all here...:)

[Guest David Ketchum]

Susan,Having the flight crew enter the initial coordinates for alignment is just a safety measure. The GPS could easily be made to interface with the IRS system and uplink the starting lat/lon automatically, but safety dictates the pilot's cross-checking the current GPS position display with a known real-world position (i.e. the departure gate). With all GPS equipped aircraft I'm aware of, the crew simply have to verify the GPS coordinates and uplink such directly to the INS without having to manually type in the LAT/LONG, so faulty inputs on the human side are thus factored out. The FMS is designed to use GPS navigation by default as it's the most accurate, but should all three GPS units fail in flight the IRS would indeed automatically provide navigational backup. All the bases are covered. :)GPS is very accurate and reliable, but nothing is completely infallible and stuff can happen. :) Having the pilot's double-check the integrity of this process keeps it safe flying.

[basa 0]

>>GPS is very accurate and reliable, but nothing is completely>infallible and stuff can happen. :) Having the pilot's>double-check the integrity of this process keeps it safe>flying. AFAIK, the GPS satellite network is finally controlled by some U.S. Defence Department, and given an emergency (security issue) situation, they could simply disconnect the system.That is why GPS-only NAV systems are not certified for commercial aviation.I think the European Union was building an independent satellite system called Galileo.