IRS Alignment

[Guest adesmith]

B747-400 QOTS.Having powered up the plane (battery, standby power, EXT or APU power) I swithch the IRS selectors from OFF to Nav and then back to ALIGN. Setting is for this to be realistic (circa ten minutes).NOW, once the countdown timers reach about 2 I get an FMC message telling me to enter IRS position. If I do this immediately and then change IRS selectors to NAV once countdown is complete then the process starts afresh with 7+ to align.I have tried waiting until the countdown timers reach 0 and dissappear then EITHER enter IRS position BEFORE IRS to NAV and also the reverse procedure (IRS to NAV then enter Position in FMC.There appears to be no consistency as to which one works regularly, sometimes one will work sometimnes it appears not to do so.I accept that maybe at 50 I am getting on a bit and have not recalled the PRECISE sequence of keystrokes, but could someone please confirm to me the correct sequence for this procedure.Other than this prob I can get from cold and dark to start up in about 15 minutes.Thanks for assistanceAdrian Smith

[Guest dt1951]

There is no need to place the selector from NAV to ALIGN. Just leave the selector in the NAV position. It will work just fine.

[Guest adesmith]

Thanks, so it works like PS1 did in that respect then?

[Guest jgreth123]

The 'ALIGN' part is actually a quick-alignment. When doing a short turn-around, you use this function of the IRS just to fine-tune the alignment. The ALIGN part takes about 30 seconds or so. It will only function that way when the IRS has been initially aligned.The proper way is to turn them from OFF to NAV. Enter the IRS position into the FMC. If the position from the last-known IRS position is off by a set amount, it will ask you to confirm it. Instead of waiting for the 2 minute mark, put in the initial IRS position, then hit CLR then put in the IRS position again on top of it. That appeases the IRS position confirmation gods.If you just pulled into a gate and shut down ( but didn't totally turn all power off ), you can use the quick-ALIGN mode. Just turn the knobs to ALIGN and re-enter the new IRS position and switch them back to NAV.I hope this helps.

[Qavion 19]

"NOW, once the countdown timers reach about 2 I get an FMC message telling me to enter IRS position. If I do this immediately and then change IRS selectors to NAV once countdown is complete then the process starts afresh with 7+ to align."Sounds like something is broken here, but as the other gentleman said, normally, you just switch your IRS selectors from OFF,straight to NAV. Only if you are departing from high latitudes do you need to select ALIGN (and leave your selectors there for a minimum of 17 minutes to allow the IRS's to better figure out where there are). If the countdown is restarting, then there is something wrong (I'm assuming the aircraft was stationary at the time?.... If the aircraft moves during alignment, the countdown will start again).Hope this helpsCheers.Q>

[Check_Airman 3]

The fault is with the PMDG norm proc. It states NAv then ALIGN then NAV.Paul

[Guest andyman]

Yes, it does state that.This is incorrect.

[Guest D17S]

Probably not a lot of help, but in the airplanes I've been dealing with for way too many years (hey, you're not getting older, your getting better!), the method is to use either Align or Nav. If IRU's MSU is set to align, once the countdown is complete the PP (present position) will be entered into a holding memory. When the airplane is ready to go, the operator selects Nav. This action will load the previously calculated PP. The thinking here is that drift will occur anytime IRUs are actually navigating (in NAV). In align, the PP calculation is completed, but not engaged. Selecting NAV simply loads the previously calculated (aligned) PP. If Nav is used, the IRU goes directly into Nav mode once the aligmnent is completed. I've seen 'em use either method. The 747-400's "Nav > Align >> countdown >? hold, !< enter, ~ clr >< re-enter/overrite > Nav" drill doesn't appear to provide this function, but I have my PMDG model set on a 10 second align. All I really know is that it doesn't work like what I'm used to in real airplanes. So far with the sim, I generally have bigger fish to fry right then and I just beat on the goofy thing until it till it takes my PP.But it sure would like to get the logical sequence behind how this alignment procedure is working. In real life, the Align/Nav switchology and sequencing makes sense. Here, I can't figure it out. "This to here and that to there." That's OK . . . but why?

[Qavion 19]

"But it sure would like to get the logical sequence behind how this alignment procedure is working. In real life, the Align/Nav switchology and sequencing makes sense. Here, I can't figure it out. "This to here and that to there." That's OK . . . but why?"The ALIGN mode is only required for departures from high latitudes and for quick realignments. For all other operations, OFF to NAV is the standard. The NAV position has an inbuilt 10 minute timer which allows the internals of the IRU to transition from the alignment mode to the navigation mode. It is a fixed 10 minutes (from the moment the selector knob leaves OFF) and doesn't take into account the fact that the IRS's may not be very well aligned (such as in the case in high latitude departures).OFF to NAV mode (directly) is simply more convenient than OFF, then ALIGN, then NAV.I'm told that on the 744 (but not the Airbus), it is better to enter the PPOS as soon as possible. It speeds up the initial alignment process (allowing a more stable platform to be achieved in the fixed 10 minute alignment period).Hope this clarifies things?Cheers.Q>

[Guest D17S]

It's so simple when it's clearly put. Thanks.

[Check_Airman 3]

"doesn't take into account the fact that the IRS's may not be very well aligned (such as in the case in high latitude departures)."What do you mean by "very well aligned"? I thought that once the correct position is entered, that's all it needs to work properly?Thanks as usual for your insight.Paul

[Qavion 19]

"What do you mean by "very well aligned"? I thought that once the correct position is entered, that's all it needs to work properly?"no, life would be much simpler if this were true :) Laser gyro systems are still a lot like the old spinning gyros. They need to figure out which way is up... and down, figure out where True North is, etc. The more time spent in align mode, the better the IRU can navigate in NAV mode.Once the IRU's are in NAV mode and navigating (after 10 minutes), the alignment basically starts degrading. It's not a good idea to leave your IRS's in Nav mode for days at a time. Please note: The IRU's can be quickly realigned at the next airport, but not all errors are removed. At some point, the IRU's will have to be fully aligned... NAV to OFF, wait 30 seconds or so, then back on again for another 10 minute alignment (with the aircraft not moving). Please note that nav radios such as VOR, DME and GPS have nothing to do with IRU alignment. They only affect the FMC's calculation of where the aircraft is. There have been 744 pilots who thought that the IRS's could be realigned with the aircraft moving, simply because the aircraft was fitted with GPS. Costly mistake. They have had to dump fuel and return to the departure airport. GPS's currently only give you position information, they don't tell you if your plane is upside down or not (like IRU's do).I'm not sure I understand the previous comments about GPS providing stabilisation/attitude information at some point in the future. The resolution of the GPS system would surely have to be in millimeters to provide something like this and there would have to be antennae on various points on the fuselage to be able to compute multiple position points to then calculate attitude. To rely on an external source of reference (GPS satellites) for attitude information would be ludicrous (IMHO).Hope this makes sense.Cheers.Q>

[Check_Airman 3]

"There have been 744 pilots who thought that the IRS's could be realigned with the aircraft moving, simply because the aircraft was fitted with GPS."LOLOLOL Are you serious, or just pulling my leg?

[Qavion 19]

"LOLOLOL Are you serious, or just pulling my leg?"Unfortunately, this has happened.One incident... Just prior to dusk. Small heading error developed between Captain's and F/O's instruments during taxy. Quick realignment carried out on threshold prior to takeoff. NAV-ALIGN-NAV.... Thust levers advanced before attitude completely restored. Loss of all 3 IRS's as a result... but takeoff continued. Pilots anxiously waiting for IRS's to magically realign. Pilots climbing using Standby Instrumentation. Eventually they figured out that it just wasn't going to happen. Fuel dumped, aircraft flown with Stby Instruments and visually (sun sets just as aircraft touches down).I think the crew eventually figured out that they had to put their IRS selectors to ATT to get back basic attitude information, but it wasn't clear if they were smart enough to enter a HDG via the CMC to give themselves ND VOR/ILS displays. Full alignment carried out on the ground, aircraft refuelled and continued on its way. Flight crew, I'm told, were relieved of duty at their destination by chief pilot and had to fly back home as passengers :(As someone said recently "Sorry to disillusion you", but there are pilots, engineers, doctors, etc... out there like that... but don't fool yourself into thinking that you'all know more than they do about flying 744's ;)Cheers.Q>

[Check_Airman 3]

Wow, I guess they didn't pay attention to that part of the AOM. Do you know how much experience each pilot had on type? I'd like to learn more about this incident. Or maybe you could point me to a website that would have some more info.ThanxPaul