View Full Version : How enter FMS/FMS initial position

18th May 2008, 14:28

We are trying to improve our SOPS. How do your SOPS require you to enter initial position on the FMC/FMS?

If your data base has gate info obviously you use it.

If your data base has only Airport, do you use this position or do you enter gate position information from your charts.

Do you have a procedure to check the consistency of the position you entered?



18th May 2008, 14:52
Load a GPS displayed position, after it has been checked to be in the "ballpark" from displayed data, eg, stand charts etc.

Mistakes can creep in loading data direct from a chart, into an FMS/INS.

Dan Winterland
18th May 2008, 16:23
Which type? which FMS? Which INS/IRS?

It all depends.

18th May 2008, 17:39
Hi Dan Winterland,

B737-500 CL. No GPS update, 2 IRS. Do you need more information?

and thanks, justlooking_tks!

18th May 2008, 17:53
Sufficient accuracy can be obtained from an ARP position entry. Any minute errors in platform alignment you generate will be insignificant, and DME/DME update will give you the correct position and a reference for FMC position. Bear in mind that initial position is used by the platform for coarse alignment and that fine-tuning takes out most platform alignment errors, with runway update (if you have it) taking out any starting pos errors. Only if you are going many miles into non-DME territory do you need to even think about gate position - and then screwing it up.:) If you are going to be reliant on extreme position accuracy then use gate position, and you are of course in the wrong aircraft - get a GPS based system:rolleyes:

I was once told that at least one major airline, operating in a DME rich environment, did 1 full align first flight and then left it in NAV all day.

Considering most of us normally finish with an ILS or radar there is no real issue. IF your route is over sparsely beaconed territory, you MAY benefit from a gate position - but make sure both of you check the entry. In many cases ARP is not THAT far from gate anyway.

Dream Land
18th May 2008, 17:59
Been on bus for many years and wouldn't recommend using any gate coordinates, I do a full align the first flight of the day and quick align at intermediate stations for non GPS aircraft and only quick aligns with GPS aircraft, we have some of each.

18th May 2008, 19:02
According to Airbus, we do a full alignment the first flight of the day using airport coordinates (after verification by looking at a chart). Then, throughout the day, we do fast alignments ONLY if residual ground speed is greater than 5 knots.

The exception is: When flying in areas where updates are not available (for example, over water) AND you are NOT GPS equipped, THEN you do a full alignment with gate coordinates, if available (again, verify, verify, verify).

It is not necessary, nor desirable, to do fast alignments after each landing (assuming the IRUs are performing satisfactorily)...regardless if the aircraft is or is not GPS equipped.

Let's assume you're flying a non-GPS-equipped aircraft. Now, you do the full alignment in the morning (first flight of the day). So, as per SOP, you use airport coordinates, which are probably just a slight bit off from your actual gate position. As the FMGCs get updates from ground-based stations, the FMGC position becomes 'refined'....that is, your FMGC position is probably fairly accurate by the time you make your first landing of the day.

So, now, contrary to Airbus SOP (assuming the residual ground speed is five knots or less.....which it probably is), well-intentioned pilots do fast alignments, again, using airport coordinates. Chances are, the new FMGC position (after this fast alignment) is more 'off' from actual position than before the fast alignment. So, this really isn't a big deal, since, with your subsequent takeoff, you get a runway update AND, after takeoff, the ground-based nav aids will, again, update and refine the FMGC position...just like before...just like it always does. So, what did you accomplish by doing the fast alignment? Nothing!!!! (Maybe you zero'd a residual 2 knot ground speed...big deal!)

The Airbus philosophy is: leave it alone as long as it's working properly. Again, the exception is for non-GPS aircraft that are about to fly a leg over an area where the gound-based position updates will not occur. THEN, you are to do a full alignment WITH gate coordinates (if available).

I fly with F/Os who think they're doing 'above and beyond the call of duty' by doing fast alignments each leg. Then, I show them, the FMCG position was closer to the actual aircraft position BEFORE the fast alignment...they made things worse...not better. (Again, this really is no big deal, since the thing updates with TOGA/FLEX...using runway coordinates...anyway.)

Every time you do any kind of alignment, you need to verify what's going into that FMGC. And, every time you do any kind of alignment, you're risking making an error.

Now, having said all of this...IF you follow Airbus SOP and verify the position of the aircraft vis-a'-vis the FMGC position (after any kind of alignment), then it's all only an issue for discussion anyway. Follow the SOPs and you can't go wrong....

Fly safe,


Swedish Steve
18th May 2008, 20:21
IF you follow Airbus SOP and verify the position of the aircraft vis-a'-vis the FMGC position (after any kind of alignment),

I wish everyone did this. I am an engineer. Three times in the last two years I have been called out to A320 aircraft because the crew could not align the ADIRUs. When I enter the flight deck the first thing I ask is "present position?".
Then the crew notice that they still have the present position in the FMC from the start of the last leg. No I don't know why but somehow the FMC has not recognised the end of the flight. So enter PPOS and the flashing align lights go out. Trouble is usually the crew have tried to do it themselves first by switching off the ADIRUs and starting again. But if the FMC PPOS is wrong you get the same result.

18th May 2008, 22:39
Thanks to all of you!


Dream Land
19th May 2008, 05:12
Pantload, thank you for your detailed explanation, I would like to further inquire your thoughts reference to the non GPS aircraft operated in Class 1 airspace, we regularly operate four sector days, you are advocating doing a full align for the first flight and doing nothing after that? I will try this method but suspect your theory about the FM position being more accurate than the ARP would be surprising. My world is operating with cadet button pushers that enjoy playing with the aircraft as much as possible, don't get me started on the VS knob, but with the amount of time between the turns I dissuade all FO's from trying to input the gate coordinates, anyone that knows the aircraft well knows that the potential for a large error is opened as soon as you start inputing Lat / Longs and that "High Accuracy" means nothing.

19th May 2008, 11:34
Sounds like I upset you and hit one of your 'hot buttons'. I'm sorry. Over the years, I've read many of your great contributions to this forum. So, what you contribute is valued.

The best response I can give is: Follow your company's SOP. That's really all I can say. I don't know your company's SOP. All I can tell you is what Airbus says. Your SOP may differ, in which case your company's procedures trump what Airbus says.

Fly safe,


Dream Land
19th May 2008, 13:25
Pantload, no, if that's how I sounded I apologize, I don't pretend to know the inner workings of the ADIRU's as much as I'd like to, it just happened to be a question that came up last week, one of my new era FO's who knows the SOP backward and forward, pulling down the compass bla bla bla did a wonderful cockpit setup in my absence and one of the first things I did was to look at the position monitor, all IR's had over a three mile error, I thought to myself that it was an error by the other pilot but then asked myself, what is the harm, as soon as the thrust levers are pushed forward everything will be fine, (15 year old A320 no GPS) operating in class one NAV, not over water. Any further comment appreciated.

19th May 2008, 13:54
Three mile error....no problem...that's on the individual IRUs. But, the actual FMGC positions should have been close to correct.

Again, if the ground speed is five knots or less....no problem.

In fact, the FMGCs look at individual IRU drift...account for their drift....as time goes on, the FMGCs actually 'learn' the IRUs and compensate.

I have to say...if we follow the checks and balances that are given us in the SOP, none of this is an issue. We check our position on the ground by looking at ground-based navaids (if possible)...(use raw data and compare it to what's in the FMGC when you stick the navaid in there)...look at the position of the aircraft on the ND, comparing it to the airport diagram...etc., etc. We also check the bearing and distance of the first fix...so, all is well when you push 'em up to Flex/TOGA (The airplane then 'knows' where it is EXACTLY...and you've verified where it's going.) (Looking at the bearing and distance of the first fix is different on GPS aircraft, since those figures are based on your actual position (at the gate)...vis-a'-vis the first fix...but that's OK.)

It's all good....


Dream Land
19th May 2008, 14:41
Great stuff, will start putting it to use!

19th May 2008, 16:44
was once told that at least one major airline, operating in a DME rich environment, did 1 full align first flight and then left it in NAV all day.

SaudiArabian, with their TriStar aircraft did so, only switching the IRU's off with a tech crew change.
Litton IRU's feeding Hamilton Sundstrand FMS/FMC's...with a moving map (12 inch diagonal) included.
Superbly accurate....circa 1977.

Litton recommended that the IRU's remain in NAV rather than switching off at each intermediate stop to enhance MTBF rates.

Did this work?

Yup, sure did.

Dream Land
20th May 2008, 03:07
Thank you. :ok:

Fright Level
20th May 2008, 08:38
The two Boeing (read Honeywell) types I've flown only use the initial position to help with alignment (in the non GPS days). They updated the position on the runway when you set T/O power, hence the need to add x00m to the position if you did an intersection departure.

I think you could be 10 miles out with the initial position and still not have problems as it would be corrected during alignment then updated on the departure runway threshold.

20th May 2008, 08:43
EDDNR - just a caution - it is only the FMS pos that is updated at TOGA, not the platform alignment, so a serious misalignment would not be sorted.

Fright Level
20th May 2008, 08:50
Thanks BOAC, back to the books :bored: I'll take a look in the Honeywell manual when I'm back home. If the platform wasn't aligned though, surely you wouldn't let the brake off until it was done as there would still be an align message somewhere? Also, I guess you only need to enter longitude accurately as it will gain latitude info from the earths rotation in the alignment time?

20th May 2008, 09:54
On the 737s, well, at least on the classic, you will get blinking ALIGN lights and a scratchpad message (VERIFY POSITION) if the position you enter is more than 4NM from the last known position on normally working IRUs. The IRUs also do an internal comparison test with sensed latitude and prompt with the scratchpad message ENTER IRS POSITION if the position you entered doesn't fit. Unfortunately the ALIGN lights blink as well if you do not enter any position on first align or if airplane movement was detected during alignment. If alignemt wasn't complete and you move the airplane you will get the flashing ALIGN lights, scratchpad message IRS MOTION and a full realing process will start automaticly.

Our SOPs require us to do at least a quick align on each turnaround and preferrably a full realign if time permits, which it usually does, min turnaround time is 30 minutes. Both on the classic (non GPS) and on the NG (all GPS equipped).

20th May 2008, 10:01
Absolutey, EDDNR. Only posted as I have heard people talk of 'taking out alignment errors' at TOGA and meaning platform errors. Incidentally, I may have misread your earlier post, but I don't think GPS takes a part in the actual alignment process in the 737, but I'm well out of date (used to lecture on IRS/INS fitted with toothed rubber bands, cogs and gyros:eek:) and would welcome input from a 'fundi' on this.

Maybe on the next extended slot delay I could try an alignment at the wrong longitude - I'm sure as you say it is only pos that would be wrong, and corrected at TOGA, but I'm not sure, for example, about heading? I'm sure that if the system allowed, one could align with no input position (as long as FDP allowed:)). It is a clever bit of kit.

20th May 2008, 13:39
If the initial numbers are off by more that a few degrees, particularly latitude, many INS/IRS can detect that on alignment, as the platform does not tilt with earth's rotation as the system expects it to with the wrong numbers entered.

I don't know how far the numbers have to be off for this to work though.