IRS Initial Position
 

I searched for identical posts and didn't find any. Apologies if this has been covered elsewhere:

As some of you have indicated, there are companies that want you to put the airport in the IRS Initial Position Page--as opposed to entering the actual Gate lat/long coordinates--to minimize the possibility of a "typo." Since many FMCs update the aircraft's position to the beginning of the departure runway when takeoff thrust is set, entering the airport position might seem to be sufficient.

But now, let's take a case where we depart KDFW (Dallas, Texas, USA) and do an Atlantic crossing. The airport reference point may be 2 miles from your gate (KDFW is huge). If we are flying without radio updating (over the North Atlantic) on a non-GPS aircraft, when we see the "IRS Nav Only" message, will the airplane "jump" to being 2 miles off course--since that's what you "told it" by initializing it to the airport rather than the gate? This is my question.

In "75/767 Mysteries Revealed" by Tom Gooch--a great guide to the airplanes--he mentions an "error vector." That is, while still over land (and using radio updating), the FMC (not the IRS's) corrects the airplane's position. The FMC looks at the initial position, but corrects the 2 mile error at KDFW by referencing VOR/DME's along the route. I'm wondering if this Error Vector stays with the airplane during the IRS Nav Only portion of the flight? Then, the initial 2 mile error wouldn't seem to materialize. Without this error vector, I am of the humble opinion that the airplane will be 2 miles off course in the absence of radio updating--since you initialized the IRSs with a position that was "off" by 2 miles.

Without radio updating, the FMC position will gradually 'blend' towards a position referenced to its alignment point, since up to then it has constantly been 'corrected' by updating. No sudden 'jump'. IE in Tom's language, the error vector will slowly reduce to zero.

If an initial position offset is able to be corrected via ground-based navaids, it would be a very strange architecture that arbitrarily throws away that correction just because there are no more navaids within range.

76mike, I don't know of any companies that put in the gate coordinates anymore, we don't even have that information. It's an amazing coincidence that I actually flew out of DFW today, departed of rwy 35L en route to MSP, but I digress.

I don't think what you're asking is a big issue in 2013. How many non-GPS aircraft do you reckon are crossing the Atlantic nowadays, especially at the air carrier level? And yes, as Dave pointed out, the 2-mile error would have been done and gone by the time you made it to TUL, so I don't think it would magically reappear over water.

Hi 76mike,

Our 320 manual says "If a high navigation performance is desired, (i.e. for long-range flights without GPS and without radio navigation updates, or if low RNP operation is expected), the crew should adjust the airport reference coordinates to the gate coordinates, provided that this data is published or available on board. In this case, the flight crew should use the slew keys successively for Latitude and Longitude, instead of inserting the coordinates on the scratchpad, (in order to avoid errors)."

Only our FM position is updated on take off by a "TO BIAS". The IRS positions remain unchanged. The TO BIAS is continuously updated by radio.
I think your "IRS NAV Only" message means you are not receiving radio position updates and the FMS will be using the last "Updated BIAS" it received + MIXED IRS.
If you then lost FMS (Back Up NAV) you would only be able to see your onside IRS POS.

http://www.blackholes.org.uk/PP/IRSPos.jpg

When we had INS only, we could update the displayed position over a fix. It remembered that position bias until we either updated it again or "Flushed the Fix" to remove it. PFM

@ Flyboyike,
It will have been done and gone whilst on the runway during take off.

@BOAC
Very strange indeed.
Which aircraft and do you have a reference please?

Flyboyike------not putting in gate co ordinaries any more??? We do and always have done since the 70's.....

We are talking about the actual IRS/INS aren't we?? The IRS/INS to my knowledge isn't updated on takeoff, only the FMC/FMGEC position. I would have thought you would still want the basic IRS/INS to be aligned with the most accurate position pre flight should they be relied apon for Nav in a redundancy situation later on.......

Info on gate Lat/Lon is readily available on Jepp or Navtech or other charts. At most Airports I fly from its displayed on the gate number sign 50' in front of your cockpit window!!

We don't, and I've never done it with two airlines now. Mind you, there is a provision on the MCDU for that, but, as I said, we don't even have that info.

Not even on the airfield charts?

Only for a select few airports, but I've never entered it, nor seen anyone enter it.

Do you mean that the stand coordinates aren't on the ground/airfield chart for every airport you operate from?

Precisely so, at least not on the charts issued to us.

Stand coordinates aren't usually provided on the jepps for most US airports. I suspect that it has to do with the type of traffic though. My informal survey suggests that only those with international traffic publish gate coordinates on the jepps. At large airports like JFK and Miami, not all gates have the coordinates in front of the plane. Again, I'm guessing it has to do with the type of traffic usually serving that gate.

More and more just a GPS update nowadays.

Yes, but what does it have to do with stand coordinates in the charting material?

We use GPS/GPS updating for around 15 years now and still had stand coordinates in every charting material we used, be it jepp, EAG (i believe its now navtech) or nowadays LIDO.

That said, we don't use them normally, just the present GPS position is accurate enough for initialization, in fact it is usually more precise than the official stand coordinates. Not an option for non-GPS airplanes though, or flying with GPS inop, in that case the stand coordinates have to be used.

And for the ones that don't have GPS?

Last shut-down stored position.

As long as the latitude input is accurate for gyrocompassing, the Kalman filter will sort things out once airborne.

Bill Buffer's B737 FMC User Guide advises against using LAST POS for IRS initialization for aircraft without GPS, since it containts accumulated drift error from previous flight.

I know it's hard to believe, but there are still aircraft flying without GPS in not so navaid-saturated environment,

They have my condolences.

One can go back to basics. Pull out your Jepp chart that shows the ramp area and measure off the lat/long coordinates(maybe compare them with the aerodrome reference point to make sure they make sense) and then input those coordinates. Other chart types, I don't know if they show the lat/long coordinates in a similar manner.

Yeah, I know.