Fmc Position Udpate

[Popolama]

hi all
last time i had a flight to Paris (LFPO) ,we arrived a little early than planned,so we had to wait around 90 mins for the slot time they gave us.
anyways,when we entered the rwy for t/o i looked at the EHSI and found an error between the plane position and the route drawn. actually the A/C was off-track to the right.
i know that there s a prompt in the t/o page that lets you udpate the position,or simply by pressing the TOGA swtiches,that's what i thought it'd happen but it didnt ! the A/c was still off track when airborne and i had a pain in the ass to get the a/c flying the route because of the nose abatement procedure in Orly for rwy 24 (fly the ils 06 backcourse until 4 dme) around 2 miles we had an FMC fail msg on the CDU and we had to do the whole SID in raw data.
my question is : why the TOGA sw didn t correct for the pos error ? what was its cause?

PS: i think because of the slot time we had the IRS were aligned around 45 mins before t/o ...could it be the reason for the postion error ?

btw it was a smith U6.0 (737)

[SWA GUY]

The predeparture time of alignment would not affect the IRS. You can align it much further than 45 minutes and not have a problem.

The IRS will only update on the TOGA cue if the system is configured for that operation. Most are set up that way, it may be a fault in the system.

You may have entered the wrong runway in the CDU...sometimes happens.

Good luck!

[alexban]

I've had the same situation in BUD ,just before takeoff I've seen a mapshift of about 40NM, (the route and airport had vanished from the screen) ,I did a position update (no change),and after takeoff,when DME updating occured, the position was corected.The IRS now showed a shift of ~40nm.
One possible cause was wrong initialization coordinates,but I've had no allerting message,and the map display was correct until shortly before take off .I thought of this,becouse after inflight alignment,the IRS deviation was about 40 nm,thus a initialization error.I don't know why no position update occured on rwy.
:ok:

[timzsta]

Is it possible in both cases, that incorrect longitude was entered into the IRS. The IRS will detect incorrect latitude insertion as it detects earth rotation and if you put in the wrong latitude the ER measured isnt correct for your position and it wont align. However if you enter the wrong longitude the IRS has no way of detecting this.

My problem now is that I am only an ATPL student. My understanding is that whilst on the ground on the 737 the FMC is getting its position from the IRS. But once you get airborne the FMC updates its position by taking DME readings. Hence in both cases you kind of jumped back on track, whilst the IRS showed you to be someway off track. Given Paris proximity to Greenwhich meridian, IMHO, in the first case there may have been E/W confusion.

Thoughts?

[Md-driver]

When you enter your init pos. and use airfield identification the FMC will use airport refence point as its coordinates. On large airfields like LFPG you can be quite far away from this point. As you start to taxi the IRS´s will calculate your position starting from the airport refence point (ARP) As you line up you will most likely see a map error.
As to why you did not get a runway update I don´t know.
Our 73´s are equiped with GPS´s so we do not get an error when taxying and we do not have to worry about position errors when doing intersection take-offs, which otherwise ( if uncorrected during runway update) would invalidate your RNP to perhaps below required values for a PRNAV departure.