IRS position is always different than the FMC position at shutdowm of the IRS due to the drift rate of the IRS system, FMC position is updated during the flight with the GPS system. if that is not available than the FMC position is updated from VOR-DME tuning. The IRS system calculates its position during the flight of the Airplane movements. If there is a really good ADIRU with a minimum drift rate than the position IRS position is the almost the same as the FMC position. But I never seen it. FMC computer and ADIRU are not linked with each other during the flight to communicate there positions. IRS system is connected to the FMC computer to show it's position on the POS INIT page 2. If you look at that page you see to position's of the IRS and FMC are different to each other.

DavidzI couldn't find any parking charts for EGKK. I did find the following on the general chart, but there is no way this can be typed on the Scratch Pad. No way to enter the degree and seconds marks. 51o 8ii 9N 0o 11ii 4W.

Go to the NATS AIS website: http://www.nats-uk.ead-it.com/public/index.php.htmlClick on IAIP at the topAerodrome Index - SpecificLondon Gatwick EGKKAircraft Ground Movement/Parking/Docking Stand CoordinatesEnter coordinates in the same format you would use for latlongs in the legs page: (Stand 1) N5109.2W00009.8 (you need to convert the seconds to decimal minutes with the UK AIP charts) There is no need to input degree/minute/second signs.

Wow!!You people are great, i ca't believe how much effort you are expending to help me with this problem. I really do appreciate it. There is enough good information in your comments back and forth to each other, to keep me busy for hours. Hopefully it will gradually get into my head and i will be able to resolve the problem. Thanks again and best regards. Ed McBain

Hi everyone I just want all of you to know. I have now loaded and successfully aligned the IRS using the Cold and Dark Panel. Without your help I am certain I would have been unable to do it. I hope someday I will be able to return the favour. Best regards Ed McBain

The POS SHIFT page can be used to manually shift FMC position to the one you will select, or it can be used to see the difference between each source and the actual FMC position (angle and NM distance)You're right, the position will always differs, as I told, but the diffrence is part of the maintenance checks, if position differs too much after a flight, the ADIRU must be replaced.Position priority for FMC:ADIRU and GPS,ADIRU and DME-DME (autotuned by MMRs)ADIRU and DME-VOR Vor is manually tuned by pilots, DME will be autotuned for the colocated DME stationADIRU and DME-LOC same of above, but regarding an approachADIRU only

The ANP calculation uses this data:

• The NAV stations in use
• GPS availability and accuracy
• Accuracy of DME range data
• Accuracy of VOR bearing data
• ADIRU drift.