View Full Version : 747-400 IRS Errors

2nd Sep 2001, 20:42
Is it common after an 11 hour flight to have IRS errors where each of them are about 4nm out from the FMS position when the POS key is pressed on the EFIS Control Panel and displayed on the ND????

2nd Sep 2001, 23:35
I assume it is just map shift you are talking about. As far as I know on aircraft that use an IRS/INS, over time away from any nav radio sources i.e. over the pacific/atlantic the IRS assumed position can drift away marginally from actual position. On the 767 at leasst, if the nav radio's are set to auto, and there is a suitable VOR/DME, the FMC will update it's position with regards to that.
I know the 767 can suffer from map shift as it has no GPS, therefore I assume the 757 can have map shift occur...777 has GPS also, so guess it doesn't have a problem..I don't know if the 747-400 has a GPS hooked up to the FMC or not as standard.
However I could be talking complete rubbish also, as I am only a wannabe..

[ 02 September 2001: Message edited by: tom775257 ]

3rd Sep 2001, 07:48
I often see one of the IMUs at 2 to 4 miles off, but seldom will all 3 be that far off.

Generally, individual IMU drift is 0.0 to 0.4 NM/hour on block-in, which would be as much as 4.4 NM on an 11-hour flight. Usually, at least 2 of them are 0.2 NM/hr or less.

3rd Sep 2001, 18:43
Assuming full IRS systems allignment previous to the take off, not severe in flight turbulence... is within the normal limits.