Hi guys!I've been wondering about this:Let's assume an aircraft, that is on the ground, parking brakes on and engines off, however with all 3 IRU's aligned. Now I know that IRU's drift over time, and that position offsets occur aswell as groundspeed indications will become less accurate over time. But what if an aircraft is not moving? Will the groundspeed drift over time aswell? Let's assume this aircraft has just flown for an hour, returns to it's base, and the IRU's keep operating in NAV mode. The residual groundspeed after setting the parking brakes is 7 kts. Will this increase if the aircraft is not moved?I know you can "reset" the GS by simply quick aligning the IRS, and that the residual GS can be used as a way to determine IRS accuracy, but I am simply wondering whether GS will wonder?My theories:GS should not wonder, as no acceleration changes take place. What about if the aircraft is serviced? People walking in the aisles? Those are tiny accelerations, but still, the IRU's are extremely sensitive. Can these tiny accelerations, over time, cause the GS indication to change (say 1 knot)?PS I posted this same message on the aerowinx forum aswell, just to get as many replies as possible.Regards,Mark

I don't have a clue but the question is interesting enough to give it a bump.My vote is:"No, but all real world procedures call for the re-alignment anyway so the question is only an academic."(Note I said "vote" because I really have no idea)Mike

I vote it will keep wandering. You say there are no (real) accelerations, but the accelerometers have noise in them, and the IRUs don't know that that is simply noise and not minute accelerations. Why do you think they would drift with movement but not without?!?Lee Hetherington (KBED)

Hi, I am at a loss regarding what to vote. Please see the answer I provided for the other thread regarding the IRS system. I know that in the old days when the IRUs were positioned on a gyro stabilized platform, errors could occur just from earth's rotation. Suppose that you park the airplane and align the gyros to point due north. To make matters more clear, suppose that the aircraft is parked for 4 hours in the same position. Now the gyros will not be pointing towards north but somewhere into space as earth has rotated. I know that in the old days every so often an electrical impulse was given to the system in order to realign the gyros. I seem to remember that this was done as often as every 15 minutes but don't quote me on it. I think nowadays laser ring gyros are used instead and I don't think they are prone to the same error. I think that the LRG will just give a continuous acceleration in around one axis that can be filtered away. Hope this helps,Boaz

Hi, Wanted to edit the post as I realize that I forgot to give my concluded vote but exceeded the editing period: I actually vote that the GS would indicate zero. I would think that the acceleration due to earth's rotation is well known and can be compensated for.Boaz

Well they dont pay me to sit in parked aircraft for very long ;-) so I dont know the answer! My guess would be yes they will drift, due to they inherent inaccuracy, and the rotation of the earth. If the errors start to get large enough then the GS will start to indicate. As you say, even small things like wind etc will slowly cause errors. On a recent windy day in Sydney several aircraft were delayed as they were unable to align IRS's with the wind rocking the aircraft.

"Well they dont pay me to sit in parked aircraft for very long so I dont know the answer! My guess would be yes they will drift, due to they inherent inaccuracy, and the rotation of the earth."This gets my vote too :-) I have aligned the IRU's and then come back an hour or so later and found that small groundspeed and position errors have crept in. No wind or bumping was involved as far as I recall.Cheers.Ian.