Hello,Does somebody know what is the IRS drift rate for IRSes used in the b737-800? I was testing this error in the simulator, and after ~10hrs I have average drift rate ~ 0.1nm / h, which seems to me a little low, at least when compared to b737 accidents caused by IRS misalignment.Jakub

I'm really not 100%, but it would be variable depending on many conditions I believe. So you can't say there is one consistent drift rate. Although, I am sure that there is an acceptable range of drift given a range of conditions and this would be determined when the INS/IRS goes through evaluation and certification.Daviersoft, or Andrea is a killer technician. Maybe he knows more about this.Edit...In regards to your test of 10 hours... If you wanna check the drift rate of just the IRS/INS, you need to disable the auto update I believe for the GPS and the Radio Navs. Correct me if I'm wrong, but under conditions where the system is getting reliable GPS and or Radio Nav signal, the INS/IRS will be updating its position based on the other two systems.JB

Edited January 27, 201214 yr by Buzz313th

Distance is given by the distance from IRS coordinates to the true aircraft position.

Distance is given by the distance from IRS coordinates to the true aircraft position.

LOL, see, I told ya he's good... Even quick ta boot.Thanks Andrea.JB

In regards to your test of 10 hours... If you wanna check the drift rate of just the IRS/INS, you need to disable the auto update I believe for the GPS and the Radio Navs. Correct me if I'm wrong, but under conditions where the system is getting reliable GPS and Radio Nav signal, the INS/IRS will be updating on the other two systems.

You might be right, but when I checked FCOM I am still not sure - is it the computed a/c position that gets updated (with priority to GPS, then radios), or also IRS position gets updated?If the latter was the case, the IRS drift should be 0 all the time when GPSes are on, because IRS internal position would be all the time updated using the computed (and very close to the true) position?

Distance is given by the distance from IRS coordinates to the true aircraft position.

Andrea, thanks, you are great as always. I need one clarification: what is the meaning of the areas 'replace - 1 flight' and 'replace - 2 consecutive flights' Jakub

To give an example, maintenance is required to replace the IRU unit before the next flight if it has drifted 20 miles on the previous 4 hour flight. For the same 4 hours flight, if the drift was 13 miles, the IRU can be released for an other flight without maintenance action. If during the second flight, the drift is 11 miles, looking at the graph provided on the previous post, we are still in the "Hatched" area, which requires maintenance to replace the unit prior to further flight.If the first flight had a drift of 13 miles, and the second one 5 miles, no action is required.As far as the GPS updating the IRU, the concept is the following : The GPS update and the Radio Update will line up the FMC position, but will not correct the IRU position. This is why you have a "Position Shift" page in the FMC that let you compare the different sensor positions relative to the FMC position. The GPS sensor position will be very close to the FMC position source that has the most impact in the algorithm used by the FMC software to compute the FMC position.Hope this makes sense.Marc Bergeron

Edited January 27, 201214 yr by Seatrend45

You might be right, but when I checked FCOM I am still not sure - is it the computed a/c position that gets updated (with priority to GPS, then radios), or also IRS position gets updated?If the latter was the case, the IRS drift should be 0 all the time when GPSes are on, because IRS internal position would be all the time updated using the computed (and very close to the true) position?

As far as the GPS updating the IRU, the concept is the following : The GPS update and the Radio Update will line up the FMC position, but will not correct the IRU position. This is why you have a "Position Shift" page in the FMC that let you compare the different sensor positions relative to the FMC position. The GPS sensor position will be very close to the FMC position source that has the most impact in the algorithm used by the FMC software to compute the FMC position.Hope this makes sense.

Excellent, then I stand corrected. So on the 73, the IRS/INS will continue to drift until you do a ground realign reagrdless of composite data from the GPS/Radios? JB

Excellent, then I stand corrected. So on the 73, the IRS/INS will continue to drift until you do a ground realign reagrdless of composite data from the GPS/Radios?JB

Yes, they are independent.

Now what about ETOPS certification? Once out of range of navaids and you lose your GPS, then you are completely reliant on the inertial system. Is there no way to manually update the IRS/INS position before transitioning out of range from ground based navaids?JB

Nevermind, I answered my own question...If I read this thread correctly http://www.pprune.or...hp/t-54016.html then it's my understanding that the IRS in the 73 does not update the raw positional data, but infact the FMC is constantly remembering the IRS drift error based off of the composite nav data. And lets say you lose the GPS and ground nav capability, then the FMC will compute from the Raw average of the two IRS, then factor in the last known IRS position error to come up with a mean aircraft position.So correct me if I'm wrong... The IRS are not updating, but the FMC is updating it's adjusted pos error for correction to the two IRS.Lemme know if this assumption is correct... Your at cruise and IRS drift is indicating hypothetically 25 miles off track. Then you lose GPS and nav radios. And your sole Nav data is now from the IRS. You might expect the aircraft position to jump to 25 miles off track. But since the FMC knew the drift error before you lost the other nav systems, you would not see a shift of aircraft position when the IRS becomes the only nav system.Interesting. Seems more redundant this way.JB

Edited January 27, 201214 yr by Buzz313th

There was a pretty good discussion on some of this info here:http://forum.avsim.net/topic/350742-technical-questions-for-experts/page__p__2133815__hl__adiru__fromsearch__1#entry2133815I wasn't sold on some of the posts, but it's interesting reading.