IRS and Autopilot

[Guest Babubhai]

Hi,I own the PMDG 747-400 addon only (latest build).During mid-flight I was playing with the IRS systems. I switched off all three IRS, and the autopilot was controlling the aircraft normally.As far as I understand the autopilot should stop functioning, when all IRS systems off.Can someone shed some light on this matter. Thanks.

[saltypete 0]

Hi, remeber to sign your posts as per forum regs.As for the autoflight system it could possibly be that without the IRS the position reference comes from the GPS. By this i mean the GPS built into the FMC and not the default GPS included with MS flight sim which is not supported. If you look at the ND in small letters at the bottom there is normally ( with IRS aligned ) in green IRS3 and above it GPS. As i understand it the queen will take position reference from the GPS firstly and from any IRS that is functioning. I could be wrong but i believe this is done for accuracy and redundancy of systems. I'm sure i will be corrected if i have this wrong.Happy flyingRegardsPete (YBUD)

[Qavion 19]

"As far as I understand the autopilot should stop functioning, when all IRS systems off."You are absolutely correct. Although there are options concerning the number of IRS's required (and specific IRS's required) for engagement in the air/on the ground, with all off, you will have no A/P or FD ops.I'm not sure how this slipped through the beta test process.Regards.Q>

[Datalink 26]

WOW that seems to be something quite major missed there, unless it was intentional.If the IRS's are selected to ATT I assume you regain basic autopilot functions?

[Qavion 19]

"If the IRS's are selected to ATT I assume you regain basic autopilot functions?"As far as I can recall (in the real world), no, you don't. I seem to recall trying it on the ground without success.Regards.Q>

[Datalink 26]

Really! :-eekI guess thats where are the flying skills come into play :)

[cowpatz 243]

I recall that once back in ATT mode you should get autopilot function back but only in basic modes. It is academic really as there would never be an occasion where you would turn all 3 IRS's off in flight. Its like asking what will the AP do if I shut down all 4 engs.CheersSteve

[Subs 0]

And just to confirm Steve, I think you have to be in reletivly level flight and not in a turn, when switching to ATT after normal IRS allignment is lost?Subs

[Datalink 26]

Correct, IRS's need a relatively stable level attitude and unaccelerated flight if they are to be switched to ATT mode.

[Guest Babubhai]

I guess it's too late asking for a fix now.

[Guest A32X]

If you buy the upgrade it'll probably be included in that...Paul

[Qavion 19]

I once tried switching to attitude (from OFF) during an aircraft tow (real world)... During the (approx) 45 seconds it took for the artificial horizons to appear, the aircraft turned through 90 degrees. The displays appeared normal.Not sure what this implies...Normally it only takes about 30 seconds for ATT. Is ATT mode delayed until stable flight is achieved... or is ATTitude degraded (until all the accelerations have been averaged out).Cheers.Q>