Sign in to follow this  
Guest Babubhai

IRS and Autopilot

Recommended Posts

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.

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

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)

Share this post


Link to post
Share on other sites

"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>

Share this post


Link to post
Share on other sites

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?

Share this post


Link to post
Share on other sites

"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>

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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>

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this