747-400 FMC Update Status Display on ND
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747-400 FMC Update Status Display on ND
Would some kind person/aka -400 guru be able to explain the following?:
The FMC Update Status display in the lower RH corner of the Navigation Display shows which nav systems are providing position info to the FMC (e.g. "IRS(3)"). When I pull the FMC CB's, the display disappears (as might be expected). However, when the aircraft is running on Standby Power, the display _remains_ in view even though the FMC's are unpowered.
I realise that the Captain's CDU, should still be operating under these circumstances, however, even though my manuals tell me that the CDU uses the onside IRU for navigation (when ALTN Navigation is available during Stby Pwr ops), the "IRS(3)" display remains. Is this a software anomaly?
Thanks.
Rgds.
Q.
The FMC Update Status display in the lower RH corner of the Navigation Display shows which nav systems are providing position info to the FMC (e.g. "IRS(3)"). When I pull the FMC CB's, the display disappears (as might be expected). However, when the aircraft is running on Standby Power, the display _remains_ in view even though the FMC's are unpowered.
I realise that the Captain's CDU, should still be operating under these circumstances, however, even though my manuals tell me that the CDU uses the onside IRU for navigation (when ALTN Navigation is available during Stby Pwr ops), the "IRS(3)" display remains. Is this a software anomaly?
Thanks.
Rgds.
Q.
Hi QAVION
That's a nasty one for this time of the morning !!
However, having consulted with my mates, re. power supplies etc., the best we can come up with is this:-
Operating in StBy power, the Capt's ND will be active as will the L FMC. The 3 IRUs will also be operative initially, powered from the APU Hot battery bus, so the IRS(3) indication is correct. HOWEVER, if you leave it in that condition for more than 5 minutes, the Centre IRU will automatically switch off and you should then be left with IRS(L) as the ND indication.
I don't have access to an aircraft or the sim until tomorrow but will give it a whirl then (if I remember)
That's a nasty one for this time of the morning !!
However, having consulted with my mates, re. power supplies etc., the best we can come up with is this:-
Operating in StBy power, the Capt's ND will be active as will the L FMC. The 3 IRUs will also be operative initially, powered from the APU Hot battery bus, so the IRS(3) indication is correct. HOWEVER, if you leave it in that condition for more than 5 minutes, the Centre IRU will automatically switch off and you should then be left with IRS(L) as the ND indication.
I don't have access to an aircraft or the sim until tomorrow but will give it a whirl then (if I remember
)
Guest
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"I don't have access to an aircraft or the sim until tomorrow but will give it a whirl then (if I remember)"
Thanks for the offer, "CJ"
I realised that the IRS's were running on the batteries, but my main concern was the statement in the books which says that L CDU uses the onside IRU (only) during ALTN NAV, yet the Captain's display is telling us that all three IRU's are active.
Does the L CDU also carry out the triple mix function? (I thought only the FMC(computer) would do this
).
The Boeing Schematics are no help, as they only show a single databus going from the L IRU to the L CDU (No interaction with the other IRU's). I'm sure the EIU doesn't do any mixing. Perhaps the EIU is simply looking at which nav systems are available... and making an educated guess at what the FMC/CDU is doing with the data and putting up the update display (sounds a bit dodgy, tho').
Thanks again.
Cheers.
Q.
Thanks for the offer, "CJ"
I realised that the IRS's were running on the batteries, but my main concern was the statement in the books which says that L CDU uses the onside IRU (only) during ALTN NAV, yet the Captain's display is telling us that all three IRU's are active.
Does the L CDU also carry out the triple mix function? (I thought only the FMC(computer) would do this
).The Boeing Schematics are no help, as they only show a single databus going from the L IRU to the L CDU (No interaction with the other IRU's). I'm sure the EIU doesn't do any mixing. Perhaps the EIU is simply looking at which nav systems are available... and making an educated guess at what the FMC/CDU is doing with the data and putting up the update display (sounds a bit dodgy, tho').
Thanks again.
Cheers.
Q.
Hi Qavion
I'm having a bit of a problem with the Alternative Nav scenario. You won't be operating initially in Alternative Nav when you go to Standby power. For the first five minutes the L FMC will still be operating with all three IRUs and, therefore, the ND IRS(3) annunciation will be correct. However, at five minutes on battery power the centre IRU switches itself off and you are then left with the L FMC using the L IRU as its position input source - that should change the ND annunciation to IRS (L).
Just transitioning to Standby power won't put you in Alternative Nav - that only happens with a DOUBLE FMC failure.
Any way I'll get back to you on this - I want to see what happens tomorrow.
I'm having a bit of a problem with the Alternative Nav scenario. You won't be operating initially in Alternative Nav when you go to Standby power. For the first five minutes the L FMC will still be operating with all three IRUs and, therefore, the ND IRS(3) annunciation will be correct. However, at five minutes on battery power the centre IRU switches itself off and you are then left with the L FMC using the L IRU as its position input source - that should change the ND annunciation to IRS (L).
Just transitioning to Standby power won't put you in Alternative Nav - that only happens with a DOUBLE FMC failure.
Any way I'll get back to you on this - I want to see what happens tomorrow.
Guest
Posts: n/a
"I'm having a bit of a problem with the Alternative Nav scenario."
Ooops.... Not surprising ... It looks like I was reading my electrical diagrams incorrectly. I thought the L FMC was being driven by the Captain's Transfer Bus (which is dead when there is only Standby Power available) but is is actually being driven by the APU Stby Bus (assuming your aircraft has one). Although the APU Stby Bus is normally fed by the Captain's Transfer bus, when power is lost from the Main Busses, it is fed by the APU Static Inverter.
I assumed both FMC's would be dead in this scenario.... apparently not (Good to know)
Sorry to bother you at this time of night.
Have fun in the sim anyway ;-)
Cheers.
Q.
Ooops.... Not surprising ... It looks like I was reading my electrical diagrams incorrectly. I thought the L FMC was being driven by the Captain's Transfer Bus (which is dead when there is only Standby Power available) but is is actually being driven by the APU Stby Bus (assuming your aircraft has one). Although the APU Stby Bus is normally fed by the Captain's Transfer bus, when power is lost from the Main Busses, it is fed by the APU Static Inverter.
I assumed both FMC's would be dead in this scenario.... apparently not
(Good to know)Sorry to bother you at this time of night.
Have fun in the sim anyway ;-)
Cheers.
Q.
Qavion
Absolutely correct!
Anyway, we did the experiment and it all worked as advertised. Gave them a total 'gennie' failure to set the condition and left it for a while to check the annunciations. As expected, (IRS3) on the ND and it stayed that way. We couldn't afford the time to wait the full 5 minutes for the Centre IRU to switch off automatically, so I manually switched it off after a minute or so. Again, as expected, (IRS L) was the annunciation.
One interesting bit of info, which I hadn't looked at before, was the indicated discharge rate for the Main and APU batteries - 64 and 61 respectively - and that was without doing any electrical system 'housekeeping'; so the 30 minute power availability is obviously no problem at all.
I'm about to post a query re 400 hydraulics - would be interested in your thoughts.
Absolutely correct!
Anyway, we did the experiment and it all worked as advertised. Gave them a total 'gennie' failure to set the condition and left it for a while to check the annunciations. As expected, (IRS3) on the ND and it stayed that way. We couldn't afford the time to wait the full 5 minutes for the Centre IRU to switch off automatically, so I manually switched it off after a minute or so. Again, as expected, (IRS L) was the annunciation.
One interesting bit of info, which I hadn't looked at before, was the indicated discharge rate for the Main and APU batteries - 64 and 61 respectively - and that was without doing any electrical system 'housekeeping'; so the 30 minute power availability is obviously no problem at all.
I'm about to post a query re 400 hydraulics - would be interested in your thoughts.
