Some more info
I have done some reading and this is what I understand. Can anybody confirm this or shed some light.
Our 744 has 3 ADC Left, Center and Right. You would assume that the Autopilot L, C or R would use the corresponding ADC but after looking in the maintenance manual that does not seem to be the case. ADC information is made available on only 2 and not 3 ARINC data busses?
The ADC spit out their information on two AIRINC data busses each with 4 channels. These busses are connected to the selected ADC. Each ADC also gets information on the fact if its AOA, TAT and/or P/S anti ice is working or not. It seems that this anti ice status info is included in the output.
Various systems that uses this output information are hereby made aware that the output may be contaminated what is not clear is what systems would still accept date from a contaminated ADC and what systems do not.
If you loose AC 1 or AC 4 you loose heating to the whole left or right side. (All left P/S, L AOA and L TAT) this is why the QRH states that you loose VNAV, LNAV, Autothrottle and that performance predictions may be unreliable.
If you loose AC 1 or AC 4 you will always have a static that is not de-iced whatever switching you make (the system always uses a static on both sides of the fuselage via the balance line) on the ADC Display select switch, that’s why, in my opinion, the QRH does not allow you to do any switching.
The Center ADC is programmed to function as a L or R ADC only. With the display switches in their normal position; Capt L ADC and FO R ADC the center ADC is programmed to function as a L ADC and the center Autopilot uses the C ADC.
If you look at the QRH PITOT HEAT CAPT you will see that the FO needs to select the C ADC and the Capt the R ADC. In fact the FO ADC then functions as a R ADC and consequently both PFD show R ADC data. FCC C always uses L ADC data on the ARINC bus.
Only R ADC and C ADC that is after the switching C ADC reprogrammed in R ADC is transmitted on the ARINC busses, hence only the R FCC recognizes its data and only the R autopilot is available.
If the FO selects ADC C then a valve physically connects the ADC to another set of P/S and the C ADC is programmed as R ADC. This is done to code the output of the C ADC as R this is done to allow the systems that normally use R ADC information to recognize the data on the ARINC bus after the witching was done.
If the FO has selected C ADC the Captain can no longer select C ADC since power is removed from the Capt ADC switch's when the FO has selected his ADC switch to C ADC. This is logical since the physical valve can be in only one position and by design only the FO ADC display select switch has control over the valve.
The question now is ???? does the AFDS change the autoland status from LAND 3 to >LAND 2< and if so when? During the approach (1500 RA) or also before the approach (EICAS > NO LAND 3)
If EICAS does not give this message then the crew should be made aware via SOP. List with requirements to initiate a CAT II/III.
It took me a day or two to read trough the maint manuals. In the end what I am after is that I do not want to be fooled by my instruments, AFDS or QRH. I realize that EICAS will give me speed and alt disagree messages but what is the value of these messages after certain ADC switching...... or course there is always the standby instruments but put all this in a CAT II/III context?
Thanks for any input.
Curly Mountains ex B 777 a very different bird
Last edited by Bitburger; 27th Jul 2008 at 19:35.