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Lists on affected BUS loss?

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    Lists on affected BUS loss?

    Hi folks,

    TITLE SHOULD HAVE BEEN: LISTS OF AFFECTED SYSTEMS PER LOST BUS.

    1.
    during my last flight I lost apparently the APU BAT BUS.
    An obvious effect of this fault was that I lost my LEFT COMM box.

    Curriously I looked up the QOTSII FCOM, but could not find an elaborate list of systems depending on specific BUSes.

    However, I found only some prosa like:
    The APU battery bus powers individual equipment items such as
    • APU DC fuel pump, engine start air control
    • cargo, first officer, and service interphones, PA system
    • APU and engine fire/overheat detection loops A and B
    • APU fire warning horn
    ... but, where can I find complete lists?

    2.
    On top why does APU BAT BUS supply the COMM box?
    Is it the idea to supply the most important components in case all other buses fail?
    Sh*t if the APU BAT BUS fails alone and the L COMM box does not work anymore.

    3.
    Also the R COMM box was affected. Is this correct?

    Thanks in advance.
    Last edited by 19ab67; 15Jul2020, 14:09. Reason: Title change
    Andre(as) Berg --- i7 8700K 12MB 3.7G. MSI Z370 Tomahawk. DDR4 16G. SSD 500G M.2. MSI RTX2080 DUKE 8G

    #2
    Originally posted by 19ab67 View Post
    Hi folks, TITLE SHOULD HAVE BEEN: LISTS OF AFFECTED SYSTEMS PER LOST BUS.

    During my last flight I lost apparently the APU BAT BUS. An obvious effect of this fault was that I lost my LEFT COMM box.

    1. Curiously I looked up the QOTSII FCOM, but could not find an elaborate list of systems depending on specific BUSes.

    However, I found only some prosa like:
    The APU battery bus powers individual equipment items such as
    • APU DC fuel pump, engine start air control
    • cargo, first officer, and service interphones, PA system
    • APU and engine fire/overheat detection loops A and B
    • APU fire warning horn
    ... but, where can I find complete lists?
    A complete list is usually only found in Boeing's Technical documentation for each particular aircraft and this is obviously not available to us in our PMDG manuals. There are no less than 18 different AC and DC busses in the B744, so if you want to have a better idea of what systems are powered by some of them then all you need to do is study the overhead and other circuit breaker panels within the flight deck. However, to prevent you getting a stiff neck you could also read Chapter 6 Section 10 in the PMDG 744 FCOM, because it will help you to learn more about the basics of the aircraft's electrical system.

    Originally posted by 19ab67 View Post
    2. Why does APU BAT BUS supply the COMM box?
    Is it the idea to supply the most important components in case all other buses fail?
    Correct. This particular failure is very rare and it should never happen in real life because of the automatic redundancy and backup the B744 has built into its electrical system.

    Let's assume the aircraft is on the ground and there is no Ground Power connected at all. The aircraft's Main and APU batteries will be the only sources of electrical power when the Battery switch is first turned on. This action connects these two batteries to their respective battery busses and also activates the most important aircraft systems before we can start pre-flighting the aircraft. The APU Battery Bus powers such things as fire protection as well as the APU's DC Pump, but before we are in a position to start the APU safely we will also need to select the Standby Power switch to AUTO in order to see the EICAS messages and aural alerts. This is where things start to get complicated, because these two actions power the aircraft's Standby Bus from the Main Hot Battery Bus. Fortunately, we are now in a position to be able to see and communicate what we are doing even at night, because we also have power to the indicator and flight deck lighting and we can talk as necessary with the Ground Crew, Cabin Crew and ATC as well. These are the main reasons why the Left VHF and Interphone systems are normally powered from the APU BATTERY BUS - and another good reason I can think of is if you lost all of your engine electrical power in flight you would still be able to communicate using VHF LEFT, even if it is only to transmit a Mayday call on 121.5Mhz!

    Originally posted by 19ab67;n72370
    3. Also the R COMM box was affected. [COLOR=#e74c3c
    Is this correct?[/COLOR]
    Thanks in advance.
    It all depends on the individual aircraft and whether or not it has been modified. For example, there are many 744's that have VHF LEFT powered from the Main Battery Bus and not the APU Battery Bus.


    Michael Codd

    Comment


      #3
      Originally posted by Michael Codd View Post
      Correct. This particular failure is very rare and it should never happen in real life because of the automatic redundancy and backup the B744 has built into its electrical system.
      Thanks, Michael.
      I agree, but in my case -- APU BAT BUS failure -- no (PMDG) redundancy kicked in... Why?
      Andre(as) Berg --- i7 8700K 12MB 3.7G. MSI Z370 Tomahawk. DDR4 16G. SSD 500G M.2. MSI RTX2080 DUKE 8G

      Comment


      • Michael Codd
        Michael Codd commented
        Editing a comment
        A lot more information is needed. For example, what were you and the aircraft doing at the time?

      #4
      Hi Michael,

      I was climbing as LH446 out of EDDF to KDEN.
      All engines running, except the APU of course.
      So, if the APU BAT BUS fails, why don't other BUSes take over?

      Thanks for your patience.

      Andre(as) Berg --- i7 8700K 12MB 3.7G. MSI Z370 Tomahawk. DDR4 16G. SSD 500G M.2. MSI RTX2080 DUKE 8G

      Comment


        #5
        The 744's DC system is fully automatic except for the Battery switch, Bus Tie switches and the Standby Power selector. There are no DC indicators or panel controls for it even though It is normally powered by the aircraft's AC system through four transformer rectifier units.

        There are four Battery Busses in the aircraft's DC system and in the case of the APU Battery Bus it requires the Battery switch to be ON when no AC power is available so that it can be powered by the APU HOT BATTERY BUS (which is hard wired to the APU Battery). However, in normal flight with AC power available the APU BATTERY BUS is powered from the No.3 DC Bus which is controlled by the four TRU's. If the No.3 DC Bus fails in flight for any reason then the APU HOT BATTERY BUS will supply the APU BATTERY BUS for as long as the APU BATTERY and APU BATTERY CHARGER are working and the aircraft's BATTERY switch is still ON.

        Therefore it is not just a simple case of one DC Bus taking over from another when one fails. You have to follow the appropriate QRH procedure very carefully (if there is one) for any EICAS messages you may see and also try and regain any lost instrumentation by use of the various transfer switches. The good news is that your issue is more likely being caused by a suspect panel state, so I suggest you can safely shut down the aircraft and start over again. If this fault still persists then submit a support ticket to PMDG giving as much information about it as you can.
        Michael Codd

        Comment


          #6
          Originally posted by Michael Codd View Post
          The 744's DC system is fully automatic except for the Battery switch, Bus Tie switches and the Standby Power selector. There are no DC indicators or panel controls for it even though It is normally powered by the aircraft's AC system through four transformer rectifier units.

          There are four Battery Busses in the aircraft's DC system and in the case of the APU Battery Bus it requires the Battery switch to be ON when no AC power is available so that it can be powered by the APU HOT BATTERY BUS (which is hard wired to the APU Battery). However, in normal flight with AC power available the APU BATTERY BUS is powered from the No.3 DC Bus which is controlled by the four TRU's. If the No.3 DC Bus fails in flight for any reason then the APU HOT BATTERY BUS will supply the APU BATTERY BUS for as long as the APU BATTERY and APU BATTERY CHARGER are working and the aircraft's BATTERY switch is still ON.

          Therefore it is not just a simple case of one DC Bus taking over from another when one fails. You have to follow the appropriate QRH procedure very carefully (if there is one) for any EICAS messages you may see and also try and regain any lost instrumentation by use of the various transfer switches. The good news is that your issue is more likely being caused by a suspect panel state, so I suggest you can safely shut down the aircraft and start over again. If this fault still persists then submit a support ticket to PMDG giving as much information about it as you can.
          Thanks, Michael, very informative!

          You can check the behaviour by yourself:
          Simply activate the APU BAT BUS failure via the FMC.
          Andre(as) Berg --- i7 8700K 12MB 3.7G. MSI Z370 Tomahawk. DDR4 16G. SSD 500G M.2. MSI RTX2080 DUKE 8G

          Comment


          • Michael Codd
            Michael Codd commented
            Editing a comment
            I have - several times!

          #7
          [QUOTE=Michael Codd;n72546]I have - several times!
          /QUOTE]

          Hi Michael,

          what was the behaviour on your side?

          1.
          I loaded my last in-flight saved situation file (somewhere short before T/D into KDEN) and armed/activated DC BAT BUS APU.
          Its effects are
          • LEFT COMM box display blanks, but not the illuminated green L, L MIC, and L VHF LED above the volume knob L.
            Should it not be completely off/blank?
          • RIGHT COMM box active display, I can enter new frequencies, but I cannot activate it by pushing neither R MIC nor R LED above the volume knob R.
            Should this one not be the backup?
          • CENTER COMM box active display, I can enter new frequencies, I can activate C MIC and C LED above the volume knob C, but the ATC menu does not acknowledge that the actually requested frequency is active, here, I cannot communicate with Denver Center on 125.95.
          (The OPTIONS SIMULATION setting for ACP CONTROLLING SIM is set to EITHER CAPT OR F/O. )

          So in essence, the RIGHT COMM box is not available, but it should be, should it not, neither can I use the CENTER COMM box, however, this might not be foreseen as backup in PMDG's setup (if I understand the OPTIONS SIMULATION setting correctly).

          If I set STANDBY POWER from AUTO to BAT, the EICAS messages BAT DISCH MAIN and BAT DISCH APU show, but CHKL gives only notes that the respective batteries provide power for about 30 minutes. So I return to AUTO.

          No hints on how to back-up the APU BAT BUS,
          and no automatic back-up.

          2.
          In order exclude the possibilty of a corrupted panel state, I loaded 744 DEFAULT sitting on the ground (at KDEN) and armed/activated DC BAT BUS APU.
          The bird is loaded with engines running.
          The same behaviour applies as described above.

          Please confirm that your 747 reacts the same or differently (how?), when activating DC BAT BUS APU.

          Thank you.
          Andre(as) Berg --- i7 8700K 12MB 3.7G. MSI Z370 Tomahawk. DDR4 16G. SSD 500G M.2. MSI RTX2080 DUKE 8G

          Comment

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