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Author Topic: Question regarding AC BUS #4 fault  (Read 330 times)

Dirk Schepmann

  • Join date: Aug 2014
  • Posts: 150
Question regarding AC BUS #4 fault
« on: Fri, 22 Jan 2021 13:02 »
Hi all,

on my last flight I had a failure of AC BUS #4. To be honest, I was a bit surprised about the consequences of this malfunction. Autothrottle was no longer available and LNAV was also dashed out.

I was crossing the Atlantic at that time and had a couple of hours ahead, so I decided to divert to the Azores to repair it.  :P

In the QRH, it is also mentioned that the loss of AC BUS #4 results in a loss of ATHR and LNAV, but I couldn't figure out why. The items powered by AC BUS #4 seem not to be related - obviously I'm overlooking something.

Can someone explain it?

Best regards,
Dirk

Will

  • Join date: May 2009
  • Location: Chicago
  • Posts: 2183
Re: Question regarding AC BUS #4 fault
« Reply #1 on: Fri, 22 Jan 2021 13:28 »
It's the various probe heat supplies (engine 4, AOA R, F/O pitot, AUX R pitot, and TAT R). Without probe heat, LNAV is unavailable, because data from the air data computers might be unreliable. This would impact things like bank angle limits, speed limits, turn radiuses around waypoints, etc.

Basic modes (HDG, ALT, etc.) are of course still available.
« Last edit: Fri, 22 Jan 2021 16:34 by Will »
Will /Chicago /USA

Dirk Schepmann

  • Join date: Aug 2014
  • Posts: 150
Re: Question regarding AC BUS #4 fault
« Reply #2 on: Fri, 22 Jan 2021 20:12 »
Thank-you Will, now it’s all clear.

Best regards,
Dirk

emerydc8

  • Join date: Jun 2015
  • Location: Tucson, AZ USA
  • Posts: 2045
Re: Question regarding AC BUS #4 fault
« Reply #3 on: Sun, 24 Jan 2021 02:56 »
Based on the old checklist I have, VNAV is inop too. You are basically back to the 747-200 days.