View Full Version : 737NG/Max systems assigned to each bus

28th Apr 2023, 01:49
hi all,

As a relative newby to the 737 (many years on various bus types) I have a knowledge gap (many actually) regarding the electrical system on the NG/Max.

In the event of a DC bus 1 or 2 failure, is there a list of systems affected by such a failure?

My company documentation does not include such a list and in fact none of our company modified manuals include guidance as to which bus powers each system.


70 Mustang
29th Apr 2023, 16:19
Are you asking about a physical failure of the bus DC1 or DC2 or just a failure of power to the bus?

There is an automatic change of power source if the normal power source for each DC bus fails.

That is explained clearly in the electrical chapter.

'our company modified manuals include guidance as to which bus powers each system.'

Indeed, over the years, the manuals have been steadily reducing the information given about the 'nuts and bolts' of the systems. Big example being MCAS omission. Truth is without constant review, remembering that kind of information is very difficult and in case something does happen, trying to out think the QRH and going outside the "legal" recommendations contained therein will be frowned upon in this ever increasing regulatory climate.

peppered throughout the manual one will find an occasional clarification of the specific bus source of various items.

at the end of the electrical chapter is a list of operating items with all generators inop. That's best you may get.

try to contact a mx individual at your airline. They can be a good source of extra information.


1st May 2023, 21:35
Are you asking about a physical failure of the bus DC1 or DC2 or just a failure of power to the bus?

Thanks for taking the time to respond.

No, my question relates to a failure of the Bus itself. I can see from our systems handbook how each DC Bus has several power sources.

My Airbus manuals disappeared from my IPAD the moment I changed fleets so I am relying on my memory but in the A320 with a DC Bus fault, ECAM would not only tell you what has failed but also which systems are lost (fuel pump/window heat for example) and what to expect upon landing (landing capability for example).

The QRH in the 737 we have does not include a checklist for dealing with the loss of a DC Bus and as you say there are SOME references to the power source of systems but there is no master list of which systems would be affected by the loss of a DC Bus.

We can figure out which Bus is lost based on the Volts of the various TR units BUT sitting there in the middle of the ocean, having established that a DC Bus has failed, there is no guidance as to which systems are lost except for the obvious significant system failures associated with a MC.

However, there are other subsequent failures that the loss of a DC Bus will cause that will not be annunciated by a fail light or a MC and have no checklist in the QRH.

A simple example that comes to mind is windshield wipers. If they are DC powered like on the A320 (and my manuals dont tell me if they are or not) then we will not have them available upon landing. HF radios also, no fail light and probably DC powered.

I fully recognize that its an unlikely occurance and that most things can be dealt with by stepping through the various obvious failures with the QRH in hand however I am surpised there is no quick reference summary page of systems remaining following such a Bus failure.

70 Mustang
2nd May 2023, 04:39
The boeing manuals used to have a long list of individual items but those disappeared long ago.

find an engineer. They might be willing and able to assist you.

I do remember in my first airline, based in Anchorage, that required us to remember all that list. We were also required to draw from memory, the individual systems' diagrams onto blank sheets of paper.

But in Europe I never encountered that requirement of systems knowledge.

i will search my books and see if i can find more info. However, as each year passes, the validity of each list will change with all the changes to the aircraft each year. Over the years i flew the 737, i watched them change from the 100s to the 800s as well as changes in between on the same versions.

3rd May 2023, 11:10

You could have a look at the 737 Tech guide (book and youtube), from Chris Brady, the 737 Handbook from Petr Smejkal, the 737 MRG from Patt Boone or the Cockpit companion. I have the printed one from one of these which as a list like that. There might be some differences to your outfit but it should give a general idea.


20th Nov 2023, 18:29
The 737 Tech guide by Chris Brady has a whole paragraph in the book that lists what each bus in the ELEC system powers.