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Copper.

Main XFER Bus 2 Failure in-flight

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Experienced transfer bus 2 failure inflight at 9000ft. Lnav, vnav, n1, a/p a selected. Identified right away what had occurred as a result of multiple system failures.

 

NNC TRANSFER BUS OFF actioned. APU was inoperative (simulated a departure with an APU M.E.L). Eventually got to the 'plan to land at the nearest suitable".

 

My question is - is the TRANSFER BUS OFF Non-Normal Checklist (NNC) incomplete? Or am I meant to address EVERY SINGLE failure independently?

 

I ask because, as soon as I landed, the ground horn was sounding; I initially thought it was one of the IRS's, but it was equipment cooling. Flicked the switch to ALTN and horn stopped, light extinguished.

 

Brian Nellis


Brian Nellis

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Hi Brian.

 

As you quite rightly pointed out you were down to one source of AC power as the APU was inoperative and the QRH instructs you to "Plan to land at the nearest suitable airport. Only one main AC power source remains"

 

Whenever Boeing leave 4 black square dots below a checklist in the QRH it's their way of saying the checklist is complete and all pilots would start some sort of discussion regarding the problem that occurred incorporating CRM and discuss any subsequent problems.

 

With regards to your failure each AC power system on the 737-8 has it's own transfer bus which during flight can be powered by either IDG from the engine or APU. As you mentioned the APU is inoperative so cannot supply AC power to either of the Transfer Bus's. Before the failure each IDG would normally be powering it's own transfer bus but if an engine generator is no longer supplying power the Bus Tie Breakers (BTB) would close and the other engine generator would supply both Transfer Bus's through the BTB.

 

By following the Transfer Bus Off QRH selecting Engine GEN switch (affected side) On (In this case No1 Eng if Gen 2 Transfer has failed) full AC power should be restored and all systems "Should" be functioning normally through the remaining source of AC power, albeit you only have one source of AC power remaining and if that fails then you'll have an interesting afternoon!

 

As you're only down to one source of AC power the electrical system is designed to shed electrical load based on electrical demand from the systems, initially the galleys and main bus 2 which power non-essential loads such as recirculation fans, equipment cooling etc. I'm not terribly sure why LNAV and VNAV was unavailable, electrics isn't my strong point regarding the 738! A/P B would have tripped due to the loss of electrical power or sensor input.

 

Making the decision to deal with any subsequent failures caused by the initial problem in reality involves good CRM. If Boeing tell you to land at the nearest suitable aerodrome it's usually for a good reason! Take a look at the Loss of Both Engine Generators checklist, all this does is attempt to restore AC power because without it the list of problems is so long there is no real check-list for total loss of AC power

 

Electrics for me is a real ball ache and I hoped I answered the question as accurately as possible.

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Whenever Boeing leave 4 black square dots below a checklist in the QRH it's their way of saying the checklist is complete and all pilots would start some sort of discussion regarding the problem that occurred incorporating CRM and discuss any subsequent problems.

 

Great nugget of information there! Nice!

 

Also, I do appreciate the wealth of knowledge you posted, but it's a forum rule to sign your name to your posts, so please do so in the future:

http://forum.avsim.net/topic/245586-you-must-sign-your-full-real-name-to-posts-to-use-this-forum-posts-without-names-will-be-deleted/

 

Thanks!


Kyle Rodgers

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Wow... Power packed post! You've wacked the nail on the head! For your info, lnav/vnav were available, a/p a was in cmd until I "completed" the checklist and reverted to manual control.

 

Turns out I neglected to do the nnc properly as I didn't try switching the affected side generator to on. Woops!

 

I was recording at the time so had time to run over where I went wrong and right after I read your post.

 

Had I done the nnc properly, as you've pointed out to me, the btb might've closed and re-powered the failed systems.

 

Nonetheless, I managed to land fine. Other major 'problem' I had, was loss of flap position indication.

 

I learned a lot from your post, so thanks a lot!

 

Brian Nellis


Brian Nellis

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Hi Kyle!

 

Sorry buddy, will do so in the future.

 

Hi Brian,

 

Remember to restore AC power as per the QRH! Haha. There are plenty of checklists in the QRH that can be ambiguous and misunderstood.

 

Rgds.

 

Sam Breese

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Hi Kyle!

 

Sorry buddy, will do so in the future.

 

Sam Breese

No prob. Just figured you hadn't seen the post about it. Thanks for taking care of it!


Kyle Rodgers

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Had the fault again yesterday. Turns out Transfer Bus 2 wouldn't accept an alternate power source (APU generator or non-affected side IDG). TRANSFER BUS OFF NNC actioned, properly this time. No choice but to land with only the remaining main xfr bus. Very creative PMDG!

 

This time, I was able to restore some functionality - as mentioned previously - equipment cooling supply to ALTN, Transponder/Altitude Reporting to system 1 etc.

 

Annoyingly, it always seems to strike when I plan an rnp ar approach and am flying from the RHS as PF. I end up in a hold at ppos then re-plan for a boring ol' ils approach from LHS as PF ;) On the positive side, how freakin' awesome is the ngx!

 

Brian Nellis.


Brian Nellis

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