Jump to content
Sign in to follow this  
neas1982

TRU failure

Recommended Posts

Hi everyone,

 

Question here regarding a recent electric failure I had on a flight.

It was a TRU 1 failure. The QRH says: DO not use AFDS approach mode and Autoland is not available

an OM B from a european airline says:

With any AP or both FD in approach mode, the cross bus tie relay will open upon glide slope capture. This results in a cascade of failures as TR1 is disconnected from its backup. Upon TOGA push all returns to normal.

Solution:

Use raw data

Max 1 FD in APP mode

AP on: VOR/LOC with V/S

 

Now after I received the failure I checked the QRH and the OM B and decided to continue and do an autoland (which I never do normally unless the weather calls for it) just for the sake of trying. (It's my curious nature, can't help it :P) Anyway upon G/S capture no failures happened and the a/c continued for a safe autoland in NZAA. It was my understanding that PMDG completely modelled the electrical system on the beautiful aircraft. Is it possible that this small detail was missed ? Or did something else cause the aircraft to behave normally?

 

Thanks

 

(This is in no way any criticism, just a simple question)

Share this post


Link to post
Share on other sites

Did it was a single channel landing? If yes, the aircraft is able to do a single channel autoland, however, you are not allowed to do it. Regulations says that you must disengage AP at DH if a single channel is used.


Regards

Andrea Daviero

Share this post


Link to post
Share on other sites

Hi Andrea, It was a coupled approach, but according to the manuals the cross bus tie relay will allready open even if the A/P is not engaged. It will open even if both F/D are switched on

Share this post


Link to post
Share on other sites

TRU #1 supply's DC power to DC BUS #1, But when TRU #1 fails, the DC BUS #1 get it;s power from TRU #2 trough the DC BUS TIE relay (R9). The DC BUS TIE relay is normally closed during a normal flight.

 

The DC BUS TIE relay (R9) opens on these condition's

* The BUS TRANSFER SWITCH is not in the AUTO position.

* The AC transfer bus 1 or 2 has no power.

* The Autopilot is in approach mode with Glideslope capture.

The DC BUS TIE relay opens during a approach to keep the DC BUS 1 and DC BUS 2 seperate and in parallel, because during autoland the power sources must seperated. due to fails safe, If one system fails it dont;s influences the other system.


Mark Scheerman

 

Boeing 737-6/7/8/900 Ground Engineer

Share this post


Link to post
Share on other sites

Ok, I tried it and I did a perfect dual channel landing with TR1 and 3 failed.

I tried another time, this time failing all 3 TRus and that time I lost the MCP and other systems.

So, there is something wrong here.

TRU fail light was on, on the meters the failed units had no outputs, but related systems where functional until total system failure.

I think you can send a ticket, they hopefully will fix this on SP2.


Regards

Andrea Daviero

Share this post


Link to post
Share on other sites

Hi,

 

I was testing the simulation of the electrical system using the 737 technical site as reference. Especially I was focusing on TRU failures and I noticed the same behavior in the simulation as reported above : it seems the disconnect relay has been missed. Because I cannot see any effect with a TR1 failure on G/S capture with auto-pilot(s) engaged. So the electrical system as simulated might need some tweaks..... Also a TR1 failure leads to both DC Amps and DC Volts beeing reported to zero, but the mesure point is supposed to be in the DC BUS 1 for TR1, so Volts should not be zero as soon as the relay is closed and compensed by TR2 and TR3.

Argh, I went to PMDG to test the accuracy of the electrical system.... and I canno't reproduce what is cited in the FCOM about DC sides isolations and on the 737 technical site. Sad.

Best regards.
William Z.
P3D PMDG 738 v1.1

Share this post


Link to post
Share on other sites

Argh, I went to PMDG to test the accuracy of the electrical system.... and I canno't reproduce what is cited in the FCOM about DC sides isolations and on the 737 technical site. Sad.

Best regards.

William Z.

P3D PMDG 738 v1.1

 

While I can somewhat understand your frustration, I think the fact that these issues don't pop up unless you're trying an obscure procedure/failure is actually more of a testament to how well everything else is simulated. Given that this is a very old topic, and someone said this has been reported, it's likely in our tracker to be updated/fixed should we do so in the future.

 

A couple things, though:

It's not usually in good forum form to dredge up a 3 year old topic.

Full names - first and last - are required in body of your posts in the forum.


Kyle Rodgers

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
×
×
  • Create New...