Right Transformer Rectifier Output

Crew write-up(s) dealing with four fire warning systems.

One APU FIRE LOOP, one LEFT ENGINE FIRE LOOP, one RIGHT ENGINE OVERHEAT LOOP, one LEFT WING BLEED LOOP. The second write-up was a fire system fail message on final.

When you dig into the problem, having four separate system faults at the same time seems odd. When tested, all of them passed..... meaning no actual loops were bad.
  
     


You have to look for something else in common with all of them..... that was the 28VDC power input which came from the RIGHT DC BUS. That leads you to the RIGHT T/R (or transformer rectifier).
  
     
  
Also of note, the issues with the loops happened on push-back. What is going on during that time? Engine start with power swap when the generators comes online. What was suspected on the fire system fail on landing? Auto-Pilot coupled approach where the A/C and D/C power bus's isolate from each other.

We compared DC voltage drop and recovery between the left and right T/R's. The right voltage dropped lower and took longer to recover. (Really hard to tell when everything happens in under a second.)

So..... when we went to replace the T/R....
  
     
  
Lug nut not fully tightened..... and a burned terminal.
  
     

None of the items repeated except the left wing bleed issue. That turned out to be a hard fault that will discussed on the forum shortly.