This is an IRU from Northrop Grumman. We use a Honeywell system but no pics avail.
The plane had been taxiing out for departure when the instruments flagged. The flight crew returned to the gate, then contract MX came out, performed a re-align of the IRUs. The flags went away and they signed it off. On push back, this time, the instruments flagged again so they returned to the gate again and called us to go check it out.
Road trips are a perk of being a Line Mechanic. There is a certain amount of pride in knowing that the company trusts us enough to go out and fix planes away from station. There is also satisfaction in the figuring out of what is wrong and using knowledge, and available tools to get a plane that is stuck at an away station back into service. Most guys enjoy road trips and I have written about them before.
Maybe-Maybe and I are of the same mind in that we like to go, fix the plane and come back as quickly as possible. We figured that this would be a quick fix and back to OAK.
Once we got to SJC and read the logbook we found out that the problem only existed on the FOs side instruments. All the instruments: ADI, HSI, RMI, and even the PWS INOP light came on. We began by aligning the plane and checking for any faults in the system. Of course we found no faults and everything worked as it was supposed to. We decide to put the new IRU and DAA in as we were sure the #2 IRU was acting up. When we put the new IRU in we found out that it was "Bad From Stock". There was a fault code that would not reset or go away. So much for a quick trip! We called MX CTRL and they got one headed our way through LAX MX. We could expect it in a little over an hour.
While we waited we kept busy by cleaning the plane (the flight crew left in a hurry and had no time to do it). We also changed some seat belts on another plane that had an issue.
When our IRU arrived we threw it in and got everything working correctly. I called MX CTRL to tell them I was signing off the plane and let them know that it had in fact been working when we arrived hours ago. We did the deed and soon enough a new crew arrived. Maybe-Maybe and I decided that since the plane had returned to the gate twice before, we would wait until it took off to make sure everything was ok. (I think you all know where this is going) The plane boards, pushes back and sits there for like 6-7 minutes. Maybe-Maybe and I are saying "That's it he's broke.....he'll be coming back". We went to OPS and asked the OPS guy to call them. Sure enough he said he was working on the same problem the plane had before.
DAMN! The freaking plane is POSSESSED!!!
They came back to the gate and said that the instruments on the FO side flagged out, but as soon as he put the APU bus back on line the flags went away!! That was it!! that was our piece of info that we were missing!! When the pilots put the plane on the #2 generator the FOs instruments flagged. The #2 generator was not powering the #2 IRU bus!!!
We called MX CTRL and I suggested we MEL the #2 engine driven generator. That was all we had to do all those hours ago when we got there and we could have been done!!
It's amazing how one little piece of info can throw off a whole troubleshooting plan. We had been chasing the wrong problem the whole time! In hind sight we figured the #2 generator must have stopped powering that bus when they first were taxiing out the first time the problem manifested itself. Of course the re-align worked because they were most likely on GRD power or APU power at the time.
We did wait until the plane pushed out to ensure they left. We got to SJC at 12:30pm and finally left at 7pm. A long day of waiting and chasing phantom problems. Oh well, we did figure out what was happening. The plane did finally go make some money.
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