Double APU Generator Fault

I was asked for some help on a 747-400 APU generator problem by a new member Taekchang Lee, who is at CVG.

This was a double generator problem as neither would show "AVAIL" with the APU running.

I was wrapped up on another aircraft and I really didn't have time to look at the prints he sent me, much less give him any direction.

He sent me two video's from the aircraft. I'm including them here.

After I had a chance, I did follow up on the problem. The generator issue was corrected. Another one, APU auto-shutdown when air bleed turned on is addressed in the second video. It shows the electrical page with both APU generators showing no volts - no freqs.

I'll get with Lee to see if he can explain the repairs.  
        
 

Good morning Mark,

I sincerely appreciate you giving me this great opportunity.

The issue began after another mechanic replaced the Kory switch, but the problem persisted. In addition, when operating the APU with the APU BLEED switch turned on, an autoshutdown occurred.

On the B747-400, there is a switch on the electrical bus page that allows you to open and close the bus. As you may know, it is similar to the GEN switches on the overhead panel (P5). I will provide more accurate references later if needed. There is also a separate switch in the P6 section that allows control of the bus. After operating this switch, we were able to determine that the issue was not related to the APU generator itself. This led us to confidently conclude that the problem was with the Kory switch on the P5 panel.

Many mechanics, when replacing a Kory switch, only replace the “guts” rather than the entire assembly. In this case, only the internal components were replaced, not the full assembly. During the replacement process, I found that the spring on pin 18 at the back of the Kory switch was not functioning properly. After replacing it, the APU GEN switch operated normally.

Additionally, I would like to share an easy troubleshooting method for APU autoshutdown on the B747.

I have actually been noticing some similar issues lately with the korry bodies. If time permits and parts available I have started to change the entire assembly instead of just LCM or Switch heads especially on critical switches. Really does not take much more time just 30 minutes or so to replace and worth it for peace of mind.