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While chasing down a shorted power supply output for the DFDAU system, we came across this......



After cutting tie-wraps and spreading things apart, we could see that we were dealing with a serious arcing event.



Each wire in the bundle had to be cleaned and inspected.

In total, we had 7 unshielded twisted trips, 2 shielded twisted pairs, and 5 single conductor wires sliced completely open or showing signs of stress. 6 of the twisted trips were three phase power wires for the PDU's (Power Drive Units) used for cargo handling (we had six corresponding breakers popped on the forward lower cargo power distribution panel). Our DFDAU 28VDC accelerometer (power and return twisted pair) was burned in half.

The image below shows the damage to a couple of the twisted trips. The ends of these wires had metal globules that pretty much verified that they had melted.



This image is poor, but it shows what I believe to be the culprit pair of wires. These two twisted pair wires had their insulation burned off exposing about an inch of bare conductor. The blue wire is open. The red was hanging on by just a few threads.



I think the blue wire exploded and sent out a "wedge" of heat and energy that damaged all the wires in its path. We didn't open up and examine the layout of the wires vs damage like we should have. The damage could have been circumferential, with all the immediate surrounding wires damaged. The fact that the red wire was still together leads me to think the wedge is more likely.

This wire pair was verified to be the power and return 26VAC excitation to the Right A/P Aileron Actuator LVDT (all right auto-pilot systems were inop and on deferral before we started our troubleshooting). It was the blue (power return) wire that let go.

These two wires and several others in the bundle were Kapton wires. The history of this particular kind of wire is not good. The damage that we observed seems to fit right along with the experiment below. (Go to 6:23)





The Kapton wires that I have encountered in the past have had a thin amber wrap around the conductor itself. Similar to what is shown below.


From Lectromec

I've seen Kapton wires on 737-300/400's, MD-80's, DC-10's, and MD-11's.

I could be wrong in my conclusions above, I'm just laying it out as I see it.

For a review, DFDAU functionality is discussed here .



We had a unit showing a DFDAU fail light with the code shown.



After gaining access to a manufacturer troubleshooting manual, we learned that there are internal power supplies (at least 2) inside the DFDAU unit. One power supply powers the unit's internal circuitry. Another power supply (in our case #2PS) powers external devices or sensors. One such item on this aircraft was a stand-alone accelerometer located in the left wheel-well.



Aircraft recorder installations can vary widely. Some use IRS accelerometer data instead of a stand-alone unit. The DFDAU's internal power supplies can also power extra syncro's and switch's. As an example, instead of a digital Flap/Slat input to the DFDAU for flap position, a syncro could be attached to a flap drive that is powered by DFDAU 26VAC and the output from that syncro going back to the unit for flap position recording.

For this particular problem, we had some burned wires in a sidewall that shorted out the 28VDC power from the DFDAU to the accelerometer.


Rotate.Aero has a copy of Teledyne's troubleshooting manual for this particular DFDAU unit. It cannot be published for proprietary reasons. A copy can be sent to individuals that might need one to troubleshoot a failed system.

No tools or test equipment needed....

When the wheel speed transducer signal from the #1 tire is being seen as the #5 tire at the Anti-Skid box, the system has a tendency to apply pressure to the "wrong" brake.



Both the 1/5 and 2/6 pairs of transducer wires were swapped between tires sets.



Is there any wonder why this aircraft has been wearing out tires on the left truck for a "long" time.

What a surprise! Connect the two together properly, secure them properly and shocking! It works as designed! In the automotive world, we come across too many electrical terminals that have poor fit due to unscrupulous people using all kinds of makeshift probes or tools, needles, nails, whatever is handy. And then the intermittent problems start. Where connections have a decent fit but still have erratic performance, we use Stabilant 22, a terminal contact enhancer that literally fills the microscopic deviations in the surface of the terminals. Airbag deployment and sensing systems are super critical of any slight changes in resistance and will trigger a warning light and associated codes. Sometimes a drop of this chemical is all the terminals require to return it to service.

All tank probes are combined inside the bussing plug. Only HiZ, LoZ, and Compensator wiring go to the processor.

These plugs are "supposed" to be snugged with cannon plug pliers or a wrench. Then safety wired in the tight direction..

The safety wire on this plug was backwards. It was not tight, but not completely loose either. When I secured it correctly, our failed fuel quantity indication problem operated normal.

Images of plug.

I would be curious to know where the wheel chocks were placed...or lack of?