Aircraft Wire - Substituting two shielded pairs for 4 conductor shielded wire in RVDT circuit.

[kontiki99]

All,

Ref subject, we were looking at replacing the wire to a flap position RVDT circuit.

OEM used one twisted shielded pair for RVDT excitation. This RVDT sensor has two secondary circuits, the 4 wires from the RVDT secondary were in a 4 conductor shielded run. Excitation is 28V, 400Hz.

The materials folks were not sure they could get the 4 conductor shielded wires on location in a timely manner. We were thinking of telling them to go with 2 twisted shielded wires, just to get the aircraft back in service, ultimately, they found the wire.

I don't see anything wrong with the substitution myself, though it's not a substitution I've approved before. I assume the only reason the OEM went with the 4 conductor shielded was to save weight.

Anyone know of a reason you couldn't swap a 4 conductor shielded wire with 2 twisted shielded wire pairs?

Thanks,

In theory, there is no difference between theory and practice however, experience suggests that in practice, there is!

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm affiliated with.

 

[VEBill]

In addition to all the 'Airworthiness' formal paperwork requirements...

One would need to be very careful to make sure that the twisted pairs were assigned correctly. Of course that applies to the original 4-wire cable too.

There doesn't seem to be any reason why the two output signals couldn't each be within their own shield. The shields would be terminated together at each end. Topologically, it's nearly equivalent; just less coupling between the two output signals.

Of course it might be better, faster and cheaper to 'Overnight' in the original PN cable than tackle all the paperwork.

 

[kontiki99]

We are the people you overnight things on. When the AOG desk gives you a long lead time and you have a freighter full of priority packages, we do everything possible, within the scope of the MEL and FAA Minor approval authority to get it out on time. The alternative is an expensive transload and recovery flight.

Having said that, when I type in, "Repair uses processes and materials equivalent to OEM specifications to restore the part to its design condition." I want it to be true.

This particular RVDT has two secondary coils, all 4 conductors were in the same shielded wire. It's one of those things. As long as I just paired the two wires from each secondary so there was no chance of coupling secondary A to secondary B, I see no possible issue.

The other side it that it's in a flap position system and we were addressing a history of flap asymmetry faults as it is. A flap asymmetry condition constitutes an in flight emergency on a revenue flight.

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[tbuelna]

There might be a difference in terms of functional fault tolerance between the two wiring arrangements. The arrangement of conductors and shielding was designed like that for a reason. And unless you have an understanding of why the designers of the system chose to use that particular arrangement, it would be best not to change it.