Lack on an equipment grounding conductor makes the use of GFCI circuitry impossible. The Military uses an 0.093" dia. SS insulated cable for static voltage drain, but this is not a "conductor". Thus, any internal fault is seen by the source circuit breakers as an increase in load current. I notice that commercial jetways also have the same problem, indeed, they carefully isolate the aircraft skin from any metal contact with the jetway mechanism. Finally, consider that many of the "Legacy" aircraft use the structure as a Neutral return for the MIL-STD-704 Grounded-WYE, 400HZ service power.