MD-83
Confusion; Distraction; Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL
General Maintenance Action
Procedure; Manuals; Aircraft; MEL
With APU already running for entire flight segment per MEL 24-04A; on taxi in we shut down right engine. Thinking that Maintenance had properly placarded for the previous write-up; we were both surprised to see the right GEN AC BUS go dead along with the First Officer gauges and all other right AC powered items.  My question is; would we have been single generator (Left AC; no Cross Tie) in flight IF we had lost right engine?  I.e. Multiple emergency situations? Was placarding the AC Crosstie Inop the right solution to the previous write-ups about APU not picking up Right AC BUS on shut down? And has replacing the AC Crosstie Relay solving the problem now? If so; then perhaps jet should have never flown with crosstie relay on placard. To my mind; it really wasn't a crosstie issue since APU does not crosstie on the ground anyway. But APU was consistently failing to pick up Right AC BUS.  I do not claim to be the systems expert; however it does concern me that perhaps if we had had the right engine fail; we could have also been single generator ops if the APU did not pick up the Right AC BUS.
An MD-83 was dispatched under MEL 24-04A because of an AC CROSS TIE fault with the APU continuously operating but while taxiing in after landing when the right engine was shutdown the Right AC BUS lost power.