Date: 201206
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Dusk
Aircraft: MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: APU Electrical
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Powerplant; Maintenance Airframe
Human factors: Communication Breakdown; Situational Awareness; Confusion; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Maintenance
When detected: Aircraft In Service At Gate
Result: General Maintenance Action
Contributing factors: Company Policy; Aircraft; Human Factors; Procedure; MEL; Logbook Entry
Primary problem: MEL

Narrative: APU Aux relay was stuck closed; which caused the Left Electrical Bus to be powered no matter what position the switch was in. When the flight crew started the left engine; the Left Engine Generator failed to pick-up the Bus because it was stuck; being [electrically] powered by the APU. The flight crew assumed they had a bad Left Generator and disconnected it; and installed a crew placard. This situation would not allow the [flight] crew to isolate the Left Bus and remove power in an emergency (relay stuck on) and also caused them to disconnect a good CSD.   This MEL was previously a non [flight] crew placardable item and was recently changed. I believe it requires troubleshooting to determine why the generator is not coming online.  The item should be a Maintenance MEL only; to accommodate trouble shooting.

Callback: Reporter stated he is a Line Mechanic who also works with avionics technicians. His concern involves the lack of adequate Maintenance Troubleshhooting prior to deferring an Engine CSD Generator on MD80s under their current flight crew placarding procedures. He understands that from a pilot's perspective; when the Left Engine Generator failed to pick-up the Left Electrical Bus after the Left Engine started; the situation may have looked like a bad CSD/Generator. So performing a flight crew placard of the Engine CSD may have seemed appropriate; but; the procedure actually locked the pilots into a situation where they could not have isolated the APU Auxiliary Bus if the APU; or APU Generator started sparking; unless the APU was shut down. And with a good Left Engine CSD unnecessarily disconnected; the flight crew would have only one generator operating.      Reporter stated there are two toggle switches on the APU Overhead Electrical Panel for the APU Left and Right Bus to supply electrical power to the main AC Left and Right Electrical Bus through the APU left and right Auxiliary Power Relays.  Avionic's Technicians tell him the same APU AUX relays have been found stuck in the 'ON' position before; they just fixed the problem; but never reported about the recurring issue.  What's important to keep in mind is the ability to isolate an electrical problem before a situation could get worse; a procedure the pilots could not perform under the conditions he describes.  Adequate Maintenance troubleshooting is the key to identifying what the problem is; in order to avoid an incorrect and possibly unsafe MEL deferral. That's why he and other Maintenance personnel believe the CSD/Generator issue should be transferred back for deferral under an MEL Maintenance Procedure; instead of just allowing a Flight crew deferral placard to be applied.



Synopsis: A Line Mechanic reports how a change in their Air Carrier MEL procedures from a Maintenance procedure to a flight crew MEL placard deferral procedure; contributed to a 'good' Left Engine CSD/Generator being disconnected on an MD80 aircraft. Lack of troubleshooting under the flight crew placarding procedures prevented the flight crew from isolating the APU Auxliary Electrical Relay that was stuck in the 'ON' position.