MD-90 Series (DC-9-90) Undifferentiated or Other Model
Maintenance Airframe; Maintenance Powerplant
Maintenance Lead Technician 1; Maintenance Technician 13
Troubleshooting
Aircraft Equipment Problem Less Severe
Flight Crew Returned To Gate; General Maintenance Action
Incorrect / Not Installed / Unavailable Part; Staffing
[A MD90] arrived for a routine scheduled maintenance overnight check. The flight crew that operated the inbound flight noted a discrepancy in the logbook concerning a hydraulic quantity fluctuation. During troubleshooting per MD90 [Fault Isolation Manual] FIM 29-30-00 air was found in the right hydraulic system. During flushing and subsequent servicing of the hydraulic system per MD90 AMM 12-13-01; it was determined that the source of the air in the hydraulic reservoir was the right elevator/rudder power control accumulator. At that time the station was out of stock of the normal allocation of one power control accumulator. It was also noted at this time that the elevator/rudder power control accumulator is not a deferrable item. It was then determined that the thrust reverser accumulator was a deferrable item and the part number of that accumulator was effective for the elevator/rudder power control accumulator position. As the thrust reverser power control accumulator was found to be in serviceable condition it was decided upon to swap the inoperative elevator/rudder power control accumulator for the operational thrust reverser accumulator and then defer the replacement of the thrust reverser accumulator as per MD90 MEL 78-33-04. The thrust reverser accumulator was installed per the MD90 AMM 78-31-02. At this time a leak check was performed of all the hydraulic and pneumatic lines associated with the thrust reverser accumulator and no leaks were noted as required per both the MD90 AMM 78-31-02 and the MD90 MEL 78-33-04. The aircraft was released for service to resume its scheduled flights the next morning. [The aircraft] then operated normally for 4 days in which time it made 18 flight cycles to the amount of 40.6 block hours of operation without trouble noted to this system. [Five days later] during the first scheduled flight of operation; on taxi-out the flight crew reported odor and haze in the cabin. It was determined that the APU inlet had become contaminated with hydraulic fluid. The source was found to be the right trust reverser accumulator leaking from the bottom of the hydraulic connector. The right trust reverser accumulator was replaced and appropriate maintenance was performed to the APU so that [the aircraft] could be returned to service and normal operation could continue. A thought regarding actions that could have prevented this from occurring; if there was a maintenance action in the MEL that required subsequent inspections repeating the initial inspection of the affected accumulator until the item was corrected; it is possible that this compromised accumulator; which developed a leak; a considerable amount of time after it was deferred per MEL; could have been detected prior to affecting the aircraft operation and subsequently the passengers inconvenience. While this particular MEL was still within the acceptable timeframe of operation; this incident could also have been avoided had the accumulator been changed and the deferred maintenance been corrected in a more expedient manner.
When the Maintenance technicians identified a defective part in a MD90 hydraulic system; they discovered that a replacement part was not available; and the system was not deferrable. They then identified a comparable part on the aircraft that was installed in a deferrable system. They swapped the units; and deferred the affected system per an approved MEL. The plan worked well until several days later when the inoperative accumulator; now installed on the thrust reverser; developed an external leak. The released hydraulic fluid contaminated the APU inlet.