Incident Overview
On 3 February 2107, before a return flight from Denpasar (Bali, Indonesia) to Warsaw, a maintenance mechanic performing ground handling identified faults in the Variable Frequency Starter Generator L1 (VFSG L1) and the Cabin Air Compressor (CAC R1). The mechanic applied deferrals according to the Minimum Equipment List (MEL) to postpone repairs.
Deferral Details
For the VFSG L1 fault, MEL 24-22-01-02 was applied, which mandated the following conditions:
- The inoperative VFSG was to be disconnected.
- Both APU starter-generator systems were to operate normally.
- The left APU generator control unit was to function normally.
- The APU battery and its charging system were to operate normally.
- The engine associated with the fault was not to be started on the ground at ambient temperatures above 45°C, provided the oil temperature of the remaining VFSG was equal to or below 80°C.
- The APU was to be started before departure and run continuously throughout the flight.
- Flight time to landing at a suitable airport was to be within 180 minutes.
- Repairs were to be completed within three flight days.
Additionally, due to diagnostic system messages, the mechanic also applied MEL 21-51-01-02-02a for the CAC R1 fault to defer its repair.
Conflict in Deferrals
According to MEL 24-22-01-02, Note 4, only the VFSG on the side of the faulty CAC could be deactivated. This meant that the two deferrals applied were mutually exclusive, as the CAC R1 deferral required the associated VFSG to be inoperative, while the VFSG L1 deferral also rendered that generator inoperative. The simultaneous application of both deferrals was therefore improper.
Post-Flight Correction
After the aircraft returned to Warsaw, the receiving mechanic performing ground handling identified the incorrect application of the MEL deferrals. Technical services subsequently replaced VFSG L1 and its power cables. MEL 24-22-01-02 was closed on 4 February 2017 (the year is presumed, though the incident date is stated as 2107). The deferral for CAC R1 under MEL 21-51-01-02-02a remained open due to a lack of spare parts in inventory.
Probable Cause
The probable cause of the incident was the application of an incorrect MEL deferral by a maintenance mechanic.
Preventive Actions
The operator recorded the event in its Safety Management System (SMS) database and classified it according to the criteria adopted by PLL LOT SA. It will be included in safety performance indicators monitored under the principles of the Aviation Safety Management Manual.