What happened
On February 3, 2017, during preparations for a return flight from Denpasar (Bali, Indonesia) to Warsaw, maintenance personnel identified malfunctions in the L1 Variable Frequency Starter Generator (VFSG) and the Cabin Air Compressor (CAC). To facilitate the flight, the mechanic applied a deferral for the VFSG repair in accordance with MEL 24-22-01-02. This specific deferral required several operational constraints, including the disconnection of the faulty VFSG, the continuous operation of the APU throughout the flight, and a flight duration limit of 180 minutes.
Additionally, due to diagnostic system messages, a second deferral (MEL 21-51-01-02-02a) was applied regarding the R1 CAC. However, a conflict existed between these two procedures: according to the notes in MEL 24-22-01-02, only the VFSG on the side of the malfunctioning CAC could be taken out of service, meaning the two applied deferrals were mutually exclusive.
Upon the aircraft's arrival in Warsaw, receiving maintenance staff identified that the MEL deferral had been applied incorrectly. While the VFSG and associated power cables were subsequently replaced and the primary deferral was closed on February 4, 2017, the CAC deferral remained open due to a lack of available spare parts.
The investigation
The investigation focused on the maintenance procedures performed at Denpasar. The inquiry examined the technical requirements of the MEL entries and the decision-making process used by the maintenance technician to implement the repairs and deferrals for the Boeing 787-800.