B737-800
Maintenance Airframe; Maintenance Powerplant; Other Maintenance Controller
Time Pressure; Troubleshooting; Communication Breakdown
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Landed As Precaution; Flight Crew Diverted; General Flight Cancelled / Delayed; General Maintenance Action
MEL; Human Factors; Aircraft; Procedure
Aircraft X called while in flight. He had a momentary fire warning light for the aft cargo pit. I advised him to follow the QRH and he subsequently diverted to ZZZ. NOTE: The B fire protection system was currently on MEL so the current fault was on the A side. He also told me that the discharge light was on which seemed odd to me because he didn't activate. Once the aircraft landed in ZZZ; I had contract maintenance pull and reset CB per maintenance notes in MEL XX-XX-C; which is what I used to get the airplane out of ZZZ. I missed step D of the MEL which should have had me INOP BOTH the forward and aft pits because one of the bottles is used. I thought because we still had a good bottle; I could dispatch with the aft pit empty. It appears that I should have used MEL XX-XX-B and deferred BOTH forward and aft pits. I found this MEL to be very confusing. XX-XX-C. The system has two fire bottles.
Maintenance Controller reported misapplying an MEL to the Cargo Fire Detection/Suppression system and the aircraft flew in a non airworthy status.