Date: 201910
Local time of day: 0601-1200
Location: MSP.Airport, MN
Light: Dawn
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: CVR
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Communication Breakdown
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate; Pre-flight
Result: General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; MEL; Procedure; Human Factors
Primary problem: Procedure

Narrative: Upon reviewing the release I noticed that the aircraft was carrying MEL 23-XX-XX ACARS System. As soon as we got the aircraft powered up and started loading the FMC I saw the Datalink Fail message and knew that MEL 23-XY-XX DATA Link Communication should also have been applied to this aircraft. While continuing my preflight checks I found the CVR would not perform its test. There is a note on MEL 23-XX-XX that says 'Note: An inoperative ACARS system could cause a CVR fault. If the CVR and/or RIPS is inoperative; refer to MEL 23-XZ Cockpit Voice Recorder System (CVR).' With this in mind I called Dispatch and had them patch me through to Maintenance Control. I advised the Controller that the CPDLC was also inoperative and that we needed to apply MEL XX-XY to the aircraft. I then informed him about the CVR failing to test. He replied that we did not need to apply MEL 23-XZ because it was already referenced to by MEL 23-XY. I did not agree with his interpretation. He consulted his 'Lead' and they still came to the same conclusion. So; we just proceeded to apply MEL 23-XY for the CPDLC. The only reason I knew we needed that MEL is because I was involved in determining that we needed to apply this MEL to our simulators. After hanging up with Maintenance Control; the Dispatcher knew I was not content with their decision not to apply MEL 23-XZ and asked if I wanted him to get the Chief Pilot [Company Communications System] on the line; which we did. Ultimately after discussing the situation with [Company Communications System]; the Chief agreed with my interpretation; but Maintenance Control still did not. I decided that I was not happy with that answer and that applying the MEL was the conservative course of action so I made a defect entry for the CVR in the logbook which necessitated the application of MEL 23-XZ.The verbiage in MEL 23-XX needs to change to guide the user to the fact that if the Datalink Communications are also inoperative that MEL 23-XY should be applied. It is not an associated MEL (as it used to be) because there are Datalink times when the ACARS may be inoperative; but the CPDLC is still functioning. This is not the first time I have had an aircraft where the Datalink was not properly documented. The vast majority of line pilots do not have any idea that they need to apply a second MEL if the CPDLC is also inoperative. Maintenance needs some training about the intention of the note about the CVR in MEL 23-XX as well. If the CVR will not test; it must be considered inoperative. I do not believe the current wording is confusing; but clearly Maintenance Control is not interpreting it the same way.




Synopsis: B737 Captain reported a lack of consensus regarding the requirement to apply an additional MEL before departing.