B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Type 2656
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL
General None Reported / Taken
Aircraft; MEL
Dispatched with a placard; MEL XXXX; for the lower Multi Function Display Unit inoperative. Placard was authorized on the previous day. Checked and complied with the MEL before departure. Specifically; we verified that the engine indications could be manually selected to either the Captains or FOs inboard display unit and that all other display units were operative. Even though we complied with the MEL before departure; during cruise the FO and I had further discussion about the MEL verbiage and the indications that we were able to see. Specifically; we were able to see all engine indications compacted on the upper MFD but were not able to view hydraulic quantity and pressure. Earlier; we initially dismissed not seeing this as a condition of the MEL placard but we increasingly thought that this didn't make sense. Decided to send a message for Maintenance to meet the aircraft upon arrival and followed up with a radio request to Maintenance to discuss the MEL placard. Made the following Aircraft Maintenance Log entry: With current placard for lower display unit inoperative unable to view hydraulic quantity and pressure. Discussed entry with Maintenance and briefed oncoming crew.Possibly an additional unaddressed system display indication failure. If this is true; it would have been better if it was caught sooner by the Flight Crew or maintenance. MEL addresses being able to see engine indications but doesn't address system indications.
During the flight we noticed that the hydraulic information (pressures and quantities) was missing and could not be pulled up by pushing the SYS switch which is normally the case. That information was missing and could not be found during or after flight. Possible software issue that does not allow for hydraulic information to be displayed.
B737 flight crew reported that with the Lower Multi-Function Display Unit deferred; all engine instruments were displayed except for hydraulic quantity and pressure on the Upper Display Unit.