Date: 202105
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower ZZZ
Aircraft component: Powerplant Fuel System
Component problem: Malfunctioning; Design
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Time Pressure; Troubleshooting; Workload
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural MEL / CDL; Ground Event / Encounter Fuel Issue; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: Taxi; In-flight; Aircraft In Service At Gate
Result: Flight Crew Overcame Equipment Problem; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Aircraft; MEL; Procedure
Primary problem: MEL

Narrative: During our taxi-out and takeoff role; the Captain and I noted a Number 1 Fuel Filter Bypass Master Caution Light.  We both felt this was part of a MEL condition entered into the aircraft's ELB; but feel the MEL procedure to be both ambiguous and poorly written.  The condition should not be available for ER [Extended Range] operations.During our pre-flight planning activities; the Captain and I noted a defect of 'Repeat. LH Fuel Filter Bypass warning light was on for 5 minutes' written up on the Electronic Log Maintenance Logbook (ELB).  Maintenance indicated that 'Fuel Filter Signal Disagree' on EEC #1 was the actual issue causing this light to indicate. Maintenance deferred the fault by placing the #1 Fuel Filter Bypass Light on MEL 7304A.The ELB indicated that maintenance replaced the #1 engine fuel filter; did an engine idle run; and noted no gross contamination of the fuel in the filter.  No further CDU faults were noted by maintenance.  The number #1 engine 'Fuel Filter Bypass Light' was placarded.  Upon referencing the 737NG MEL; as far as we could tell; this is all the maintenance action items that need to be completed.As for the flight crew actions to be completed for MEL 7304A; we must only assure that the:1. 'Remaining fuel filter pressure warning system must operate normally.'2. 'Maintenance is required to verify malfunction is in fuel filter bypass warning system on initial deferral.'We noted that the flight crew actions for this MEL are rather sparse: Our normal pre-flight activities would ensure that the #2 Fuel Filter Bypass Light was extinguished and maintenance indicated they performed step 2.During our pre-flight briefing the Captain and I discussed this MEL; verified that the system was not needed for ER operations - as our flight to Destination would; of course; fall under this category.  Unfortunately; a complete discussion and briefing about this item may have fallen slightly short as we became mildly distracted with a separate and unrelated new maintenance fault in the parking brake accumulator system that was noted during our flight deck pre-flight activities.  In turn; our flight became delayed at the gate in ZZZ as the parking brake system needed to be deferred using another MEL.  Additionally; we waited for connecting passengers from an inbound ZZZ1 flight.We pushed back from the gate in ZZZ started to taxi out for a departure on Runway XXR.  During a turn on our taxi to the runway; we both noted that the Master Caution light illuminated for the Number 1 Fuel Filter Bypass Master Caution Light.  I canceled the light and the Captain decided he wanted to stop in a safe area to deal with the possible issue. I agreed and ground control in ZZZ allowed us to stop on Taxiway 1.As the Captain stopped and set the parking brake; we both stated how confused we were about the fuel filter bypass light illuminating.  I checked the 737QRH for this light and it only indicated that 'erratic engine operation' may occur 'in flight'.The Captain and I discussed how it was extinguished after engine start; and only illuminated momentarily on taxi out; during a turn; and it was now extinguished again.  We referenced MEL 7304A again from the 737NG MEL.  Nowhere could we find whether the 'inoperative/placarded' light should remain illuminated; extinguished; or momentarily indicate.  We discussed that most MELs similar to this deferral would explain to the flight crew that a caution light may illuminate.  (See example MEL 78030: 'NOTE: The 'Master Caution' and 'ENG' MAY illuminate when performing master caution recall.')The Captain decided to call Dispatch and conference in maintenance while stopped on the taxiway.  We told our Dispatcher about our issue and he brought in Maintenance on the phone line.  The initial maintenance personnel transferred us to the 'Specific area.'  The 'Specific area' advisor told us that this momentarily illumination was normal and part of the MEL; the write-up was already in the ELB; and that it is 'OK to continue.'During our conversation the Captain; Maintenance; Dispatch; and I came to the conclusion:1. There is no maintenance procedure to deactivate this light.2. The issue is already indicated in the ELB.3. Maintenance stated that was a normal and 'very common' occurrence on the 737 fleet and we are OK to continue.After finishing our conversation with maintenance and dispatch; the Captain and I amended our takeoff briefing to be prepared for this light to illuminate during the takeoff roll.  I; as the PNF; would indicate it was this light; cancel the alert; and the takeoff would continue in both the low speed and high-speed regimes.During the takeoff roll; at approximately 120 kts (at least 20 kts below V1); the Number 1 Fuel Filter Bypass Master Caution Light did illuminate.  I called out that the fault adding 'expected!'; canceled the Master Caution Light; and we continued the takeoff.  The light extinguished itself shortly thereafter.  The light never illuminated again throughout the rest of the flight.  The flight continued uneventfully to ZZZ2.


Second narrative: The maintenance release had an open write up for the 'fuel filter differential pressure warning system'.  We read both the maintenance release and the MEL (7304A). Not much information other than the other system must be operative and if you do get the other filter bypass light; you should assume both engine filters have an impending bypass condition. Startup was normal. On taxi out; we got the left fuel filter bypass light; the side that was written up. It caught us off guard; because no where did it say that the light might come on. We pulled over and got a phone patch to Maintenance Control to discuss the light. Maintenance Control said it was normal with the problem and we were OK to continue. The light went and we taxied for takeoff. At approximately 120 knots; the light illuminated again. We continued the takeoff and the light went out shortly after we were airborne. The light did not illuminate again until taxi in.


Synopsis: B737 NG First Officer reported differences between MEL narrative and the actual indications of the aircraft systems when departing on an ETOPS flight.