Date: 201909
Local time of day: 1201-1800
Location: KZAK.ARTCC, HI
Weather elements / visibility: Turbulence
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A KZAK
ATC / advisory: Center KZAK
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness; Confusion; Communication Breakdown
Second reporter function: Captain; Pilot Flying
Anomaly: Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Contributing factors: Human Factors; Procedure; Weather
Primary problem: Human Factors

Narrative: There was forecast to be some turbulence along route. Around XA20 we discussed climbing because we were starting to get moderate chop and light turbulence. The Captain sent the CPDLC (Controller Pilot Data Link Communication) message and requested FL360 due to weather. While waiting for instructions; we noticed an aircraft behind us on TCAS (Traffic Collision Avoidance System) at FL360. I verbalized that the request will get denied due to traffic. At XA23 we got the message that said 'UNABLE REQUESTED ALTITUDE DUE TO TRAFFIC' all in caps like all ATC CPDLC messages. Below that line there was 'Climb and maintain FL350' in all lowercase. We saw the unable and accepted the message. The climb instructions were not observed and we maintained FL340. At XB15 we received a message to confirm altitude; you were cleared to climb and maintain FL350 about an hour ago. The Captain and I then went through all the log and found the missed clearance. We coordinated with ATC and determined we would stay at FL340. The confusion on my part came into effect because we [were] used to seeing instructions from ATC in all caps and the previous message below [was] in lower case. The message that cleared us to climb and maintain FL350 was not in the same format as other messages we received. The UNABLE CLIMB portion in all caps and then the climb and maintain in lower case led me to believe that was the previous message. I went back though all of our ATC messages and they all followed that same format with ATC in bold and our response in lower case.


Second narrative: At XA21 we entered an area of moderate turbulence; so we sent a CPDLC (Controller Pilot Data Link Communications) request to climb to FL360 due to weather. At XA23 we received a response in large lettering; 'UNABLE REQUESTED ALTITUDE DUE TO TRAFFIC;' followed by small lettering; 'climb to and maintain FL350.' We missed the clearance to climb to FL350 and maintained FL340. At XB15 we received a CPDLC message asking us to confirm altitude; stating we were cleared to FL350 about an hour ago. At XB16 we responded to inquiry; stating we saw unable FL360; do you want us at FL350 or can we stay at FL340. At XB22 [Center] responded; 'WE DID UNABLE FL360 BUT CLEARED YOU TO FL350 AT THAT TIME. You can stay at FL340.'I believe the difference in the size of the lettering on the XA23 CPDLC message led to the error of missing our clearance to FL350 along with the fact that we had not requested FL350. Nice of [Center] to do; but we missed it. I will not be reading any more CPDLC messages off the First Officer's FMC (Flight Management Computer). From now on I will select ATC on my FMC; review silently and confirm with the First Officer that we've seen the clearance in its entirety. Yes we acknowledge [Center's] XA23 message; but perhaps [Center] could have asked us to confirm altitude sooner?


Synopsis: B737 flight crew reported missing clearance from ATC to climb.