Date: 202106
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: IMC
Light: Daylight
Aircraft: B737 MAX Series Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Taxi
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Communication Breakdown; Distraction; Workload; Situational Awareness
Second reporter function: Captain; Pilot Flying
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: After starting the #1 engine during a single engine taxi out; I shut down the APU. This was done immediately after my workload increased significantly. Since we were on a MAX we chose to start the second engine early on the taxi out to [Runway] XXR; but engine motoring continued until we were crossing Runway XXL. My attention was split between monitoring the engine start and looking outside during the runway crossing. By the time we were clear I was concentrating on the remaining taxi distance for our three minute warmup. I went through the normal After Start Flow without thinking about leaving the APU running. My mistake was not caught by either of us on the Before Takeoff Checklist; only on the climb out through about 8;000 feet.We should have slowed down. There were many factors causing us to rush. The gate was unusual for (Company) operations; and we had to deal with broken jetway stairs and ground power that was disconnected during boarding. These were both stress inducers; and we should have recognized that we were being pushed into the Yellow before we even left the gate. The Captain was unfamiliar with the airport. I could have done a better job walking him through the taxi procedures. I should have realized this would cause a distraction while starting the second engine. We were on a MAX departing for an ETOPS flight; and fuel is almost always a consideration during taxi out. We chose to delay APU start at the gate and single engine taxi to conserve as much as possible. We even briefed the earlier start because of lengthy MAX engine motoring; but it still wasn't early enough to be complete before crossing [Runway] XXL. We should have started the engine earlier so I could concentrate on one thing at a time. Both the Captain and I were new to ETOPS flying. We were in and out of the Yellow during preflight as we worked through the extra preflight procedures. We should have given ourselves more time to complete these tasks.


Second narrative: During taxi out; at an unfamiliar airport; a delayed engine start of engine #1 was performed by the F/O; due to a long taxi to the runway. During the After Start Flow performed by the F/O; he inadvertently shut the APU off; which was required for this ETOPS flight. I missed the fact that the APU was OFF during the 'Before Takeoff' checklist; and a normal takeoff was performed. During the climb; at approximately 7000 feet; it was discovered that the APU was off; so I started the APU prior to reaching 10;000 feet. The flight continued without incident. At no time did the aircraft fly in Class II operations with the APU off.


Synopsis: B737 MAX flight crew inadvertently shut down the APU during taxi prior to an ETOPS flight.