Date: 201810
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Distraction
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: After arriving early; we stopped short of the J-line and waited several minutes for our ground crew to position and marshal us to the gate after an uneventful flight and landing. We parked the aircraft at [the] gate and accomplished the parking flow and checklist. I then opened the flight deck door and proceeded to say goodbye to the deplaning passengers. I immediately noticed excessive noise from outside the forward entry door. It was then I looked back into the cockpit and realized we failed to shut down the left engine. I immediately shut it down and the noise ceased. Apparently; we accomplished the parking checklist without visually verifying the left start lever was in the cutoff position.


Second narrative: The flight was unremarkable up until parking at the gate. The flight arrived 19 minutes early and it was a short taxi from Runway XXL to the gate. The Captain ordered the shutdown of the #2 Engine during taxi and I shut it down normally. He had started the APU and I switched electrical power to the APU in the normal sequence. After another minute; I closed the #1 Engine bleed valve; turned on the APU Bleed Air Switch and checked the packs switches in auto to provide more air to the cabin. We stopped just short of the gate to wait for the marshaling crew then taxied forward and parked at Gate XX. He set the parking brake. I don't remember specifically if the Captain ordered the shutdown of the #1 Engine but I assume he did and that I simply overlooked it and immediately accomplished the rest of my parking flow items. A few seconds later; he called for the Parking Checklist. I announced it and we did the required challenge and response of all steps; including 'Engine Start Levers;' 'Cutoff.' I usually look at each switch/lever as I read the checklist. I remember specifically doing a visual check of the start switches and transponder. I did not realize I had not placed the #1 Start Lever in Cutoff. I called the checklist complete and we both continued with other post flight duties. The jetbridge was positioned against the aircraft and the main cabin door was opened. Passengers started deplaning. The Captain had donned his jacket and hat and was standing outside the flightdeck doorway saying goodbye to every passenger; as he normally does. I was looking out the left window to see if external air was hooked up when the Captain quickly stepped forward and reached by me in order to Cutoff the #1 Engine. He said he had noticed the unusual noise level and then realized the engine was still running. That was the end of the incident. No one was hurt; no equipment was damaged; and no one else mentioned to the flight crew that they had noticed the error. I suspect the ground crew realized it but the Captain acted before they had a chance to say anything.


Synopsis: B737 flight crew reported neglecting to shutdown the left engine after parking the aircraft at the gate.