Date: 202201
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ramp ZZZ
Aircraft component: Coalescer Bag
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Troubleshooting
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew
When detected: Taxi
Result: Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Gate; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Started #2 Engine only after pushback for single engine taxi with a sharp left turn from gate. The air system was configured with both packs supplied by APU air during after start flows and remained in that configuration for taxi. Upon the start of taxi we both started to smell something bad. Not smoke; not acrid; but foul. At first I wondered if the First Officer had a gastrointestinal event but it was stronger and different than what would be expected from that. Then; he mentioned that he smelled it too. It was the smell of previously documented cases of wet sock or gym smell. The aircraft; coincidentally; had been written up on it's previous flight for this smell. We stopped our taxi on taxiway Alpha and were eventually moved short of D on A. The packs were turned off at this point. The First Officer donned his O2 mask and I opened my window for ventilation We coordinated with station MX; Ops; and Ramp Control for a gate return. I wrote up the event the best I could with the 4 lines provided in ACARS. MX met us after the parking check and I filled out the MX fume checklist. The smell was only in the cockpit and lingered until we left the aircraft. You could smell it more after leaving the cockpit and then returning as our noses were getting used to the smell. I called the Chief Pilot and texted via to Dispatch that we were a gate return after I received a query from Dispatch via ACARS.


Second narrative: [Flight crew experienced] intense and dirty sock smell on taxi-out.Duration [was] short]; a few minutes intense smell; lingering longer in cockpit. After engine start (only number 2 engine was started for taxi); initial power application to taxi-out (35-40% N1); smell noticed within one minute of power application; intensified for the next few minutes; then First Officer went on 100% O2 and Captain opened cockpit window.Smell; Dirty sock and/or a bad biological odor.Bleed and pack configuration:- During engine start; all bleeds and crossfeed open.- After start checklist; engine bleeds turned off and both packs turned on.Flight attendants were polled and none of them smelled anything in the cabin.  We returned to the gate with the Captain's window open and the packs off.The APU and both packs had been running all during boarding with no smell noted.The aircraft had been written up the previous flight for fumes on descent. Maintenance could not duplicate it and had cleared the previous write up.Smell seemed to stay confined to the flight deck. Only after blocking in and opening cockpit door did the flight attendant say she could smell it; and indeed; the cockpit; upon returning to it; smelled like a 'truckstop bathroom'.


Synopsis: Air carrier flight crew reported a fume event of dirty socks during departure taxi. The flight crew executed a ground gate return.