Date: 202001
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch; Descent
Airspace: Class B ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Aircraft

Narrative: Aircraft had engine wash [the] night before flight. During taxi and takeoff noticed foul stench from aircraft conditioned air. Went from bad during takeoff roll to worse on initial climb. Slowly dissipated but not without crew consideration of in flight return.  Finally tolerable at about 6-10 thousand feet and didn't notice again until descent below 10 thousand feet. Odor was foul but not threatening.


Second narrative: When the thrust was pushed up for takeoff a bad; musty odor came through the flight deck vents. It smelled like an old; musty; wet basement. As we climbed above the clouds (around 1;000-2;000 [feet]) the smell began to dissipate. It was bad enough that I reached down and partially closed my gasper and concurrently asked if anyone else (we had a jumpseater) could smell that. If the smell had lasted any longer; I thought about calling for the Smoke; Fire; and Fumes QRC but it had begun to dissipate by that point. We talked about the smell and decided it was probably related to the foggy; humid conditions in ZZZ. It was a 'wet pack' smell. It did not smell like smoke or have an acrid; or burning scent. On descent into ZZZ1 as we began to pass through a cloud layer (somewhere around 18;000 [feet]) the smell came back. As soon as we passed out of the clouds the smell began to dissipate. It was at this point that the Captain decided to make an ELB entry. Once at the gate we talked with a mechanic to pass on the conditions that we noticed the smell: 1. engine bleeds on 2. first noticed at a high thrust setting but later came back at a lower setting 3. when in a moist environment (clouds and fog). A little while later; ZZZ1 Flight Ops called and asked if I could meet with a mechanic to discuss the event again since I was waiting for a deadhead flight and they were trying to troubleshoot. As we were discussing where to meet; they realized the plane had undergone a compressor wash on the overnight and attributed the odor to that. It made sense because the Captain had told me he was surprised on the walk around that the engine lips had some type of greasy liquid on them; but thought it must have been residual deice fluid. He also had noticed on the preflight that the engine bleeds had been turned off; which is an unusual configuration to find while preflighting.


Synopsis: B737 flight crew reported a 'foul stench' during taxi and takeoff which dissipated and returned on descent. Captain suggested the likely cause was a 'compressor wash' on the overnight layover.