Date: 201912
Location: ZZZ.Airport, US
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Physiological - Other
Second reporter function: Pilot Flying; First Officer
Anomaly: Flight Deck / Cabin / Aircraft Event Illness / Injury; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant; Person Flight Crew
When detected: Taxi; In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We started both engines and turned off the APU. Upon taxi; I noticed an acrid/dirty socks/musty smell when I added power. As I backed off the power on taxi; the smell seemed to dissipate. On power up taking the Runway (XXL); the smell returned. I made the decision to continue the takeoff and said to the First Officer that if this doesn't dissipate and go away we should return and he agreed. Once airborne; it got even worse and became a significant fume event. We donned O2 masks and goggles; I [advised ATC] due to fumes in the cockpit and we returned to land on [Runway] XXL. The First Officer flew and landed. I talked to ATC and ran the Fumes/Odor/Smoke Checklist. We landed uneventfully 2;000 lbs overweight.  Holding short of the runway; I called back to the Purser and asked him if he smelled anything. He said he didn't but would check in the back. I told him what we had experienced and that we would be returning if it continued. According to the flight attendants after landing; the smell was significant in the back of the plane and they said their eyes were burning as well. Upon arriving back at the gate; we were met by ARFF personnel and [Company] Maintenance. Firefighters boarded the plane and noticed the fumes as well; which were still present. The mechanic said they had done a 'compressor wash' the previous night and that it was his opinion that oil may have been freed from the compressors and entered the cabin. I was handed a Smoke/Fumes/Odor Form to fill out; which I did and returned it to Station Operations. Although some of the flight attendants complained to me about headaches and eyes burning; they declined to be checked out and told Inflight Operations they were fine and good to continue. I called the [Duty Manager] and we completed the human factors checklist. My Chief Pilot called to check on us and we said that while we may change our fit-for-duty status; for now; we were OK to continue. After talking about this more with the First Officer; we decided we were no longer fit for duty and advised the [Duty Manager].


Second narrative: During pushback from Gate XX at ZZZ; #2 engine was started normally; then First Officer began start of the #1 engine. After #1 engine light off; ground personnel told the Captain that an access panel had opened on the #2 engine. Upon the completion of the #1 engine start cycle; the number #2 engine was shut down; and ground personnel re-closed the panel. The number #2 engine was re-started without incident. Upon initial taxi; the Captain and First Officer smelled a strong oily; dirty socks smell. We checked with the cabin crew and nothing was noticed outside the cockpit. Fumes seemed to subside some during taxi to Runway XXL. Upon taxiing into position near the runway; with 40-45% power settings; the fumes seemed to reappear. Captain discussed the fumes with me and we decided to takeoff; and if the fumes persisted; we would [advise ATC] and return to ZZZ.Almost immediately after takeoff; the fumes became very strong. Crew immediately donned oxygen per the immediate action items and [advised ATC]. ATC provided amended clearance; and vectors to return to RWY XXL. First Officer; who was Pilot Flying; continued flight duties while Captain ran appropriate checklists; and coordinated our return with the requisite entities. First Officer executed an uneventful landing. Captain taxied back to Gate XY. Maintenance personnel boarded the aircraft along with fire crews; and said they had conducted a compressor wash prior to our event and suspect it was residual oil. After deplaning; Captain and First Officer discussed events with [Duty Manager] and Chief Pilot. Despite passing the company 'Fume Event Checklist'; Captain and First Officer declared themselves no longer fit for duty.


Synopsis: B737 flight crew reported a significant fume event shortly after takeoff resulting in an air return.