B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 220; Flight Crew Total 720; Flight Crew Type 710
Situational Awareness
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Air Traffic Control Provided Assistance; Flight Crew Returned To Departure Airport; Flight Crew Landed in Emergency Condition; Flight Crew Landed As Precaution; General Flight Cancelled / Delayed
Aircraft
Sleep; layover; and crew brief all uneventful beginning day 2 of a 2 day pairing with the same flight crew. [Maintenance release] listed engine swap out for Number 1 engine and ETOPS verification required. We briefed the engine change; examined on preflight and found no issues. Further preflight briefing discussed monitoring the engine for any abnormalities. Preflight and engine start normal and pushed from gate a few minutes early. Started both engines due to short taxi and the fact that the engine had been changed out so we wanted time to assess how it was doing. No issues for engine start or before takeoff checks. All indications and smells appeared normal. Cleared for takeoff. Engine spooled normally. At rotation (above V1); I smelled a faint odor from what appeared to be the FO (First Officer) air vent and determined I would safely get the aircraft airborne and out of a critical phase of flight before further assessment. There were no other cockpit indications or lights that I observed. Approximately 100 feet off the ground the flight attendant call button rang twice to notify the PM (Pilot Monitoring) of a smell in the main cabin.  The smell went from faint to noticeable. The odor smelled like a burning fluid vice electrical type smells.  We [advised ATC] with departure and complied with critical action items for smoke and fumes in the cockpit. ATC leveled us at 5000 ft and set us up for a downwind to return for a visual arrival. As part of troubleshooting and seeing no visual smoke or fumes; we checked and the smell dissipated within approximately 2-5 minutes. Our CRM and piloting focus was on getting an aircraft safely on the ground; rather than running further items in the QRH beyond critical actions. The aircraft was airborne for 11 minutes total.  I continued to fly the aircraft while the Captain worked radios and consultation with the flight attendants. They felt the odor subsided as well. I landed the aircraft on speed and softly (albeit overweight due to the nature of the emergency) and cleared [the runway] where we were met by emergency crews.After touchdown; we instructed the passengers to remain seated and continued with the smoke/fumes checklist in the QRH. I then discovered that the recirculation fans had been off (incorrectly); but I don't suspect that any increased load on the packs caused the odor that subsided.  Airflow volume remained normal throughout. We left them off per the QRH. We opted to not troubleshoot isolating each packs in latter steps of the checklist. No further smells on flight deck or cabin other than some lingering odor. Shut down engines and ran the APU for fire crews to assess. No further indications of a problem inside or outside the aircraft.We consulted with maintenance; who suspected residual fluids due to the engine change. Maintenance approved a single engine taxi on the number 2 engine back to parking.  We returned to the gate without incident and deplaned the passengers after appropriate checklists.After parking checklist we were met by maintenance. One member speculated that the engine change had not had a full test cell engine run due to a change in company or practices. I have no way to personally corroborate this; but it seems quite reasonable that IF this is correct; such an odor would have been detected prior to flying it with passengers on board.
At rotation there was a strong fume/smoke smell. Immediately upon lift off flight attendants in rear reported the same. I donned oxygen masked [advised ATC]. Fume/smoke smell dissipated around the pattern. Immediately returned to [the airport] for a smooth; minimal sink rate overweight landing. Fire Crews met us; no evacuation.
B737 flight crew reported that they smelled smoke in the cabin right after lift-off.