Date: 202005
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Oxygen System/Crew
Component problem: Design
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: After starting the #2 engine; the pushback crew noticed a large amount smoke behind the engine which quickly went away.  Because there was very little wind. Some of this smoke entered the aircraft before we finished starting the #1 engine. Other than a slightly odd smell; it seemed to disperse quickly.  I verbalized this to the pushback crew and the First Officer. Additionally; I mentioned to the FO (First Officer) that I would call the flight attendants and make an announcement over the PA. Before taxiing; the F/A's (Flight Attendants) called to report the smell and that there was some possibility of smoke or haze in the cabin. I communicated through the F/O that we would check back with them after we taxied a ways to see if it was improving or getting worse. I also made a PA to the passengers.  At this time I believed the problem was that we had ingested exhaust fumes from the engine start. After taxiing for about four minutes; I checked back with the F/A's and the front two could neither see nor smell anything. The back two said they could still smell something; but not see anything.  They also stated that it was not getting better; but neither was it getting worse. I discussed the situation with the F/O and he felt there was no reason not to go; but thought one more check with the F/A's might help.  Since we just talked to them and they were alright with going; I elected to continue. We executed a normal takeoff and upon approaching the cleanup altitude; the odd smell returned and I stated that this was something that required us to return to ZZZ. Simultaneously; the F/A's also reported the smoke and fumes had returned. I am not sure of the precise order of events; but we donned our oxygen masks; notified ATC; and requested a return to the airport.With our masks on; the F/O and I struggled to communicate with each other and ATC. Basically the design shoots so much oxygen into the mask that it overpowers the microphone inside it.  We both operated the mask correctly and spent some effort and time verifying the settings; but nothing worked. For this reason; I elected to make a relatively quick return to the airfield. In all my experience (43 years); I have never had a problem like this with a mask.  I asked Maintenance Control if I should write both masks up and said no because they frequently get complaints like this. We flew a box pattern and made an uneventful landing with CFR (Crash Fire Rescue) truck giving us a good check out.  After coordinating with the company and flight attendants and briefing the passengers; we taxied to the gate. During the taxi we did not experience anymore smoke and fumes. Maintenance technicians checked the aircraft out; informed us that the 'B' system hydraulic system was over serviced and had entered in the ventilation system where it produced the smoke and fumes. Continued flying would not have improved our situation; so I was glad we made a prompt return. I think the decision to take the aircraft flying was not good in retrospect; but all of us thought there was no reason; at the time; to not go flying.  I have had similar situations just like this and the problem went away once there was sufficient air moving through the cabin. Looking back; even though there was contradictory inputs from the F/A's; there was still enough to warrant a return to gate. The problems with O2 masks is unacceptable.  Maybe they would have worked better at higher altitudes (we only climbed to 6000 ft); but it made the flight deck workload very high and challenging.




Synopsis: Flight Crew flying 737-800 aircraft encountered smoke and fumes in cabin after takeoff.