Air Carrier First Officer reported a fume event shortly after takeoff. After trouble shooting and evaluating the situation flight crew returned to departure airport.
Synopsis
Air Carrier First Officer reported a fume event shortly after takeoff. After trouble shooting and evaluating the situation flight crew returned to departure airport.
Narrative
I was on my third revised assignment of the day. Scheduling changed my schedule and gave me approximately 50 minute notice for [a long] flight. I was rushed; in preflight; cockpit setup; etc. was completed normally. We were delayed approximately one hour due to passenger connection issues; baggage and other reasons I'm sure. Normal start up and taxi out. I was the Pilot Monitoring and we were cleared for takeoff. Everything was normal until shortly after rotation. The A-Flight Attendant called us and informed us of an unusual odor in the cabin. They said it was worse in back. The Captain notified ATC we needed to level off and work through some issues. I reduced temperature on packs. The flight crew also started detecting the unusual odor. It was different than a warm pack odor; but didn't smell like electrical; pneumatic; or hydraulic type fires. At that point a deadheading First Officer (F/O) got up and went to the rear Flight Attendant (F/A) station and was feeling around for a fire. The smell was getting worse; but the deadheading F/O spoke with me and thought it smelt like a warm pack. Passengers were looking around in a concerned manner also we were informed. Discussed situation with the Captain and at this point we had used enough fuel that we would be short fuel at our ETP (Equal Time Point); so we elected to return to ZZZ. We were overweight and discussed holding to burn off fuel; but were concerned if indeed there was a fire in an unmonitored space; we could aggravate the situation; so we would return for an overweight landing. I asked the F/A if an evacuation was needed upon return to ZZZ and informed me yes; but the A-F/A said no. We did not [request priority handling] or request equipment; but Tower rolled the equipment as a precaution. We ran checklists; reviewed QRH for overweight landing information and performed a normal landing. Landing configuration was AB2; flaps 30. Normal landing using reverse thrust and no brakes applied until approximately 60 knots. Upon rollout; I ran a QRH brake cooling and it returned a no special procedures result; The emergency equipment followed us to the gate. We asked if they noticed any smoke and they replied negative smoke. Proceeded to the gate with a normal shutdown. When Maintenance arrived; they performed a brake temperature check and said brakes were extremely cool.
Second reporter narrative
On takeoff from ZZZ; shortly after rotation; the First Officer (F/O) and I noticed a burning odor. At 1;000 ft. AGL we got a call from the Flight Attendants telling us about this same odor. She told the F/O that it was really bad and the passengers were looking worried. At about 5;000 ft. we told ATC about the odor and we asked for a vector and a level off to give us time to assess the situation before continuing over water. We continued southeast bound and leveled off at 8;000 ft. MSL. The odor was strong; was apparent throughout the cabin and not the normal smell that sometimes comes from the packs. The Flight Attendants and a deadheading pilot checked around the cabin and galleys and didn't find any indications of fire; smoke; or fumes. Because of this we did not run the QRH. The odor was slowly going away. I spoke to Dispatch on Company frequency while trying to decide if we can continue on; but then the odor was returning; just not as strong. At that point I realized we had burned through the 1;072 pounds of our ETP fuel and the F/O and I decided the safest plan was to return to ZZZ. I told this to Dispatch and ATC; and we got vectors back to the ILS XX. We briefed the approach; the F/O got the ATIS and the landing data for an overweight landing; and we completed the Descent and Approach Checklists. Because we were unsure of the origin of the odor; we felt a little urgency to get the aircraft on the ground. There was lots of rain and wind; and we were IMC the entire flight; so we were busy. After landing; the F/O got the brake cooling data for autobrakes 2; and it said to continue to the gate as normal. ZZZ Tower had the fire trucks standing by. They did a visual inspection and found no indications of fire or smoke. We didn't have a chance to talk to the Passengers until we arrived at the gate; but I believe the Flight Attendants did in flight. I still don't know what the odor was from or if it was normal; but I would still make the same decision to return; instead of continuing on [a long] flight with an unknown odor in the cabin.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.