Narrative:

We pushed back off the gate and started the engines. I placed the right pack in high because it was a little warm in the cabin and planned to place it in the normal position; which I did before takeoff. Right as we were about to taxi; I noticed an odor. To me it smelled a bit like a baby diaper and I thought it may have been a smell from the lav; and didn't say anything about it. We taxied out and I don't really remember smelling it. On takeoff I noticed the smell getting a little stronger but I just figured it was due to the increased bleed air feeding the packs. As we started our climb all indications were normal. At no time did I think the smell was overpowering. Somewhere around 9000'; the captain said; 'hey they're getting smoke in the back'. I didn't have my service interphone on; so I assumed he must have been listening in. Then the flight attendants called and the captain asked me to answer it. He said he had the aircraft. They told me they were getting a smell of smoke pretty strong in the cabin; but no visible smoke. I relayed this information to the captain and he decided to notify ATC and return to the field. ATC gave us a left turn and a descent; but I don't recall the altitude. We donned our O2 masks and started the qrc. At this point; the flight attendants called back again and had a commuting captain on the line. He said that it smelled like a recirc fan possibly going bad; but not a musty smell like one of the flight attendants had suggested. After relaying this info to the captain I noticed all the automation was now off and we were is the left turn back to ZZZ. I suggested turning back on the automation and gave him autothrottles; level change and LNAV. We continued running the QRH in between talking to the flight attendants. I asked what runway we were going to and the captain said xxr. I pulled up the chart and set myself up. At this point we were being vectored and I was cognizant of the fact that no PA had been made. I quickly told the passengers that due to the odor in the cabin we were returning to ZZZ and would be on the ground shortly. We landed and crash fire rescue equipment (fire department) was waiting. The captain said something about hot brakes so I quickly ran the data to see if it called for the brake cooling program to be run in the ACARS. I made sure to use our current fuel load and not the default destination fuel. We were not overweight so it was not required. Crash fire rescue equipment checked the outside of the aircraft including brakes and said all appeared normal. We elected to take off our masks at this point since there was no visible smoke or odor I could smell when I pulled my mask away from my face. We taxied to the gate uneventfully and deplaned normally through the forward passenger door.preventative measures: in the future; any time I smell anything remotely out of the ordinary; I will make sure to verbalize it. The captain said he had thought it smelled like a burnt bagel. If we had talked about this before taking off; maybe we would have decided to go back to the gate or at least see if the flight attendants were smelling it in the cabin. Once in the air; it becomes a much bigger problem; due to the time constraint of wanting to do everything right but also just get the aircraft back on the ground; in case the problem gets worse. The oxygen mask adds a serious level of difficulty to communication. The ptt switch on the yoke is probably the best to use; versus the intercom switch on the audio panel.

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Original NASA ASRS Text

Title: B737-700 First Officer reported an uneventful emergency air return due to a a fume event during initial climb.

Narrative: We pushed back off the gate and started the engines. I placed the right pack in high because it was a little warm in the cabin and planned to place it in the normal position; which I did before takeoff. Right as we were about to taxi; I noticed an odor. To me it smelled a bit like a baby diaper and I thought it may have been a smell from the lav; and didn't say anything about it. We taxied out and I don't really remember smelling it. On takeoff I noticed the smell getting a little stronger but I just figured it was due to the increased bleed air feeding the packs. As we started our climb all indications were normal. At no time did I think the smell was overpowering. Somewhere around 9000'; the Captain said; 'Hey they're getting smoke in the back'. I didn't have my service interphone on; so I assumed he must have been listening in. Then the flight attendants called and the Captain asked me to answer it. He said he had the aircraft. They told me they were getting a smell of smoke pretty strong in the cabin; but no visible smoke. I relayed this information to the Captain and he decided to notify ATC and return to the field. ATC gave us a left turn and a descent; but I don't recall the altitude. We donned our O2 masks and started the QRC. At this point; the flight attendants called back again and had a commuting Captain on the line. He said that it smelled like a recirc fan possibly going bad; but not a musty smell like one of the flight attendants had suggested. After relaying this info to the Captain I noticed all the automation was now off and we were is the left turn back to ZZZ. I suggested turning back on the automation and gave him autothrottles; Level Change and LNAV. We continued running the QRH in between talking to the flight attendants. I asked what runway we were going to and the Captain said XXR. I pulled up the chart and set myself up. At this point we were being vectored and I was cognizant of the fact that no PA had been made. I quickly told the passengers that due to the odor in the cabin we were returning to ZZZ and would be on the ground shortly. We landed and CFR (Fire Department) was waiting. The Captain said something about hot brakes so I quickly ran the data to see if it called for the brake cooling program to be run in the ACARS. I made sure to use our current fuel load and not the default destination fuel. We were not overweight so it was not required. CFR checked the outside of the aircraft including brakes and said all appeared normal. We elected to take off our masks at this point since there was no visible smoke or odor I could smell when I pulled my mask away from my face. We taxied to the gate uneventfully and deplaned normally through the forward passenger door.Preventative Measures: In the future; any time I smell anything remotely out of the ordinary; I will make sure to verbalize it. The Captain said he had thought it smelled like a burnt bagel. If we had talked about this before taking off; maybe we would have decided to go back to the gate or at least see if the flight attendants were smelling it in the cabin. Once in the air; it becomes a much bigger problem; due to the time constraint of wanting to do everything right but also just get the aircraft back on the ground; in case the problem gets worse. The oxygen mask adds a serious level of difficulty to communication. The PTT switch on the yoke is probably the best to use; versus the intercom switch on the audio panel.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.