B737 Flight Crew reported a fume event of unknown source during initial climb. The Flight crew completed appropriate QRH procedures and performed an air turn back.
Synopsis
B737 Flight Crew reported a fume event of unknown source during initial climb. The Flight crew completed appropriate QRH procedures and performed an air turn back.
Narrative
We operated Aircraft X from ZZZ - ZZZ1 and had to return to field shortly after takeoff due to a fumes event. The flight started normally and we were not rushed for time as our previous flight from ZZZ2 arrived on time. After we had pushed and started a southeast taxi; we noticed that the autothrottles somehow became disconnected. We reengaged the AT (Autothrottles) only to have them come off once again after about 10 seconds. At this point we had left the ramp and were taxiing south on [Taxiway] XX. We started the number one engine and after the number one engine was started and stabilized; the AT stayed on and we had no further issues with it. We did have a contingency brief on what our plan was should it disconnect on our takeoff roll; but it stayed on for the remainder of our flight till my FO (First Officer) disconnected them for landing. My FO was the pilot flying and after a normal takeoff from Runway XX; I was finishing up our after takeoff checklist when I started to smell a burning smell of some sort in the flight deck while climbing through approximately 1;000 ft. AGL. I confirmed this with my First Officer; who said he smelt the same burning smell; during our debrief he would later tell me he also had slight irritation in his eyes around the time we noticed the fumes. We performed our memory items for smoke and fumes followed by running the QRC items for smoke and fumes. My First Officer was the flying pilot so I handed ATC comms to my FO while I checked in with our flight attendants. I advised them of the situation and asked if they had anything in the galley ovens that might be burning. We were told that there was no food cooking in the galley oven but they would check for any residual food. My FO and I had a brief short discussion about the AT disconnecting a few times while taxiing out southeast and wondered if it might be related to that. The AT continued to work fine so I then continued on with the checklist for smoke and fumes and directed our FAs (Flight Attendant) to turn on and off various cabin lights per the checklist. After confirming no remaining food in the ovens that could be burning; our aft FAs did confirm a burning smell of some sort; while our forward galley FAs confirmed no strong smell but they did smell something on takeoff but thought it was fumes from prior departing aircraft. Since the burning smell was still present and we could not identify where the fumes were originating from; we decided to return to ZZZ.My FO continued to handle the PF (Pilot Flying) duties as well as ATC comms; while I sent a quick message to Dispatch and ran through the briefing with our Lead Flight Attendant. Afterwards; I continued further in the checklist as far as I could; both packs off below 10;000 ft. at this point in the smoke and fumes removal checklist; until I decided that time was of the essence and I could not continue any further as we were being vectored back for Runway XXC; the runway we requested as we had just landed on it a few hours prior. After loading the FMC for a return to field; responding to Dispatch via ACARS; and a quick brief for [Runway] XXC; I then made an announcement to our passengers about the issue and our return to ZZZ. We finished our descent checklist to make sure we didn't miss any items and after discussing the overweight landing situation; I decided it was best to allow my FO to continue flying as he felt comfortable with the situation and we made an uneventful landing in ZZZ. We were overweight but I don't exactly recall the number other than it was by about 4000 lb. We cleared [Runway] XXC and talked to Airport Rescue and Firefighting who verified there was no visible smoke and fumes before taxiing back to our gate uneventfully.
Second reporter narrative
I was the PF (Pilot Flying) on this leg. We elected to taxi out single engine and the autothrottles disengaged several times with only one engine running. After the second engine start; there were no more issues with this system. We briefed a contingency plan for the takeoff roll that we would continue if they disengaged.After departure; while retracting flaps; we both noticed fumes/a burning odor in the flight deck. We performed the smoke/fire/fumes QRC procedure; and then I took over the radios while the Captain called the flight attendants. We elected to continue climbing and stay on course for the time being. The flight attendants confirmed the burning smell and stated there was no burning food in the ovens. They double-checked this prior to diverting.We were unable to identify the source of the fumes and the Captain continued running the QRH procedure while I [requested priority handling] with ZZZ Departure. We were cleared direct to the field. I set up for an overweight landing on Runway XXC. We notified the flight attendants; passengers; Dispatch and ATC. A normal overweight landing was completed. Upon exiting the runway we confirmed there was no sign of fire with Airport Rescue and Firefighting. We taxied back to the gate under normal operations.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.