Date: 201411
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: Mixed
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Distraction
Second reporter function: First Officer
Anomaly: Deviation - Speed All Types; Deviation / Discrepancy - Procedural Weight And Balance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; Flight Crew Landed in Emergency Condition; General Declared Emergency
Contributing factors: Human Factors; Aircraft
Primary problem: Aircraft

Narrative: 'B' Flight Attendant (FA) reported fumes of burning plastic in aft galley; likely source gasper vents.  I directed the First Officer (FO) to open the Smoke/Fire/Fumes on the Aircraft Checklist in case we chose to go that path.  The FO checked in again with the 'B' FA to gain more information if the fumes persisted.  Once we were confident the smell was not the result of deicing; we donned oxygen masks in accordance with the immediate actions; and initiated the Smoke/Fire/Fumes on the Aircraft Checklist. To shorten up the story; we created a plan to recover the aircraft at the nearest suitable field.  I continued to fly and talk with ATC and he continued to manage the checklists and communication with all parties except ATC.  As the FO calculated landing data; I provided him with a landing weight of 122;000 pounds.  He seemed confused by this number but had little time to process why as we were very busy running the Smoke/Fire/Fumes on the Aircraft Checklist (latter portion of trying to ID the source of smoke); Descent Checklist; and communicating with the cabin; ATC; and Flight Ops.  Only after landing did I realize the error calculating landing weight and approach speeds.  Since neither one of us cleared out the FMC routing; the enroute fuel burn calculation contributed to an unrealistic landing weight (122;000 vice 142;000).  Had either one of us been in the Green we would have simply used the gross weight as landing was imminent. Post-flight; we calculated that we introduced a 10-knot error into the approach calculations. I do not believe we approached the planned (erroneous) speed until over the threshold. This was the FO's first leg after an extended time off and only my second trip after a six-week absence. Crew Coordination could have been better.  Upon receiving a clearance direct to the airport; I programmed and executed the clearance while the FO was otherwise occupied.  He was notified of our routing and altitudes whenever he returned to the ATC frequency.  However; I/we could have also changed the FMCs destination to the new airport. Assertiveness and backing each other up:  I feel that our expedited recovery reduced/prevented the time to think of what we had already accomplished and if anything was left out.  As a result; our ability to trap the weight calculation error was nullified.  The FO may have dismissed the weight calculation inconsistency because a) of my perceived experience and b) task loading.  The ongoing communication demands with Flight Ops; Cabin crew; and in his spare time; ATC. We should have managed the FMC programmed routing in a manner to avoid any confusion on fuel burn or landing weights.  Enter the revised destination on Route page 1.  Continue to talk with the other Flight Crew Member to flesh out concerns or misunderstandings.


Second narrative: On climbout; we experienced fumes on the aircraft. Handled the emergency in accordance with the QRH. On return to the airport; I entered incorrect gross weight. We were very busy dealing with checklist; ATC; Flight Attendants; and the aircraft. The Captain computed the gross weight; which I agreed with; however; we must have entered a gross weight based on continuing the flight to destination. We computed a gross weight roughly 20;000 pounds lighter than what we actually weighed. This gave us a VREF/Tgt speed of 10 knots slower than what we should have used. Captain was Pilot Flying and kept the approach a little fast due to being high.


Synopsis: B737-700 flight crew reported making an error when figuring landing gross weight during their return to departure airport because of fumes in the aircraft.