Date: 201208
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Troubleshooting; Situational Awareness; Physiological - Other; Distraction
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant; Person Maintenance; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; Flight Crew Took Evasive Action; Flight Crew Landed As Precaution; General Maintenance Action; General Release Refused / Aircraft Not Accepted
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: The APU was inoperative and we required a ground start unit. Starting Number 1 Engine first we got smoke and a very foul odor (smelled like dirty gym socks) in the flight deck. As soon as Number 1 [Engine] was started the smoke and smell begin to decrease. We believed the cause was the start unit and continued with start of Number 2 [Engine]. We made a PA to the passengers indicating it was not the airplane; but the starting unit causing the smoke and smell. We advised Ground Operations to have the ground start unit checked. As we taxied out for takeoff; the flight deck and cabin improved; but did not completely clear of the foul odor. As we raised the landing gear after takeoff; we got a Master Caution; Flt Control and HYD annunciations. There were no lights overhead. Checking hydraulic indications and aircraft handling; everything was normal. Right about 10;000 FT the flight attendants called and said that during the takeoff and climb the cabin had gotten smoky and smelly again. I decided to return to the departure airport and advised Departure Control. The First Officer flew the aircraft and I ran the Smoke in Cabin Checklist; advised airport Ground Operations and passengers before returning to normal flight duties for approach and landing. We requested the fire equipment personnel observe the airplane as we landed for any indications of a problem. After clearing the runway; we checked with the flight attendants as to the cabin condition and were advised that it had gotten better. The smoke was mostly gone; but the bad smell was still there. As there were no indications of a continuing problem; we continued taxi into the gate. Maintenance met the aircraft and said the airplane really smelled bad as did the ground start unit. They also said they could smell an additional smell of hydraulics. We de-planed aircraft at gate and checked on the passengers to see if everyone was OK. No one indicated any problems. We proceeded to a new gate and aircraft and continued on. I am not sure what could be done to prevent this in the future. If the ground start unit was in better condition; we would not have had the original smoke and smell issue. At this time I do not know what caused the return of smoke and smell to the cabin during and after takeoff or if there was any connection between this and the momentary hydraulic lights after raising the gear.

Callback: The Reporter reaffirmed the odor but believes that the smell was from the ground start unit which was particularly 'stinky' during preflight and ground start. He did not know why the APU was MEL'ed. At the gate; maintenance personnel thought that a slight hydraulic fluid odor was present in the cabin. The hydraulic system abnormals which illuminated the Master Caution were momentary and all returned to normal during the return to the departure airport. Maintenance found no hydraulic system anomalies.

Second narrative: Masks were donned and I leveled the aircraft at 12;000 FT while the Captain informed ATC that we would need to return to the airport. The Captain initiated checklist items for smoke and fumes as I coordinated with ATC for return to the departure airport. Fumes appeared to dissipate during the approach. Approach; landing; and return to gate were uneventful.


Synopsis: A B737-700 developed a dirty gym socks odor during engine start which cleared somewhat. After takeoff as the landing gear was raised; momentary Master Caution; FLT CTL; and HYD annunciations were seen with no secondary indications. During climb; the odor reoccurred and the aircraft was returned to the departure airport.