Date: 201201
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Pressurization Control System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Human factors: Training / Qualification; Physiological - Other; Confusion
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: About 5000' AGL; we got amber Auto Fail light and a green Standby light.  Seeing the green Standby light; we continued the climb and began checking the QRH and gauges.  We noticed a cabin altitude of around one to two thousand; a normal differential pressure and the confusing one was the cabin rate was showing a descending cabin of around 500' a minute; but we were climbing at around 4000' a minute and the cabin was not climbing looking at this gauge.  Believing we were still pressurizing because of the STBY light; we continued to climb.  I was flying and the Captain was in the QRH.  Passing through around 13;000'; I began to feel tingling in my arms and legs.  Having experienced symptoms of hypoxia before; I immediately donned my O2 mask and started a descent.  I told the Captain we were not pressurizing and to get his mask on; within a few seconds the tingling was gone.  The Captain noted the cabin altitude per the gauge had just dropped to zero; when it should have been at around 3000'; or at least climbing with us; a completely different from what it should have been doing.  I coordinated with ATC a descent to 10;000'.  We reached an altitude of around 13;800 feet as a Top of Climb.   The Captain decided we would return to departure airport.  I coordinated again with ATC while the Captain talked to the F/As.  We removed our masks at 10;000'; and returned uneventfully.  No Passenger issues were noted.  The next day I spoke with Maintenance Control and they said it was a pressure control failure.  They replaced it that night and the aircraft was back in service. I was once told in Training that if you have a green light on the pressurization panel; 'you're good.'  Obviously this is not true.  It seems that at lower altitudes it's hard to recognize a dual pressure control failure.   We probably should have leveled off at 6000' just to make sure we were actually pressurizing.


Second narrative: On climbout passing 5000'-7000'; the Cabin Pressurization panel Auto Fail amber light illuminated; followed shortly thereafter by the green STBY light.  I referred to the QRH; Auto Fail Checklist; and ran the checklist.  The pressurization panel indications were: Cabin pressure 1000'; cabin pressure differential climbing through the three o'clock position; cabin rate of climb -500 fpm.  The first two were normal; the last was opposite of normal indications.  The checklist (and systems study) indicated that the system had automatically switched to the alternate or secondary pressurization system.  Passing 10;000'; system indications were essentially the same.  Cabin differential had increased slightly to the four o'clock position and cabin pressure was reading just above sea level.  At 13;500' the First Officer (PF) nosed the aircraft over and coordinated for a descent to 10;000'.  He indicated that he was feeling tingling in his fingers; one sign of possible early onset of hypoxia; thus we donned the cockpit O2 masks.  I felt nothing.  We accomplished an uneventful return to departure airport.  No Passenger or Crew complications were noted; except for the First Officer's reported sensations.  No Cabin Pressure Warning Horn sounded.  Aircraft altitude reached 13;500'-13;800'.  Cabin Pressure Indicator did not exceed 1000'.  Cabin masks did not deploy.  Again; I felt no adverse sensations; though the cabin air did seem a little thin prior to donning masks.  Thus; physically; it appeared that the cabin had not pressurized; though the indications on the pressurization panel (except for rate of climb) indicated it had.  Confused?  Maintenance fully debriefed.


Synopsis: B737-300 Flight Crew experiences an AUTO Fail with a green STBY light climbing through 5000 feet.  The QRH is consulted and the crew continues climbing with normal pressurization indications except for the cabin indicating a descent.  At 13000 feet the First Officer believes he is experiencing hypoxia and dons his oxygen mask and noses over.  Flight returns to departure airport uneventfully.