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.
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.
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 reporter 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.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.