Narrative:

We started our morning with -fzra. All runways were shut down when we arrived at the airport. This put us over two hours late [departing]. We had an aircraft swap; which was an originator and covered in snow. The fuel was over our planned and would take too long to defuel us. Also; the forward and side windows were covered in a foot of snow. The captain tried opening the side window to break the snow outward; however snow fell inside and into the window tracks. We had to call maintenance to clear the snow out of the side window tracks. Afterwards; the captain spent some time with the dispatcher to try and accommodate all the passengers. When all this was reconciled; we were almost four hours late. The captain was the pilot flying; so he briefed and we ran the before start checklist. With all the snow on the aircraft; we had a lengthy deice process. Ran all the deice checklist from the card and taxied for takeoff.climbing through FL200; we received a cabin altitude warning horn and light. We quickly donned our oxygen masks. I was not very familiar with the oxygen mask and panel on the -800. All the practice I had in the simulator was the rapid descent with other models where you select mask and can go hot microphone on the audio panel. So; the established communications did not go as smoothly as anticipated. I am not trying to make excuses; just trying to learn from mistake. I looked up and noticed the captain had turned the autopilot off and was descending with nothing in the altitude selector. Since I did not think I was transmitting to ATC; I looked at cabin altitude and was not much over 10;000 ft and was worried about our descent and if ATC was aware. Because of the mask problem; I did not hear him ask for lower altitude and since I did not see a lower altitude in altitude select window; I asked for lower altitude and reset selector.the captain while descending asked for a turn toward [the airport] and called for the qrc. As I was running the checklist; he was informing the passengers and flight attendants. When I reached pressure controllers-manual; I noticed switch in manual. The next step was to manually close the outflow valve and; after confirmation from pilot flying; I closed it. The next step to call for was passenger oxygen on. I hesitated for a moment and it dawned on me that we possibly made a mistake (there was never an auto press fail light). We were near 10;000 ft at this point and I selected altitude on pressurization panel. The aircraft started pressurizing normal. I noticed the cabin pressure going down below 10;000 ft and said 'oxygen masks off.'the captain transferred controls and radios to me so he could work with the dispatcher and maintenance control. It was at this time that I told captain that I think we departed with the pressure controller in manual and that I selected altitude and everything was working as normal. I told him I think we may have made a mistake. The captain received authorization for an overweight landing due to the uncertainties. The captain asked me to have the emergency vehicles stand by for hot brakes. The captain did a great job of leading and keeping all parties informed. The captain briefed the ILS and overweight landing procedures. He took the aircraft controls back and did a great job with a low impact landing using full length and minimum brakes.we cleared the runway and had emergency personnel check our brakes. Received the all clear and safely taxied to terminal. After; at terminal; we checked that the green indicator for manual pressurization was working; and it was. We discussed how impractical it was that we did not see this on checklist. I agreed and a part of me second guesses that in the struggle with the oxygen mask; I somehow ran the qrc twice and during the stressful event; thought it was already in manual. I honestly do not think this is the way it happened; but I also don't see how we missed the green light.oxygen mask preparation and different typesof mask and audio control panel set up. I will be much better prepared because of this event for future situations. This is not the time to figure out the differences in masks. All of my practice events were in other aircraft model sims; where you select mask and I/C switch on audio panel. This event will make me a better prepared pilot in the end. Also; no warning for manual mode besides switch position and green light. From a human factors position; green means go. This could cause confusion. Also; no recall or amber warning of manual.

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Original NASA ASRS Text

Title: B737-800 flight crew and the Dispatcher involved reported a cabin altitude warning horn passing through FL200 and return to the departure airport. The First Officer believes that they may have taken off with the pressurization panel set to manual rather than auto.

Narrative: We started our morning with -FZRA. All runways were shut down when we arrived at the airport. This put us over two hours late [departing]. We had an aircraft swap; which was an originator and covered in snow. The fuel was over our planned and would take too long to defuel us. Also; the forward and side windows were covered in a foot of snow. The Captain tried opening the side window to break the snow outward; however snow fell inside and into the window tracks. We had to call Maintenance to clear the snow out of the side window tracks. Afterwards; the Captain spent some time with the Dispatcher to try and accommodate all the Passengers. When all this was reconciled; we were almost four hours late. The Captain was the Pilot Flying; so he briefed and we ran the Before Start Checklist. With all the snow on the aircraft; we had a lengthy deice process. Ran all the deice checklist from the card and taxied for takeoff.Climbing through FL200; we received a Cabin Altitude Warning horn and light. We quickly donned our oxygen masks. I was not very familiar with the Oxygen mask and panel on the -800. All the practice I had in the simulator was the rapid descent with other models where you select mask and can go hot microphone on the audio panel. So; the established communications did not go as smoothly as anticipated. I am not trying to make excuses; just trying to learn from mistake. I looked up and noticed the Captain had turned the autopilot off and was descending with nothing in the altitude selector. Since I did not think I was transmitting to ATC; I looked at cabin altitude and was not much over 10;000 ft and was worried about our descent and if ATC was aware. Because of the mask problem; I did not hear him ask for lower altitude and since I did not see a lower altitude in altitude select window; I asked for lower altitude and reset selector.The Captain while descending asked for a turn toward [the airport] and called for the QRC. As I was running the checklist; he was informing the Passengers and Flight Attendants. When I reached Pressure Controllers-manual; I noticed switch in manual. The next step was to manually close the outflow valve and; after confirmation from Pilot Flying; I closed it. The next step to call for was Passenger oxygen on. I hesitated for a moment and it dawned on me that we possibly made a mistake (there was never an auto press fail light). We were near 10;000 ft at this point and I selected ALT on pressurization panel. The aircraft started pressurizing normal. I noticed the cabin pressure going down below 10;000 ft and said 'oxygen masks off.'The Captain transferred controls and radios to me so he could work with the Dispatcher and Maintenance Control. It was at this time that I told Captain that I think we departed with the pressure controller in manual and that I selected ALT and everything was working as normal. I told him I think we may have made a mistake. The Captain received authorization for an overweight landing due to the uncertainties. The Captain asked me to have the emergency vehicles stand by for hot brakes. The Captain did a great job of leading and keeping all parties informed. The Captain briefed the ILS and overweight landing procedures. He took the aircraft controls back and did a great job with a low impact landing using full length and minimum brakes.We cleared the runway and had Emergency personnel check our brakes. Received the all clear and safely taxied to terminal. After; at terminal; we checked that the green indicator for manual pressurization was working; and it was. We discussed how impractical it was that we did not see this on checklist. I agreed and a part of me second guesses that in the struggle with the oxygen mask; I somehow ran the QRC twice and during the stressful event; thought it was already in manual. I honestly do not think this is the way it happened; but I also don't see how we missed the green light.Oxygen mask preparation and different typesof mask and audio control panel set up. I will be much better prepared because of this event for future situations. This is not the time to figure out the differences in masks. All of my practice events were in other aircraft model sims; where you select mask and I/C switch on audio panel. This event will make me a better prepared pilot in the end. Also; no warning for manual mode besides switch position and green light. From a human factors position; green means go. This could cause confusion. Also; no recall or amber warning of manual.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.