Narrative:

Flight was filed ZZZ to ZZZ1. This was the fourth day flying with the same captain. It was our third leg of the day and our second on [this] aircraft. Weather in both locations was clear with light winds out of the north. We pushed from the gate in ZZZ early with 13 passengers. Because of the light load; I pitched to 15 degrees on takeoff to avoid any chance of over-speeding the flaps. On the initial climb; I accelerated to 230 and continued. Through 5;000 ft.; I noted a creaking sound in the airframe and mentioned it to the captain. Around the same time; he noted the pressurization panel. The cabin differential and cabin altitude were pegged to zero while the cabin showed a slight descent. Rapidly approaching 10;000 ft.; we noted that if we weren't pressurized; we should get a master warning chime through 9;800 ft. There was no warning chime. The caution panel remained dark. We began to wonder if it was the gauges that had failed. We discussed this for a moment and agreed to bring maintenance into the situation. The captain called maintenance on satcom; and I took over ATC and continued to fly.reaching FL200; we leveled and set cruise power. I had AUX2 turned down so I could focus on flying and ATC; so I didn't hear the captain's conversation with maintenance directly. Shortly after leveling; the captain asked me to request 15;000 ft. While still on the call. The concern then was a potential overpressurization since we didn't know what the cabin differential was. I told ATC that we needed a descent to 15;000 ft. For a potential pressurization issue. We were cleared: 'descend and maintain 16;000. Expect lower shortly.' the controller followed up by asking if we needed any assistance. I responded; 'not at this time.' I set the alerter for 16;000; pressed altitude sel; verified altitude sel with the captain while he was still on the call; and started a descent.the call with maintenance was completed shortly after starting down; and the captain recapped the conversation with me. Maintenance was fine with us continuing to ZZZ1 at a lower altitude. He asked for my thoughts on it. I asked if we had performance for it; and he said maintenance checked with dispatch and we were fine at the lower altitude. We agreed to reassess the situation at the lower altitude and make a decision from there. Shortly after; the captain asked 'how are you feeling? I think I'm feeling a little lightheaded.' I responded; 'yeah; I think my fingers are tingling a little. How about we go down to ten?' we agreed; and I told ATC that we needed 10;000 ft. Once we were cleared; I switched to IAS mode and reduced power to flight idle. The autopilot responded aggressively; so I disconnected it momentarily to establish the descent and reengaged it once we were stable. We used oxygen during the descent and discontinued use reaching 10;000 ft. The captain asked the flight attendants what conditions were in the back: if the flight attendants were experiencing any symptoms; if the passengers were experiencing any symptoms; if they noticed any abnormalities or heard any hissing noises? Meanwhile; I was handed off to approach. The captain and I circled back. He informed me that the flight attendants had reported physiological symptoms that were commensurate with our own--starting in the descent. I don't recall any complaints from passengers; and there were no abnormal noises in the cabin. I reported that we were level at 10;000 ft. Talking to [approach] with a correct altimeter setting.at that point; we were approaching the valley west of ZZZ2. The captain noted that we were still at least 130 miles from ZZZ1. We both came to the conclusion that we didn't know what was wrong with the plane; specifically; and we didn't know why we weren't feeling right. Neither of us were comfortable with continuing to a higher-density airport at a lower altitude under the current circumstance. We agreed that a diversion to ZZZ2 was the best course of action. The captain began coordinating with dispatch; the flight attendants; and addressed the passengers. I advised ZZZ2 approach that we needed to plan a diversion to ZZZ2; needed delay vectors; and a lower altitude. We were given present heading; 8;000 ft.; expect visual [runway] xxl. ATC asked if we required assistance. I repeated; 'not at this time.' by now; ZZZ2 was off of our right wing; so I slowed to 200 KIAS to allow time to set up and brief the approach and began a descent to 8;000 ft. During that time; the TRACON went through a shift change. I changed the destination in the efb and requested performance for ZZZ2. I also set up the FMS for the RNAV xxl.once each of us had everything taken care of; the captain and I circled back to brief the approach. We acknowledged that [there] were several threats in front of us and outlined the visual approach backed up with the RNAV xxl. By this point; we were on a high downwind for xxl. We completed the descent and approach checklists and descended for the approach. We were left fairly high on the base turn; so I increased our descent rate until we were in a better place for the approach. The controller vectored us at a sharp angle for final and asked if we had the airport in sight. The captain responded that we needed vectors to final; and we were given a sharp turn to intercept xxl then cleared for the visual approach. The FMS began overshooting the turn; so I disengaged the autopilot and made the corrections to get back on profile. We continued a stable approach to a normal landing. Once clear of the runway; we contacted [operations] for a gate assignment; continued to the gate; and deplaned. After recounting the anomalies to maintenance personnel; the crew met in the back to debrief and discuss the events that had happened. No additional info needed for [this report].in the sequence of events that unfolded; it was interesting that there was no singular trigger that indicated that we were approaching a [critical situation]. Our expectation for depressurization or lack of pressurization was a cabin press annunciation associated with a master warning. That expectation wasn't realized; so we decided to bring maintenance into the discussion. In the time that it took to address each level; with what seemed like appropriate action at that time; the situation escalated or more information was learned requiring a further escalation of action. I think CRM played a key role in being able to mitigate the rolling events that resulted in a diversion on an already short flight. From my perspective; it made a huge difference that the captain had confidence in my ability to handle the airplane; our flight path; and communication with ATC. That allowed him to handle all of the communication with company and flight attendants all while keeping the passengers informed. I especially liked the fact that he continually asked for my input; opinion; and feelings on decisions. We established a collaborative environment in which we quickly assessed where we were; assigned expectations for the next few moments; completed those tasks individually; gathered information; and then circled back to evaluate and repeat the process. I also recognize that we both felt empowered; even for me from the right seat; to say I'm not comfortable with this and stop the operation. In retrospect; I am concerned that the trigger for the master warning failed in tandem with the instrumentation on the cabin pressure control panel. I feel like the warning system should be independent of the system itself. Given that; having the visual pressurization check on the after-takeoff flow and the cruise flow helps trap that threat. Going forward; if there's ever any question of pressurization; I would level at a lower altitude and assess the situation. In hindsight; because we hadn't received a master warning annunciation; we believed that we were in fact pressurized and sought additional input before making further decisions.

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Original NASA ASRS Text

Title: Q400 flight crew reported aircraft failed to pressurize due to a failed cabin pressure controller and crew began to feel hypoxic effects; requiring a decent to a diversion and a precautionary landing.

Narrative: Flight was filed ZZZ to ZZZ1. This was the fourth day flying with the same Captain. It was our third leg of the day and our second on [this] aircraft. Weather in both locations was clear with light winds out of the north. We pushed from the gate in ZZZ early with 13 passengers. Because of the light load; I pitched to 15 degrees on takeoff to avoid any chance of over-speeding the flaps. On the initial climb; I accelerated to 230 and continued. Through 5;000 ft.; I noted a creaking sound in the airframe and mentioned it to the Captain. Around the same time; he noted the pressurization panel. The cabin differential and cabin altitude were pegged to zero while the cabin showed a slight descent. Rapidly approaching 10;000 ft.; we noted that if we weren't pressurized; we should get a Master Warning chime through 9;800 ft. There was no warning chime. The caution panel remained dark. We began to wonder if it was the gauges that had failed. We discussed this for a moment and agreed to bring Maintenance into the situation. The Captain called Maintenance on SATCOM; and I took over ATC and continued to fly.Reaching FL200; we leveled and set cruise power. I had AUX2 turned down so I could focus on flying and ATC; so I didn't hear the Captain's conversation with Maintenance directly. Shortly after leveling; the Captain asked me to request 15;000 ft. while still on the call. The concern then was a potential overpressurization since we didn't know what the cabin differential was. I told ATC that we needed a descent to 15;000 ft. for a potential pressurization issue. We were cleared: 'Descend and maintain 16;000. Expect lower shortly.' The controller followed up by asking if we needed any assistance. I responded; 'Not at this time.' I set the alerter for 16;000; pressed ALT SEL; verified ALT SEL with the Captain while he was still on the call; and started a descent.The call with Maintenance was completed shortly after starting down; and the Captain recapped the conversation with me. Maintenance was fine with us continuing to ZZZ1 at a lower altitude. He asked for my thoughts on it. I asked if we had performance for it; and he said Maintenance checked with Dispatch and we were fine at the lower altitude. We agreed to reassess the situation at the lower altitude and make a decision from there. Shortly after; the Captain asked 'How are you feeling? I think I'm feeling a little lightheaded.' I responded; 'Yeah; I think my fingers are tingling a little. How about we go down to ten?' We agreed; and I told ATC that we needed 10;000 ft. Once we were cleared; I switched to IAS mode and reduced power to flight idle. The autopilot responded aggressively; so I disconnected it momentarily to establish the descent and reengaged it once we were stable. We used oxygen during the descent and discontinued use reaching 10;000 ft. The Captain asked the flight attendants what conditions were in the back: if the flight attendants were experiencing any symptoms; if the passengers were experiencing any symptoms; if they noticed any abnormalities or heard any hissing noises? Meanwhile; I was handed off to Approach. The Captain and I circled back. He informed me that the flight attendants had reported physiological symptoms that were commensurate with our own--starting in the descent. I don't recall any complaints from passengers; and there were no abnormal noises in the cabin. I reported that we were level at 10;000 ft. talking to [Approach] with a correct altimeter setting.At that point; we were approaching the Valley west of ZZZ2. The Captain noted that we were still at least 130 miles from ZZZ1. We both came to the conclusion that we didn't know what was wrong with the plane; specifically; and we didn't know why we weren't feeling right. Neither of us were comfortable with continuing to a higher-density airport at a lower altitude under the current circumstance. We agreed that a diversion to ZZZ2 was the best course of action. The Captain began coordinating with Dispatch; the flight attendants; and addressed the passengers. I advised ZZZ2 Approach that we needed to plan a diversion to ZZZ2; needed delay vectors; and a lower altitude. We were given present heading; 8;000 ft.; expect visual [Runway] XXL. ATC asked if we required assistance. I repeated; 'Not at this time.' By now; ZZZ2 was off of our right wing; so I slowed to 200 KIAS to allow time to set up and brief the approach and began a descent to 8;000 ft. During that time; the TRACON went through a shift change. I changed the destination in the EFB and requested performance for ZZZ2. I also set up the FMS for the RNAV XXL.Once each of us had everything taken care of; the Captain and I circled back to brief the approach. We acknowledged that [there] were several threats in front of us and outlined the visual approach backed up with the RNAV XXL. By this point; we were on a high downwind for XXL. We completed the descent and approach checklists and descended for the approach. We were left fairly high on the base turn; so I increased our descent rate until we were in a better place for the approach. The controller vectored us at a sharp angle for final and asked if we had the airport in sight. The Captain responded that we needed vectors to final; and we were given a sharp turn to intercept XXL then cleared for the visual approach. The FMS began overshooting the turn; so I disengaged the autopilot and made the corrections to get back on profile. We continued a stable approach to a normal landing. Once clear of the runway; we contacted [Operations] for a gate assignment; continued to the gate; and deplaned. After recounting the anomalies to maintenance personnel; the crew met in the back to debrief and discuss the events that had happened. No additional info needed for [this report].In the sequence of events that unfolded; it was interesting that there was no singular trigger that indicated that we were approaching a [critical situation]. Our expectation for depressurization or lack of pressurization was a CABIN PRESS annunciation associated with a Master Warning. That expectation wasn't realized; so we decided to bring Maintenance into the discussion. In the time that it took to address each level; with what seemed like appropriate action at that time; the situation escalated or more information was learned requiring a further escalation of action. I think CRM played a key role in being able to mitigate the rolling events that resulted in a diversion on an already short flight. From my perspective; it made a huge difference that the Captain had confidence in my ability to handle the airplane; our flight path; and communication with ATC. That allowed him to handle all of the communication with company and flight attendants all while keeping the passengers informed. I especially liked the fact that he continually asked for my input; opinion; and feelings on decisions. We established a collaborative environment in which we quickly assessed where we were; assigned expectations for the next few moments; completed those tasks individually; gathered information; and then circled back to evaluate and repeat the process. I also recognize that we both felt empowered; even for me from the right seat; to say I'm not comfortable with this and stop the operation. In retrospect; I am concerned that the trigger for the Master Warning failed in tandem with the instrumentation on the cabin pressure control panel. I feel like the warning system should be independent of the system itself. Given that; having the visual pressurization check on the after-takeoff flow and the cruise flow helps trap that threat. Going forward; if there's ever any question of pressurization; I would level at a lower altitude and assess the situation. In hindsight; because we hadn't received a Master Warning annunciation; we believed that we were in fact pressurized and sought additional input before making further decisions.

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.