Date: 202004
Local time of day: 1801-2400
Flight conditions: VMC
Light: Night
Aircraft: Q400
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
Aircraft component: Pressurization Control System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Illness / Injury
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Landed As Precaution; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Our aircraft and aircrew were in good operating order. Ceilings and visibility departing ZZZ were unrestricted and we were about to finish the final leg of our 4 day trip. The First Officer was the pilot flying and the Captain was the pilot monitoring for this leg.Just prior to passing through 10;000 ft. on our climb-out the Captain checked the pressure indication control panel which indicated the cabin was pressurizing at a rate of about 500 fpm; which isn't unusual for a departure from a near sea level airport; combined with the high climb rate of our light aircraft. What did seem odd was the readings on the other two instruments; the differential gauge was pegged at zero; as was the cabin altitude gauge. These two instruments clearly were not in agreement with each other; or with the pressurization rate gauge. In talking with the First Officer as we climbed through 10;000 ft.; we thought that another way to confirm if we were pressurizing was if the Cabin Press master warning was triggered through 9;800 ft. Also we confirmed that the bleeds and packs were on with normal flow control setting. We did discuss that we were both experiencing popping ears and the First Officer thought he might have heard a creaking noise from the airframe. As we received no master warning; we thought we were probably pressurizing as the rate indicator showed; but possibly had faulty cabin differential and cabin alt gauges. At this point this Captain informed the First Officer that he was going to call Maintenance to help troubleshoot the gauges and to get a second opinion as to if we were in fact pressurizing as the crew had thought. The Captain handed the radios over to the First Officer.Maintenance control informed this Captain that there was not anything they could guide the flight crew in with respect to troubleshooting the pressure indication control panel; and that they could evaluate the system on the ground in ZZZ1 but there was not anything that could be reset or diagnosed in flight from their standpoint. At this time we were climbing through about 15;000 ft. and the panel was reading the same as it had previously; there was a slight cabin pressurization rate with no Cabin Press warning. The Captain and First Officer held a discussion between themselves if they were feeling any symptoms of hypoxia and both pilots were free of such physiological effects. The Captain relayed to Maintenance that there seemed to be something wrong and it was the Captain's concern that the pressure vessel could exceed limitations of maximum pressurization differential and for safety of flight wanted to descend back down to 15;000 ft. At this time we were climbing through approximately 16;000 ft. Maintenance asked the Captain if the call could be forwarded to Dispatch to make sure there was enough fuel on board to safely operate the flight to ZZZ1 at a lower altitude than planned. The Captain agreed and the call was forwarded.While waiting for the call to be picked up by Operations the Captain once again did a self check and felt no physical effects of hypoxia. The Captain asked the First Officer if he was feeling hypoxic; which the First Officer stated he was not. The Captain then asked the First Officer to call the flight attendants to see if they; or the passengers were feeling off at all; which might indicate high or low pressurization problems. According to the First Officer the reports from the back were that all was seemingly normal. The flight attendants also reported that they did not hear any whooshing or whistling sounds that might indicate a leak in the pressure vessel. We had just leveled off at 20;000 ft. with no master cautions or master warnings. The pressure indication control panel read the same here as it had at the other points indicated in the climb.When Operations picked up their line they quickly ran performance numbers and we agreed that in the interest of safety we would descend to 15;000 ft. While still on the phone with Operations the Captain asked the First Officer to request a descent with ATC. This request was immediately granted; if it was not; it was this Captain's intent to [request priority handling] and inform ATC of our descent to 15;000 ft. Before we reached 15;000 ft. on our descent the Captain started feeling light headed and experienced tingling in his fingers and increased popping in his ears. When asked; the First Officer reported similar altitude effects. It was our assumption that since we were descending the symptoms would not develop too severely as we were descending into more dense air. We then called the flight attendants back and they reported similar hypoxic effects and also frequent ear popping.At about 16;000 ft.; we decided to continue our descent down to 10;000 ft. which we had no problem obtaining clearance for with ATC. At this point the Captain determined we were about 130 miles South of ZZZ1; which was our filed destination. We were also almost directly over ZZZ2. The Captain did another self assessment and felt that it was probably best to not continue to ZZZ1; but wanted to check with the rest of the crew to see how they were feeling about a diversion. The First Officer also reported that he felt like the more conservative approach was to divert to ZZZ2. The Captain specifically remembers feeling that it was irresponsible to continue the flight as planned with an undiagnosed pressurization problem; but with multiple crew members reporting symptoms of hypoxia. This Captain also thought about the fidelity of the operation and did not want to be too aggressive in a manner that would bring unneeded scrutiny on our company if the decision was made to continue and people arrived in ZZZ1 feeling sick. The Captain decided then that in the interest of safety; as the most conservative decision; the flight would divert to ZZZ2. He then called Dispatch back and informed the Dispatcher that we would be diverting to ZZZ2. The Captain asked the First Officer to request vectors from ATC back to ZZZ2 and to inform them that we would be diverting. The Captain then called the flight attendants to inform them about our upcoming change of plans and to make sure everyone in the back was feeling okay. The flight attendants reported feeling relieved that we were diverting and that nobody in the back was reporting feeling sick. This was good news and the Captain determined that emergency services would not be necessary upon landing. Descending through 10;000 ft. both the Captain and First Officer began feeling light headed and were experiencing mild cognitive decline. The pilots then took turns briefly donning oxygen masks in an attempt to raise our blood oxygen levels and stave off hypoxic effects which was helpful. The Captain made an announcement to the passengers informing them that due to a mechanical difficulty we would be making a brief stop in ZZZ1 for safety.The Captain and First Officer then came together to ensure that we were on the same page and to catch each other up. We briefed our approach and ran the descent and approach checklists. The First Officer then landed the aircraft at ZZZ2; once clearing the runway we received a parking spot from ZZZ2 operations; proceeded to the terminal and terminated our flight.This was a challenging situation to identify and manage because of the nature of the failure that we experienced. In talking with maintenance personnel in ZZZ2; the nature of the failure that we experienced; a faulty cabin pressure controller; not only provides information to the gauges we use to monitor the pressurization status of the aircraft; a component inside of it also triggers the master warning for a high cabin pressure altitude if the airplane loses pressurization. When we thought we were using independent systems to verify the pressurization status of the aircraft; we were in fact relying on the same system that was presenting multiple failures throughout many aircraft fail safes; which generate from the same place. In the future; I think it is important when dealing with an undiagnosed pressurization problem to strongly consider descending even if not presented with a cabin press master warning. Also having truly standalone pressurization reporting and pressurization warning systems is important for the safety of crew and our passengers.Reflecting on this situation I also worry that I might have fallen prey to confirmation bias. I had readings somewhat consistent with normal aircraft operation which made me comfortable continuing with the standard flight profile while at the same time using my resources; my crew; the passengers; Maintenance; and Dispatch; to diagnose the problem and make decisions. While my First Officer and I had correctly identified an undesired aircraft state existed and were actively and vigorously trying to trap and mitigate the ambiguous instrument readings and conflicting physical symptomology; stopping our climb earlier would have proven the best trap strategy. On paper these events seem somewhat far apart and isolated; being reported in thousand foot As they unfolded on the flight deck though; the events and timelines are much more compressed. For example the time between 10;000 ft. and 15;000 ft. seems dramatic but only takes a couple of minutes with the aircraft performance that we experienced on that flight day. Combined with the short nature of our flight; without the benefits of experiencing a master warning or master caution that would have driven us into different checklist driven actions; I believe we mitigated and trapped the problems with our Aircraft effectively. I believe we achieved this through quality use of CRM with amongst and between cabin crew; and with our ground based colleagues in the Maintenance and Dispatch departments.




Synopsis: Q400 Captain reported that a pressurization problem resulted in a diversion.