Date: 201901
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization 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 Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; General Physical Injury / Incapacitation; General Maintenance Action
Contributing factors: Human Factors; Procedure; Aircraft
Primary problem: Aircraft

Narrative: Passing through approximately 23;000-24;000 feet we experienced a CABIN ALTITUDE Warning light on the Forward Panel with the associated warning horn. We donned the O2 masks; established communications [and] requested a descent to 10;000 feet with Center. We completed the QRC/ QRH Checklist items for CABIN ALTITUDE WARNING and established communication with the flight attendants and were told that the cabin O2 masks had not dropped. The cabin pressure gauge was giving what we determined to be anomalous/unexpected pressure readings and ATC cleared us to 10;000 feet and to continue the LNAV arrival track. After arriving at 10;000 feet; we removed our masks and proceeded to land. We discussed the situation and determined we would not need CFR (Crash Fire Rescue) resources to assist us upon landing and told Approach and Tower we were a 'no Assistance required' aircraft. We did request paramedics to be on-hand in the gate area since the flight attendants stated that a few (six to eight) passengers were complaining of pressure in their ears. As we descended from 10;000 feet; the First Officer adjusted the pressurization to decrease cabin pressure to provide relief. Because we were focused on the arrival; the cabin altitude was found to be in the negative range (overpressure) due to the fact that we were more concerned with our descent than controlling a comfortable pressure. We landed uneventfully and made a maintenance logbook entry which outlined the pressure discrepancy. Because we did not discuss the specific altitudes noted when the event occurred; I wrote in the logbook that the event occurred around 19;000 feet (which is what I recalled). The First Officer later told me he thought the altitude at the time of the event was between 23;000-24;000 feet. We debriefed the flight attendants and they told us that they heard 'pressure/air leaking noises from around the Aft Service Door around the time of the event. After debrief with the Gate/Ops Agent I learned that only two passengers discussed their health concerns with paramedics and were consulted; but not treated. The passengers proceeded (on their own) out of the terminal. It was only after I filled out the maintenance logbook that I wrote the wrong event altitude in the writeup; though I was told that the exact altitude information was not essential to maintenance trouble-shooting. This was an aircraft mechanical issue. There is nothing the flight crew could have done to prevent the event. On an administrative note; the QRC Checklist instructs flight crews to 'move outflow valve to OPEN or CLOSE as needed to control cabin altitude and rate' but how to do so to promote passenger comfort is not outlined in the QRC or in Associated Ops Manuals. The system; in Manual mode; is extremely sensitive and was not easy for the FO (First Officer) to manipulate with assurance that the actions were going to be more than just a 'best guess.' As a result; we inadvertently over-pressured the Cabin Altitude as we descended below 10;000 feet. This may have forestalled any additional passenger discomfort.




Synopsis: B737-700 Captain reported a loss of cabin pressure descending through FL240.