Date: 202107
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Flight conditions: VMC
Weather elements / visibility: Wind
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Climb
ATC / advisory: TRACON ZZZ
Aircraft component: Pressurization System
Component problem: Malfunctioning
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Critical

Narrative: Preflight; taxi; takeoff and initial climb were normal. We were flying the ZZZZZ RNAV departure when at approximately 15;000 feet the aircraft displayed a Level 2 alert; CABIN INFLO LO. I was acting as Pilot Monitoring so I began to reference the QRH; but the alert extinguished after only a few seconds. As a matter of course my First Officer and I took note of the QRH procedures for this alert should it reappear. A few moments later we heard a loud squeal seemingly emanating from below the flight deck. This in itself wasn't entirely unusual in an aircraft of this vintage; but the noise was soon followed by a level 1 alert; CABIN RATE. I again referenced the QRH and we found that; indeed; cabin altitude was rising. Passing approximately FL 210 we heard a loud BOOM and the cabin altitude increased its rate of climb. We shallowed our rate of climb to around 1;000 fpm to allow us to process the problem. Given the rate of increase in cabin altitude it was clear to me that we were about to exceed 10;000 feet; and I verbalized to my First Officer to get ready to don O2 masks. This was followed by a Level 3 alert; CABIN ALTITUDE. We donned masks; established crew communications; and I instructed our jump seater to don her mask as well (and comm was established with her). I asked ZZZ Center for a level off at FL 240 (our next available FL). Roles were established with my First Officer continuing as Pilot Flying and I as Pilot Monitoring. We followed QRH procedures for the CABIN ALTITUDE alert but it was clear that the cabin altitude could not be controlled. I [advised] with ZZZ Center and requested an immediate descent and turn back to ZZZ. We were given clearance to 9;000 feet and a left turn back to the airport. My First Officer began an IDLE descent with full spoilers while I set up navigation. Once in contact with ZZZ Approach we were offered any runway landing south. Given that our weight was well below max landing weight we accepted RWY XXR. During the descent I built and briefed the ILS approach; and as cabin altitude went below 10;000 feet we doffed masks and resumed communication on boom mics. I instructed our jump seater that she could do the same. ZZZ Approach inquired as to whether I wished to continue as a [priority] aircraft; but given that cabin altitude had stabilized and we had no other issues I declined the status (CFR did not roll). At approximately 25 nm from the airport I assumed the role of Pilot Flying. My First Officer did an excellent job in depressurizing the aircraft during the remaining descent IAW limitations. Landing; rollout and taxi in were uneventful. After landing it was discovered that the Avionics Compartment Access door under the nose of the aircraft had come open in flight. Maintenance discovered that the Avionics Compartment Access door seal failed allowing the door to open in flight. I don't have a good answer for this one. These are aging aircraft and anomalies are to be expected. I am unaware of current maintenance practices in regards to inspecting these door seals; but perhaps a greater emphasis needs to be put on those inspections.




Synopsis: Air carrier flight crew reported experiencing a pressurization problem during climbout. The appropriate QRH checklist was completed and the aircraft returned to the departure airport.