Date: 202007
Local time of day: 1801-2400
Location: ZLA.ARTCC, CA
Flight conditions: VMC
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight phase: Climb
ATC / advisory: Center ZLA
Aircraft component: Pressurization Control System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Communication Breakdown; Other / Unknown
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural MEL / CDL
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance; General Maintenance Action
Contributing factors: Human Factors; Aircraft
Primary problem: Aircraft

Narrative: Aircraft dispatched with deferred pack and deferred auto pressurization controller. Previous inbound flight (different crew) had experienced pressurization problems. During climb I monitored pressurization performance due to previous issues. Upon leveling at planned and MEL limiting FL350 the pressurization system had achieved only 7.0 differential where 8.6 psi would have been normal. When engine power was reduced to cruise setting the cabin altitude began to climb and cabin pressure decrease. I advised ATC of problem and requested FL250. Previous crew had been successful at flying at FL240. We were cleared to descend to FL250. Before reaching FL250 the cabin altitude exceeded 10;000 ft. and was continuing to climb. The crew executed the procedure for high cabin altitude and [advised] ATC along with request for lowest available altitude. 15;000 ft. was assigned due terrain and [priority] descent initiated to that altitude. Although Cabin Automatic INOP was not annunciated; the applicable QRH procedure was executed with concurrence from Dispatch and Maintenance Control. Using manual pressurization mode; partial pressurization was achieved. A climb to FL190 was accomplished with pressurization stabilized at 5.4 differential and a cabin altitude 4;000 ft.Satisfied that the integrity of the aircraft was not in doubt; adequate fuel existed with data from Dispatch; and concurrence from Maintenance Control; the [event] was terminated and flight continued to planned destination. Side note for ATC: continual and insistent interrogation from ATC to 'state intentions' is not helpful. I can't tell you when or the frequency on which it occurred; but I am confident we were asked no less than four times what our intentions were. Our primary attention during this event is to stabilize the aircraft. After that is accomplished then the crew can assess options and advise ATC accordingly. I don't take [advising] lightly. When I say '[advised ATC]' I need ATC to clear the path above; below; and either side and then JUST STAND BY. Once the situation is stabilized; then all the players can collaborate to assemble a path forward. The constant interrogation from ATC only slows the assessment and decision making process.Mechanical failure. Inadequate troubleshooting at origin. Misapplied MEL. Overzealousness to dispatch aircraft. [Need] resistance to dispatch aircraft using any plausible and available MEL.


Second narrative: Our aircraft was previously written up with a cabin pressurization problem. In addition the right pack and Auto 1 cabin pressurization were MEL'd out of service. First Officer was Pilot Flying with Captain as Pilot Monitoring. Taxi; takeoff; departure and initial climb out were uneventful. Pilot Monitoring continuously monitored cabin altitude as we climbed. Although the pressurization seemed a little behind schedule; it was mostly keeping up throughout the climb. Upon leveling off at FL350 crew noticed that the cabin pressure continued to climb and was approaching 10;000 feet. Pilot Monitoring requested and was granted immediate descent. Pilot Flying initiated quick descent; however cabin pressure exceeded 10;000 feet up to 12;000 feet. Pilot Flying and Pilot Monitoring donned O2 masks and established comms. Pilot Monitoring [advised ATC] and ran cabin altitude QRH. Pilot Flying leveled at 15;000 due to mountainous terrain. Captain; in coordination with Maintenance Control and Dispatch; ran Cabin Automatic Inoperative checklist and was able to control cabin pressure with Manual selected. Once determined that cabin pressurization could be maintained with manual control; a collaborative decision was made to continue on to destination. The Captain terminated the [event] with ATC. The rest of the flight through engine shutdown was uneventful.  Unknown aircraft mechanical issue. Prior maintenance may have misdiagnosed/miscorrected original problem.


Synopsis: B757-200 flight crew reported experiencing critical inflight pressurization malfunction caused primarily due to misdiagnoses and misapplied MEL at departure airport.