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 inoperative 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.

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Original NASA ASRS Text

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

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.

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.