A Red Sea Airlines Boeing 737-800, registration SU-RSB performing flight 4S-3501 from Hurghada (Egypt) to Porto (Portugal) with 170 passengers and 10 crew, was in the initial descent through FL330 towards Porto when the crew initiated an emergency descent, the passenger oxygen masks were released, due to the loss of cabin pressure. The aircraft levelled at FL100 about 5 minutes later (average rate of descent 4600 fpm) and continued to Porto for a safe landing on runway 35 about 25 minutes after initiating the emergency descent.
The aircraft remained on the ground in Porto for about 23.5 hours, then positioned to Cairo (Egypt) climbing up to FL300. The aircraft was still on the ground in Cairo about 60 hours after landing in Cairo.
On Oct 11th 2023 Portugal's GPIAA reported the occurrence was rated an incident stating:
At 22:15, while flying in the vicinity of Zamora - Spain, about 200 miles (~30 minutes) from the destination airport (LPPR) flying at flight level 370, the crew received a failure indication (master caution auto-fail) in the aircraft's pressurization control system.
The crew would not have been able to manually control the cabin pressure and subsequently received the cabin altitude alert.
At 22:20 while flying at FL324, the Portuguese air traffic control service (ATC) was contacted to descend, the crew having resorted to oxygen masks, a procedure that made communications with ATC difficult. At 22:23 while passing FL200 on the descent, the crew declared an emergency (Mayday) and requested descent to FL100 due to pressurization problems.
At 10:25 p.m. at 10,000 feet, according to the flight commander's report, he contacted the purser to check for any problems with the passengers and crew, and received information that everyone on board was well.
The flight continued depressurized and at low altitude (FL100) until the final descent to Porto airport where it landed at 22:48 with no further issues.
The GPIAA reported the aircraft did not suffer any damage, however, the Cabin Pressure Controller #1 was deactivated under minimum equipment list requirements. All oxygen masks and generators had been used.
The GPIAA further stated:
The absence of occurrence reporting by the operator as determined by EU legislation and ICAO best practices, the delay and lack of detail to the questions posed via EAAID without proper independent factual support (e.g. FDR data, procedures, etc.), prevent a solid reconstruction of data and facts necessary for the present incident assessment process.
...
Regarding the Boeing 737 pressurization system architecture with the three independent operating modes, this allows a level of redundancy precisely to avoid a depressurization at altitude and consequent need for an emergency descent.
On the other hand, and even admitting the simultaneous and improbable failure of the three systems, from the moment of the alleged failure still with the aircraft flying in FL370 or in the following moments during the beginning of the descent, the crew actions in deciding to put on the oxygen masks and not following the procedures provided for in the manufacturer QRH with immediate descent to FL100 are not understood, choosing to descend to FL200 and only then declare an emergency 8 minutes after the fault was indicated.
Based on the available collected data and after analysis of the facts in the occurrence context, GPIAAF decided not to proceed with a formal safety investigation, beyond the analysis carried out in this evaluation process, considering the process closed
