ERA15LA328 · Gulfstream Aerospace G IV UNDESIGNAT

The pilot and copilot were conducting a repositioning flight. According to the pilot, during cruise at flight level 430, they observed a red "9.8 CABIN DFRN" warning message, indicating a maximum cabin differential pressure of 9.8 pounds per square inch differential or greater, followed by a red "DOOR MAIN" indication. The pilots donned oxygen masks and referenced the airplane's emergency checklist. They then heard a loud "bam" sound in the cabin and immediately initiated a descent. The pilots opened the cabin pressure outflow valve manually and leveled the airplane at 12,000 ft mean sea level. The pilots continued the flight unpressurized and landed without further incident.

The day after the accident, the airplane was flown for a short and uneventful repositioning flight, after which it was examined and structural airframe damage, including a cracked floor beam, dimpled areas in the floor boards, damaged structure between ribs, and damaged wing links, was found. An examination of the outer fuselage revealed that the cabin pressurization relief/safety valve (CPRV) static port, located above the CPRV, was completely plugged with a foreign material resembling dried dirt from a mud dauber. According to the airplane manufacturer, a blocked CPRV static port would render the CPRV inoperative due to its inability to measure the cabin-to-atmosphere pressure differential. The cockpit aural warning speaker was also found inoperative, which may have delayed the pilots' ability to recognize the overpressurization condition. The airplane's digital flight data recorder indicated that the crew acknowledged the initial warning message 89 seconds after its illumination. No other mechanical anomalies were found with the airplane's pressurization system. The airplane's structural damage was not repaired after the accident; therefore, the reason for the overpressurization condition could not be determined.

Verbatim NTSB analysis and probable cause from the NTSB dataset