Gulfstream IV / G350 / G450
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 60; Flight Crew Total 20006; Flight Crew Type 178
Distraction; Human-Machine Interface; Troubleshooting; Communication Breakdown
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem; General Declared Emergency
Aircraft
While cruising at FL400 the cabin altitude warning sounded and the cabin altitude was 9;200 FT and climbing. After both pilots checked in on 100% oxygen the outflow valve was manually closed and since the cabin altitude continued to climb an emergency descent was initiated. ATC was advised; but had difficulty understanding my transmission from the oxygen mask microphone. Another aircraft understood and relayed to ATC. The checklist was run from the top and I removed my mask as soon as possible to use another microphone. We determined the left engine bleed had failed and the flight was continued at a lower altitude with the isolation valve open and bleed air being supplied from the right engine. I overlooked changing the transponder code to 7700 until requested by the Controller due to the difficulty of communicating through the mask. The transponder code was changed at the following Controller's request. It should be noted that the oxygen masks and microphones tested good prior to the flight and another aircraft was able to understand our transmission.
A G-IV flight executed and emergency descent when their pressurization system failed at FL400.