A g-iv flight executed and emergency descent when their pressurization system failed at FL400.

Attributes
ACN	1024591
Time
Date	201207
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Gulfstream IV / G350 / G450
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Pneumatic Valve/Bleed Valve
Person 1
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 60 Flight Crew Total 20006 Flight Crew Type 178
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Procedural Published Material / Policy

Narrative:

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.

Original NASA ASRS Text

Title: A G-IV flight executed and emergency descent when their pressurization system failed at FL400.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 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.