Date: 201408
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Weather elements / visibility: Turbulence; Rain
Light: Night
Aircraft: HS 125 Series
Operator: Fractional
Operating under: Part 91
Mission: Ferry / Re-Positioning
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization Control System
Component problem: Failed
Reporter organization: Fractional
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Landed As Precaution; Flight Crew Diverted; General Declared Emergency
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

Narrative: We were on a ferry permit to ZZZ1 at FL410. We were deviating around convective weather and were IMC at night in moderate to heavy rain; and light to moderate turbulence. SIC noticed that the cabin altitude needle showed cabin altitude rising rapidly. He suggested we request a lower altitude to try to arrest the cabin altitude. ATC assigned us FL390. As I started the descent SIC said we should put O2 masks on; which we did. A few seconds later the CABIN ALT and Master Warning alarms activated. We initiated an emergency descent. I handled the flying duties while my SIC ran the appropriate checklists and dealt with ATC. Once we reached 12;000 MSL we removed our O2 masks and started looking for a suitable airport. We determined that ZZZ was the best option. We began an RNAV approach; but broke out and landed visually. There was no tower or ATC at this airport at the time we landed. Post flight inspection did not indicate any cause for the depressurization.


Second narrative: While enroute to ZZZ1 at FL410 while acting as pilot not flying; I took notice of the pressurization 'triple gauge' needles moving. The cabin altitude needle was climbing away from approximately 7;500 FT upwards toward 9;000 FT and the differential needle had decreased to about 7 on its indicator. I advised the left seat flying pilot (PIC) that we NOW needed to don our Oxygen Mask and start a descent to a lower altitude as we were starting to lose pressurization. After donning Oxygen masks and establishing communications with ATC; the red Cabin Altitude light and Horn came on indicating a cabin level of 9;300 FT. The PIC agreed; and I declared an emergency with Center since I was working the radios and put 7700 in the transponder. ATC cleared us down to 12;000 as I began running the depressurization checklist and subsequent checklist(s). After leveling off at 12;000; ATC advised us that ZZZ airport was ahead of us approximately 40 miles and asked if we wanted to go there. We looked it up in the plates and agreed it was the nearest suitable airport to land per the field length and weather conditions. I advised ATC that we would like to go there to land. He cleared us to proceed to the holding fix for GPS Runway 30. We programmed the FMS for an RNAV GPS Runway 30 approach. After the approach briefings; landing performance assessments and checklist were completed we advised ATC that we were ready to land. He then cleared us for the approach. We shot the approach in VMC conditions and landed uneventfully. After the shutdown checklists were accomplished; I secured the aircraft with chocks and gear pins while the PIC notified the company about the emergency deviation landing via cell phone. Note: There were no passengers on board as we were on a FAA ferry permit to ZZZ1 for maintenance repairs for 'fuel leaking.' After shutdown while retrieving our crew bags we detected a strong odor of fuel vapors in the cabin. That too was reported in by the PIC.


Synopsis: Hawker 900 flight crew experiences a loss of cabin pressurization at FL410 and declares an emergency before descending to 12;000 FT. The crew elects to divert to a suitable alternate.