Date: 201209
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Weather elements / visibility: Haze / Smoke; Turbulence
Light: Daylight
Aircraft: Citation I/SP (C501)
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Squat Switch
Component problem: Failed
Reporter organization: Corporate
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Troubleshooting; Training / Qualification; Physiological - Other; Distraction; Confusion; Communication Breakdown
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew FLC complied w / Automation / Advisory; General Declared Emergency; General Maintenance Action
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Aircraft

Narrative: We had just experienced severe turbulence less than 5 minutes before when the 10;000 FT cabin horn sounded. We quickly donned our oxygen masks and the PIC declared an emergency with the ARTCC and indicated we were going to perform an emergency descent. I disconnected the autopilot and pitched over to the appropriate attitude to execute the emergency descent and confirmed that the throttles and speed brakes were in the proper position. It appeared that the ARTCC may or may not have understood what the PIC had told them so I also informed them that we were making an emergency descent. The ARTCC told us to maintain several altitudes; (FL240 and FL210); and I again informed them of the nature of our descent and they finally cleared us to 15;000. We requested 11;000 as a final altitude and terminated the emergency and requested to proceed to our destination airport. One thing we had forgotten to do was change our transponder to 7700. After accessing the situation we believe our pressurization system had failed; but at 11;000 FT it began to operate properly. The PIC had maintenance troubleshoot the system but they could not find anything definitively wrong with it. We continued on our trip at altitudes not requiring pressurization and it worked properly thereafter. As a precaution when we deadheaded home we remained at altitudes were we could use our masks and not have to descend. The pressurization system has performed perfectly since the incident described above.


Second narrative: Total rapid loss of pressurization; loud air noises; executed all emergency procedures in accordance with checklists; advised Center of deviations from assigned altitude and received clearance to descend to requested 11;000 FT cruise altitude. No injuries; damage or passengers involved. [We] proceeded to our destination for normal landing and pressurization maintenance servicing; follow on replacement of left landing gear electric squat switch. No further problems.


Synopsis: A CE-501 squat switch failed at FL350 causing a rapid loss of pressurization; so an emergency was declared and the flight descended to 11;000 FT then continued to its planned destination.