Date: 201310
Local time of day: 1801-2400
Location: SDM, CA
Flight conditions: VMC
Light: Night
Aircraft: Cessna Citation Sovereign (C680)
Operator: Fractional
Operating under: Part 91
Flight phase: Climb
Airspace: Class E SCT
ATC / advisory: TRACON SCT
Aircraft component: Integrated Audio System
Component problem: Improperly Operated
Reporter organization: Fractional
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Fatigue
Anomaly: ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Preflight and taxi were standard and uneventful. Our clearance involved departure on Runway 26R from San Diego's Brown (Brown) Municipal Airport SDM with published obstacle departure procedure. After SIC [announced] departure on CTAF frequency; initial climbout radio communications transitioned to me as pilot not flying. I was very active in continuing to scan outside in those initial moments after liftoff. For some undetermined reason; when I looked to Comm 2 VHF radio I believed the radio was now already switched to SCT Departure Control (ATC); and checked in. My belief was reinforced by the fact that ATC responded to my call by identifying themselves and issuing a vector as I verified climbing to 4;000 FT. There was an anomaly to sort out in the cockpit as the SIC indicated he could not hear ATC. ATC then issued a further clearance to climb and for lateral navigation. I read back the clearance issued that included '...climb and maintain 11;000 FT.' The SIC later verified he had heard me read back 11;000 FT though he was still not hearing ATC. In the climb between 4;000 and 11;000 ATC discovered I had been communicating with them on Brown airport's CTAF frequency 128.25 and asked me to switch to 119.6 where we quickly reestablished communication with each other with the standard check-in procedure. Somewhere along this time period the SIC indicated he is now hearing ATC as well. While climbing and near 10;000 FT; ATC queried us as to what altitude we were climbing to. I responded: '...11;000 FT.' ATC then said we were cleared to 10;000 FT; whereby the SIC made an immediate but smooth level off response commensurate with the fact we had passengers in back. In the post flight briefing; the SIC and I recall rounding the top of the level-off correction at approximately 10;600 FT. When we were stable at 10;000 FT I made a query with ATC indicating that I recalled having read back 11;000 FT on the climb altitude portion of the clearance. There was no real response to that other than ATC's reestablishing current altitude assigned of 10;000 FT. There was no other traffic conflict in the area and apparently low air traffic volume in the region. There was no request for us to make a telephone call after the flight and we were later transferred to other ATC frequencies down the line with no further events.There are human factors regarding the day's schedule that are material. The altitude event occurred at twelve hours and thirty minutes (12:30) of duty already served in that work day. We were on the fifth of five (5) legs assigned at the beginning of the work day and approaching midnight local time. Providing a highly varied and dynamic flying service; the first leg in the day started well into a two-hour scheduled slide; and on our fifth leg--containing the events that are the subject of this report--our passengers were late; as is common and part of the corporate service we provide. However; it all takes its toll. We concluded the day at thirteen hours and thirty-six minutes of duty; while still enroute to a hotel not local in nature. As a crew we both believed that I had read back 11;000 FT and that I was not corrected if indeed we had been cleared to 10;000 FT. Indeed; 10;000 FT was our filed cruise altitude. When in any doubt of a clearance altitude I would query the controller per SOPs. There were multiple anomalies of communication radio management; crew communication; and apparently not being corrected by ATC on altitude readback. Time was compressed and the normal query was not made in time. It is my professional opinion that for our part it was not the lack of a desire to do a good and professional job; but the onset of fatigue as a result of the day's schedule that was the overriding factor in the events occurring that are the subject of this report.




Synopsis: CE680 Captain reports a communication mixup departing SDM late at night after a long duty day. Several radio communications to ATC are attempted and apparently completed using the CTAF while listening on the ATC frequency. The crew believes the assigned altitude is 11;000 FT and ATC stating 10;000 FT; resulting in an altitude deviation.