Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine
Flight Crew Last 90 Days 20; Flight Crew Total 5100; Flight Crew Type 400
Communication Breakdown; Time Pressure
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Weather
On IFR flight plan to ECP. Weather was rapidly deteriorating accompanied by severe thunderstorms and moderate to extreme precipitation. Aircraft is equipped with NEXRAD weather; but not on-board sweeping weather radar. Approximately 30 miles from severe reported weather (as reported by prior aircraft and confirmed by NEXRAD equipment) I attempted to change destination to CEW to avoid entering extreme weather exceeding capability of aircraft. Requested change of destination from filed destination (ECP) to CEW approximately three times over 5 minute period. ATC did not acknowledge transmission; and descended aircraft to assigned altitude of 17;000. During descent to 17;000 ATC advised to level off at 18;000. I then advised ATC that if I could get descent below 18;000 I would cancel IFR clearance and precede VFR. ATC advised 'standby' and then assigned 17;000. During descent below 18;000 I requested to 'cancel IFR' more than once to remain VFR prior to entering extreme weather and precede VFR; but transmission was not acknowledged. After announcing to Controller I was canceling IFR aircraft descended below last assigned IFR cleared altitude of 17;000. Aircraft descended to approximately 16;300 MSL at which time ATC advised assigned altitude was 17;000. Better flight planning and earlier decision making regarding a more suitable alternate would have mitigated problem. This would have given ATC and me more time to sequence and plan aircraft's intended course of action.
EA500 Captain reports communication difficulties with ATC while attempting to change destination or descend and and cancel IFR due to weather. A deviation below the ATC assigned altitude of 17;000 FT occurs prior to the Controller acknowledging IFR cancellation request.