BE90 pilot reports encountering moderate icing during vectors for approach to SNA. The wing deice boots are unable to keep up and the pilot deviates from assigned course and altitude to avoid worsening conditions before informing ATC.

Narrative

Aircraft was on initial arrival to SNA in heavy IMC. Radar on board indicated light precipitation ahead; however aircraft experienced moderate rime and mixed icing; as well as moderate turbulence which made for difficulty in holding aircraft altitude and speed. Controller was trying to steer flight away from weather and bring aircraft down to final approach altitude and intercept course for ILS 19R in SNA. De icing boots were working; but unable to keep up with the amount of ice accumulating on aircraft. Hail was suspect at this time and pilot deviated from assigned course and altitude to avoid further or deteriorating conditions. ATC noticed the deviation and requested aircraft status before pilot had time to advise ATC of time and safety sensitive deviation. Aircraft re-established altitude and heading to continue arrival to approach without incident. Pilot was under heavy workload and keeping the aircraft away from an unforeseen weather situation that was possibly threatening to the flight. ATC was very efficient and proceeded to keep the flight in line for a short vector and rapid decent onto the ILS. Continued vigilance of immediate potentially threatening weather and fast calls to ATC to parlay pertinent needs for deviation is essential to flights in this situation. Heavy pilot workloads and busy controllers during inclement weather continue to be challenging; in this situation pilot and controller worked together to keep aircraft safe from weather and obstacles. Pilot must be vigilant on informing ATC of deviation prior to deviation from clearance.