Date: 201404
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Weather elements / visibility: Icing
Light: Night
Aircraft: Dash 8-100
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Time Pressure; Training / Qualification; Confusion
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Exited Penetrated Airspace; Flight Crew Landed As Precaution
Contributing factors: Aircraft; Procedure; Weather
Primary problem: Weather

Narrative: On climbout between 12;000 feet and 17;000 feet we encountered severe icing. Upon entering icing conditions initially at about 12;000 feet; we adjusted climb rate to approximately 500 FPM as to keep a low angle of attack (due to icing conditions). We were initially cleared to 20;000 feet; but after noticing the ice building at a much greater than usual rate and a dramatic drop in IAS; we disengaged the autopilot. After autopilot disengagement at approximately 17;000 feet; we could only hold straight and level flight; and the aircraft started to pitch up and down for a neutral elevator position (indicating to me that the tail was significantly iced up). We already had charted torque set; so we started a slight descent and requested priority handling from ATC. We reported to ATC we were encountering severe icing; and would have to descend immediately to 10;000 to exit icing conditions. Once we were at 10;000 feet; the ice melted off and the aircraft performed normally. We then requested more direct routing; and recalculated our fuel burn. After making the necessary calculations; we contacted Dispatch; and informed them of the exact location and altitude of the severe icing encounter. We then communicated to Dispatch our current fuel; altitude; and fuel burn so we could recalculate our fuel burn to our destination (this was an over 500 mile reposition flight; with bad weather and alternates; so we needed to watch this closely). After [receiving] a new alternate; and calculating our new fuel burn; the mutual decision was made to stop at an intermediate station to refuel. We then refueled; and proceeded on to our filed destination without further incident.Ice accumulation was extremely rapid; associated with a dramatic drop in IAS over a short period of time coupled with degraded aircraft performance. All anti-ice/de-ice/ice protection systems were on well prior to entering icing conditions (5 on; warm up; prop heat -10 and below; windshield heat full hot; side window heat on; igniters manual; and boots to fast).This was a freak encounter with severe icing conditions. We immediately exited icing conditions. If we had disengaged the autopilot sooner; we would have been able to detect the abnormal flight characteristics in a more timely fashion. The statement that the autopilot masks the performance degradation is absolutely true. In this example it COMPLETELY masked any abnormal flight characteristics that the aircraft was exhibiting. Next time we are getting anything more than light icing; I will be sure to hand fly the aircraft.


Second narrative: [Report narrative contained no additional information]


Synopsis: A DHC-8-100 climbing through 12;000 feet to 17;000 feet encountered unanticipated severe icing which the de-ice equipment was unable to remove. The aircraft airspeed deteriorated at rated power and aircraft pitch up and down with tail icing so the flight descended out of icing and diverted for fuel.