Date: 201901
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: Icing
Aircraft: Cessna 402/402C/B379 Businessliner/Utiliner
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Taxi
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Commercial
Second reporter function: Captain; Pilot Not Flying
Anomaly: Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Weather
Primary problem: Weather

Narrative: On climbout from ZZZ we initially picked up trace mixed icing. As we climbed through about 5;000 feet we were picking up heavier ice. The ice was shedding off the aircraft and causing it to yaw back and forth as the ice would shed. At this point [we] decided it was time to go full power and continue climbing out of the ice and blow the boots. Most of the ice came off at that point. We got on top at around 7;000 feet where we were able to shed the majority of the ice. We continued and landed at [our destination] without incident. We made the decision not to return to ZZZ that evening due to the ice as going missed at ZZZ and climbing back into the ice would be poor.


Second narrative: I and my First Officer were conducting a flight from ZZZ to [our destination.] It just so happened; that our flight was empty; so this was going to be a reposition flight with just the two of us. The current weather at the time was VFR; with overcast bases at 4;000 feet. Since we had just flown the flight into ZZZ; we knew that there was a decent amount of ice in these clouds and that the tops were between 6;800 feet and 7;400 feet. Since we had [mountains] enroute; staying low was not an option today; so we were going to fly up to 9;000 feet; which would keep us away from the mountains and clouds and would give us some nice tailwinds to [our destination.] The ice didn't seem to be much of a problem on the way in so we figured it would be about the same on the way out.We took off from ZZZ runway XX and proceeded direct [to our destination.] My First Officer was the pilot flying on this leg; while I was taking care of the radios and monitoring. We picked up our IFR clearance in the air; and started turning on our deicing equipment in preparation for climbing through the cloud layer. The first part of the flight was nominal; and aircraft was climbing at a rate of 500 feet/minute with the manifold pressure at 29.5;' tachometer set to 2;700 RPM; and fuel flow set to 120 PPM. Then at about 4;000 feet; we entered the clouds and began to pick up the ice.At first everything seemed pretty standard; you see your windshield start to fog up with ice except for the portion heated by the windshield heater; the wings start to accumulate ice; and the props start to sling ice toward the nose of the plane giving the occasional 'Bang!' sound. Nothing I haven't seen or heard before flying the [from] ZZZ a little more than a year now. However; it wasn't long before things started to change. The airplane was picking up ice quick; airspeed was slowly bleeding off and it wasn't long before it was at a 1/2' thickness; and we started to blow the de-ice boots. The de-ice boots took off about half the ice that had accumulated; and the aircraft barely regained any speed it had lost.The aircraft continued to take on more ice during the climb; and airplane did something I had never before experienced. As the pieces of ice being flung off the propeller became larger; the airplane started yawing. It felt like if someone was briskly applying pressure to once side of the rudder; then to the other side a couple of seconds later. Then as the ice came off the boots; you could feel the airplane shudder. My copilot mentioned to me that the sensations were a little nerve-racking and that he didn't like it. I also felt uneasy about the how the aircraft was reacting. Then a large piece came off the right propeller; and the right engine wobbled for about 6 seconds before it normalized itself. I took a look at my airspeed indicator; and are airspeed was now down to 120 knots. With this much ice; and that slow speed; I had it. I commanded my First Officer to add full power; and he did just that. Our speed came up to 140 knots. By this point we must have been about 6;000 feet; and we still kept the plane climbing at 500 feet/minute the whole time. Conditions continued until we broke out at 7;000 feet.When we broke out; we continued our climb to 9;000 feet and our flight to [our destination.] We reported the event over the radio to ATC and company as severe icing. The reason I elected to call it severe icing was because; at the time; I felt if we would have stayed in those conditions any longer; it would have become a potentially life-threatening situation. Looking back at the icing severity definitions; I'd say it was the right call; because; when the right engine started wobbling; that was the point where the de-ice equipment was not keeping the ice hazard under control. When I think back on it; I'd say we were lucky; because; this flight only had my copilot and me. This flight was not carrying any additional weight of passengers or cargo when this happened; and yet the aircraft got as slow as 120 knots. In the climb when it could have easily held 140-150 knots in normal conditions with just the weight of two pilots onboard. Weather was moving in from the west that day. The surface winds can cause the lake effect from [a nearby lake] to bring in heavy snowfall in the direction of ZZZ. It also brings with it a lot of moisture which can help increase the likelihood of encountering ice. Furthermore; the aircraft's slower speed in the climb out of ZZZ also helped expose the aircraft to more ice than what was previously experienced in the descent.


Synopsis: C402 flight crew reported encountering severe icing on climbout.