Cessna Twin Piston Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft; Weather
While flying back from an unsuccessful maintenance reposition flight [we] experienced severe icing.  It was not known the level of icing until we were on the ground.  The icing charts/forecast did not have severe or moderate icing forecast for our route of flight. We were IMC for almost the entire flight and did not begin to pick up any ice until roughly 30 miles from (destination). When we began to encounter icing all of the Anti-Icing equipment was already on and the boots had not been used yet. We noticed a line of white on the leading edge during descent and I decided to blow the boots on the base leg once it appeared to accumulate enough on the wing.  The boots inflated but the ice did not depart the leading edge of the wing.  Our thought was that it must not have been enough on the wing for it to have broken.  At this time we had not heard any ice being shed from the propellers. When I put the gear down on the ILS approach and moved the props forward it sounded as if a shotgun was being fired rapidly. That was the ice coming off the props and hitting the side of the nose.  We were in the clouds after putting the gear down for about 1000 more feet and landed normally. We knew we had ice on the airplane and I was planning on going right back out.  As we were parking; [another employee] was walking to his plane and waited for us to park; when he saw our wing and I saw his reaction; I knew that we had carried significantly more ice in than we thought. When I inspected the plane afterwards I decided to not go back out that evening.  The plane was re-positioned to the hangar and when maintenance saw it; started testing the anti/de-ice equipment.  They asked if we were going to write the plane up and I said no because we had indications that everything was working normally.
Cessna twin engine aircraft pilot reported accumulating significant ice during flight with a great deal present after landing.