As a First Officer flying both the Twin Otter and Dash 8 in Alaska I have seen some icing. 99.9% of the time the de-ice boots have worked great, but I have had ice bridging occur once. In most icing test the boots are in great condition. I think a major factor in ice bridging is the condition of the de-ice system. Fly an aircraft that has a couple of patches on the boots, has not been ice-xed in a while and dirty boots the ice seems to stick to it.
We were flying a 200 Series Twin Otter(less power than the more prominent 300 Series) from Bethel to Anchorage over the Alaska Range. It was late spring, early summer and the leading edge was covered with a combination of old deice fluid, dirt and bugs. The MEA between Sparrevohn (SQA) and Anchorage is 12000 feet and a MOCA of 10,300 feet. The 200 series Otter is usually temp limited or at the stops at altitudes above 10,000 resulting in torque settings around 37 or 38 pounds of torque. The outside air temperature was around -20 Celsius. We got into some light rime ice just past SQA at 13000 feet and cycled the boots (to early?) with ice estimated at around 1/8 to 1/4 inch. The boots inflated but most of the ice did not shed. We decided to let it build up a little more ant try it again. This time with a little more than a ½ inch of ice we cycled the boots with the same result. Climbing was not an option so we descended to 11,000 feet and continued to cycle the boots with out any real improvement. Although there seemed to be less icing at that altitude. We were in the ice for about fifteen to twenty minutes before we could descend to 6000. By that time we had accumulated about 1 1/2 inches of ice and slowed considerably. Once in the clear and with a head of steam in the descent we started shedding the ice as we descended through about 7,000 and the temperature was still below freezing. The boots seemed to be working fine after we exited the icing condition and got into warmer temperatures. I don’t know why we couldn’t shed the ice. I think part of it may have been the condition of the boots.
The incident above happened before we changed our Ops Specs in accordance with the FAA’s new recommendation. In both the Twin Otter and the Dash 8 our ops specs now have us activate the boots upon entering icing conditions. We leave them on until at least one complete cycle has been performed after exiting icing. I have not seen “ice bridging” occur since implementing these procedures. But I have seen more residual ice on the boot after the first cycle or two. But the end result was the boots were able to keep shedding most of the ice and not allowing it to build up. I must admit though I am not totally convinced that this procedure will prevent ice bridging.
On another note, climbing out of Kodiak last week we got into some freezing rain at -15 Celsius. That is the coldest temperature I have ever seen freezing rain. The cloud didn’t look threatening and the weather radar barely showed some green.
