Carb Icing

[FireRescue85]

Hey guys,

I frequently fly in cold weather due to living in the Northeast (it's usually below 40 degrees F here for 6 months out of the year) and encountered an issue the other day. I was on a slow descent through some cold weather and would have to fly for an extended time through some low level clouds. At about 3,500 ft AGL the flight status was as follows: OAT of -8 C, thin clouds at my altitude from about 1,000-5,000, manifold pressure was holding at 21'', intercoolers were set to warm, carb filters were on, and turbos were running at 6.5 setting. After a few minutes of this configuration, I suddenly started to lose manifold pressure in all engines and the flight engineer said all four engines have problems. My carb temp was at 5 C, cold but (theoretically) not cold enough for ice. I was able to correct the problem by pushing turbos to 88, and after playing with the throttle for a bit was able to obtain a manifold pressure to 35" and a carb temp of 13 C. Once the carbs warmed up and the ice melted, the engines no longer had problems and full power was restored.

My questions are this: Is it possible to experience carb icing even if your carb temp is above 0 C ? And is there cold weather that could cause the B-17 to be grounded? I read that B-17 bombing raids over Germany were suspended due to snow and cold weather at times.

[WB_FlashOver]

Howdy FireRescue,

I've been in northern Russia as of late during my Europe tour (heading east thru Europe). I've been having a heck of a time keeping carbs warm especially during descent. At times I'm running my turbo selector at 10. Yes, your carbs can freeze at temperatures as high as 15-21C depending on the humidity. I try to keep carbs at or above 15C at altitude where moisture is low and higher carb temps at lower altitudes where the humidity is higher. Another trick, if you're still having issues with turbos at 10, is to increase RPM's. This will help the turbos compress air and thus heat it. I've ended up descending and approaching at 2300 RPM's and this seems to help. Temps on the ground have been around 10C or less with heavy fog and clouds which requires 6+ on turbo selector while idling and taxiing.

Roger