Very enlightening, thank you. That might explain the loud noise I
heard while flying from PR to Savannah, Ga. in a C-130 (8 hour flight,
no movies), de-icing using exhaust heat. That was loud and unexpected.

Hector

On 7/31/12, JRodgers <jrodgers113@gmail.com> wrote:
> Airplanes - piston types and jets - have combustion air heaters to
> pre-heat the air to remove/prevent ice formation inside the throat of
> the carb/turbine. A short story about this kind of ice.
>
> Once in Alaska, while on a trip as co-pilot in a Skyvan, was at 12,000
> ft over the Cook Inlet and coming home to Anchorage. Had been cleared to
> the Anchorage VORTAC for a hold outside Anchorage prior to landing and
> began decent. We were in the tops of clouds and began picking ice, and
> it would get worse as we descended through it to below the freezing
> level. The captain reached over and flipped both smile heaters switches
> to the solenoid valves that routed heat from the turbine compressor
> sections  into the stainless plenum around the air intake of the
> engines. Those plenums were coated with ice. The instant that ice melted
> that slug of water went directly into the engine and snuffed out the
> flame in both engines. At 12,000 feet both engines had quit. We suddenly
> became a very heavy glider with a pee-poor glide ratio. WE were out of
> 7000 feet  and nearing 6000 and descending fast before we managed to get
> the engines started again. Scared the bejeebers out of us. Everything
> was OK after that, except for the seat of our pants and the cones in the
> captains and co-pilots seats. An official change in operating procedures
> followed, along with some pilot training - Only one smile heat switch at
> a time was to be turned on to eliminate dual engine failure.
>
> Carb ice works the same way. Turn on a carb heater on a piston aircraft
> - the engine will run rough until it swallows all that water from the
> melting ice. Pilots learn to tickle the carb heat, so the ice goes away
> slowly, otherwise the engine may quit.
>
> Same goes on these carbs. But with no Carb air heaters, they can be a
> major PITA in wet, cold air. Ice can build in the throat of the carb and
> simply choke off the combustion air.
>
> It's all about Bernoulli's Principle. Carb has a venturi, venturi's drop
> the pressure, increase the air flow speed and drop the temperature. If
> weather is cool and moist, heck -even warm and moist - if the temp in
> the throat of the carb goes below freezing, whatever moisture is in the
> air will begin to plate out as ice in the carburetor. Thus the need for
> a carb air heater of some sort.
>
> John