Originally Posted by
Crash one
Under normal conditions. The butterfly controls the airflow, the airflow controls the fuel flow. If the butterfly is closed/throttle down, the mixture ratio remains the same, otherwise we would be running rich at low throttle and lean at full power.
Therefore if the ice builds up, as I believe it does, around the butterfly effectively closing the throttle then the same should apply, mixture stays the same, by ratio.
The ice doesn't form around the butterfly, it forms around the venturi. When the air passes through the venturi constriction it accelerates, so its pressure and temperature drop. That causes the ice to condense out. The condensed ice reduces the area of the venturi causing the air to travel faster than it would otherwise do for that throttle setting. To the fuel metering system that looks the same as a higher throttle setting, so more fuel flows causing a rich mixture.
The increased flow velocity causes a greater pressure/temperature drop in the venturi, which reduces the temperature further and increases the rate of ice formation, which in turn reduces the choke area and increases the flow rate, dropping the temperature further and so on causing a runaway condition. That's why once the ice starts to form it can build to dangerous levels very quickly.
PDR