It's been around for years, but still remains an issue A


wise old aviation head once said: “As long as aircraft have carburettors, there’ll be carb icing.” — He’s right, as any


avid reader of the AAIB’s incident report summaries will know.


So why does it still happen? If you recall the graph about carb icing risks, you’ll know that it isn’t supposed to form quite as quickly on a cold day compared with a summer one, which is true theoretically because cold air holds less moisture than warm air, but in reality it’s best to assume that carb ice is always a potential threat in UK flying whatever the season. As many will know, the first signs with a fixed-pitch propeller can be a slight drop in rpm and performance (airspeed and/or altitude). The loss can be sneakily smooth and gradual, and the usual reaction is often to open the throttle slightly to compensate. While this might restore power it’s only hiding the loss as ice builds in the venturi, until there’s rough running, vibration and loss of performance until the engine stops. With a constant-speed propeller, and in a helicopter, the loss of power tends to be large before an rpm reduction occurs. Icing here can be even more insidious, but there


THE HOT FACTS OF CARB ICE


will probably be a drop in manifold pressure and a reduction in performance. While the primary detection of carb icing in many fixed-pitch aircraft is the rpm gauge together with the ASI and altimeter, for constant-speed propellers the primary detection instrument is the manifold pressure gauge. In steady level flight an exhaust gas temperature gauge, if fitted, might show a decrease in temperature before any significant decrease in engine and aircraft performance.


The general threat of carburettor icing


theory is based on the physics of fuel vaporising and mixing with air in the venturi-based carburettor before entering the engine cylinders. The lower pressure in the venturi plus the natural cooling from a liquid vaporising (latent heat of energy transfer) contributes to a significant temperature drop often to freezing values. If there’s one thing even the clear air in our UK atmosphere is rarely short of in significant quantities, it's water vapour ready to solidify as ice should carburettor temperatures drop below zero. Water-cooled engine bodies do, however, tend to cool less quickly when power is reduced, lessening the icing severity. Coolant directed around


the carburettor body might also maintain the venturi temperature above freezing. So, what to do about it? Some engines do, of course, have electric heaters which increase the temperature of the carburettor, and a similar effect may be obtained in a liquid-cooled engine by directing the flow of coolant. On other air-cooled engines, selecting the alternative air source by-passes the normal intake filter and supplies air heated in an exhaust heat-exchanger to melt any ice. Fuel-injected engines generally have an alternate air intake located via a valve downstream from the normal air intake. Alternate air is warmed by engine heat, and doesn’t normally pass through a heat- exchanger.


So select hot air as a matter of routine at regular intervals to prevent ice build-up; if there’s a drop in rpm or manifold pressure, or rough engine running and when carb icing conditions are suspected. Apply carburettor heat at regular intervals for long enough (at the very least 15 seconds, but considerably more in certain aircraft) to pre-empt the loss of engine power or restore power to the original level. If icing has caused a loss of power and the hot air disperses it, re-selecting cold air should produce an increase in rpm or manifold pressure; absence of carb icing should produce no increase in rpm or manifold pressure beyond that noted prior to the use of hot air. Remember, selecting hot air when ice is present might at first make the situation appear worse as the engine runs roughly while the ice melts and passes through the engine. If this happens do resist the temptation to return to cold air so that the hot air has time to clear the ice — that might be in the region of 15 seconds, which can feel like a very long time.


Judging by the large number of successful flights in the UK, we have demonstrated that we can cope, but it is worth being careful to avoid proving that wise old aviation head’s adage about carb ice…


SUMMER 2019 CLUED UP 33


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