Piston Engines - Icing — Page 120, Lesson 156

Let’s pick up with the fuel-injected engine, because it’s a nice contrast to the carburettor we’ve been discussing. The fuel-injected engine does not have the problem of ice forming at the Venturi — that’s the throat in a carburettor where air speeds up and cools dramatically, which is the classic spot for what we call refrigeration icing. But — and this is the key point — the fuel-injected engine is not immune to icing. Other parts of the system may accumulate ice, and the result is a similar loss of power. So where does the ice gather on a fuel-injected engine? Let’s walk through the three spots. First, fuel icing may gather at the bends in the system — that’s where the fuel lines change direction, and ice crystals can collect there. Second, impact icing may form at the impact sensing tubes — these are the little tubes that sense the air pressure for the fuel metering, and ice can build up on them as air impacts them. Third, ice can form on the air filters, particularly when flying in cloud at low temperatures. Now, the important operational point: the alternate air system fitted to these engines should then be selected, and the icing drill followed according to the aircraft check list. So the procedure is — select alternate air, then run the drill from the checklist. Now let’s move to diesel engines, because they handle icing differently again. Diesel engines do not suffer from icing in the same way as conventional piston engines. Why? Firstly, there is no carburettor, and therefore no Venturi to attract the refrigeration icing associated with float chamber carburettors. So that whole class of icing — the cooling-induced ice at the Venturi — simply doesn’t apply. But diesel engines still have their own icing problems, and they solve them in specific ways. Impact icing at the air-inlet filter is overcome by the use of what are called ‘ice-guards’. These effectively by-pass the filter when it becomes blocked with ice — so instead of the ice choking off the air, the guard routes the air around the blocked filter. Then there’s a different problem: fuel-solidification, known as ‘waxing’. This is where the fuel viscosity increases due to low temperatures — the fuel gets thicker and more viscous as it gets cold, and it can solidify or wax up. That’s overcome in two ways: by putting additives in the fuel, or by using fuel-heaters in the fuel-lines or filters to ‘pre-heat’ the fuel. So additives change the fuel’s behaviour, and heaters physically warm it before it reaches the engine. Now let’s move to the operational procedures — the practical side of using carburettor heat. There are several phases of flight to consider, and each has its own rules. First, ground operation. Use of the heat control on the ground should be kept to a minimum. Why? Because the air is not filtered — when you apply carburettor heat on the ground, you’re drawing in unfiltered air, and that may feed dust and dirt into the system, causing additional wear on pistons and cylinders. So you want to minimise that. However, a function check of the heater control should be made before take-off. Here’s what you should see: rpm should drop approximately 100 rpm when heat is applied, and return to the selected setting when turned OFF — that’s the cold position. So that 100 rpm drop is your confirmation the heat system is actually working. Now take-off. If icing is evident on the ground before take-off, use heat to clear the ice — but return the control to OFF, cold, before applying take-off power. Then check that normal take-off power is available. So the sequence is: clear the ice with heat, then go back to cold, then apply power and verify you’ve got full power. Climb. Do not use carburettor heat during the climb, or at power settings above 80% — that’s approximately 2500 rpm. So above that power setting, you keep the heat off. Then flight operations — the general rules for when you’re airborne. Be aware of conditions likely to cause carburettor icing: damp, cloudy, foggy or hazy days, or when flying close to cloud, or in rain or drizzle. Those are your danger conditions. Look out for an unaccountable loss of rpm or manifold pressure — that’s your first sign something’s wrong. Make frequent checks for icing by applying heat for a period of between 15 to 30 seconds, noting first the selected rpm, then the drop of rpm as heat is applied. So you watch the rpm drop when you apply heat — that’s normal. Listen to the engine noise and check the outside air temperature. Now here’s the diagnostic part: should rpm increase whilst heat is applied, or the rpm return to a higher figure than the original when you re-select to cold, then ice is present. Think about that — if ice was blocking the intake, melting it with heat actually improves the airflow, so the rpm rises. That rise tells you ice was there. And the rule is: continue to use heat while flight in icing conditions continues. So you keep the heat on as long as the icing conditions persist. Let me just tie that together. The fuel-injected engine moves the icing problem away from the Venturi but doesn’t eliminate it — you’ve got bends, impact sensing tubes, and air filters, and the answer is alternate air. The diesel engine avoids Venturi icing entirely, but deals with impact icing via ice-guards and waxing via additives or fuel-heaters. And in the cockpit, carburettor heat is a tool with specific rules per phase: minimal on the ground, clear ice before take-off then back to cold, no heat in the climb above 80% power, and in flight, use those 15 to 30 second checks to detect ice by watching for an rpm rise.