Ice and Rain Protection — Page 253, Lesson 319

I want to walk you through the two main ways we protect an engine air intake from ice: the engine hot air system, and the electrical heating system. Let's start with the hot air system. In an engine hot air system, the air is bled from the compressor. That means we tap off some of the hot, compressed air that the engine is already producing. This air is then fed via ducting into the air intake nose cowl — that's the leading edge of the intake. From there it flows through the inlet guide vanes of the engine, and in some engines, also through the nose bullet, which is the pointed spinner at the very front. After circulating through the intake cowl and the guide vanes, the air is exhausted either to atmosphere or back into the engine air intake. The flow of that hot air is regulated by electrically operated control valves. Those valves are actuated by control switches on a cockpit panel, so the pilot turns the system on and off from the flight deck. One important point: an air temperature control system is not usually provided in a hot air system. We don't regulate the temperature of that bleed air — we just use it as it comes off the compressor. Now let's move to the electrical heating system. Here, instead of bleed air, we use heating elements. These elements are either resistance wire or sprayed metal, and they are bonded to the air intake structure. The power supply required for heating is normally three-phase alternating current — so three-phase AC. Let me describe the arrangement used in a widely used turbo-propeller engine. The elements are of the resistance wire type, and they're formed into what's called an overshoe. That overshoe is bonded around the leading edge of the air intake cowl, and also around the oil cooler air intake. Now here's a key distinction. Both anti-icing and de-icing techniques are employed. Anti-icing uses continuously heated elements — they stay hot the whole time, preventing ice from forming. De-icing uses intermittently heated elements — they heat up in cycles, allowing ice to form a little and then shedding it. So the continuously heated elements give anti-icing, and the intermittently heated elements give de-icing. The elements are sandwiched between layers of glass cloth impregnated with resin. In some systems, the elements may be sandwiched between layers of rubber instead. And in all cases, the outer surfaces are suitably protected against erosion by rain, and against the effects of oils, greases, and so on. Now, how is the power distributed? The power supply is fed directly to the continuously heated elements. But for the intermittently heated elements, and for the propeller blade elements, the power goes via a cyclic time switch unit. That unit controls the application of current in selected time sequences. Those sequences are compatible with the prevailing outside air temperature conditions and the severity of icing. The time sequences which may be selected vary between systems. For the system shown in Figure 12.13, there are two sequences. The first is called 'Fast'. It gives one complete cycle — that's heat on, then heat off — of 2 minutes, and it's used at outside air temperatures between -6°C and +10°C. The second is called 'Slow'. It gives one complete cycle of 6 minutes, and it's used at outside air temperatures below -6°C. So think about it: when it's warmer, around freezing, we cycle quickly at 2 minutes. When it's colder, below -6°C, we cycle slowly at 6 minutes. Finally, there's an indicator light, and in some cases an ammeter, provided on the appropriate cockpit control panel. That's to indicate correct functioning of the time switch circuit — so the pilot can see the system is actually cycling properly. Let me also show you the heater mat construction. In Figure 12.13 you can see the heater mats themselves, with the glass cloth layers, the electrical elements, and the distinction between the intermittently heated elements and the continuously heated elements. That's the physical build of what we just described. So to summarise the whole picture: hot air systems use compressor bleed air through the cowl, guide vanes, and nose bullet, with electrically operated valves. Electrical systems use resistance wire or sprayed metal elements bonded to the structure, powered by three-phase AC, with continuously heated elements for anti-icing and intermittently heated elements for de-icing, timed by a cyclic switch unit with Fast and Slow sequences.