Ice and Rain Protection — Page 247, Lesson 308

Let's pick up with the Beta Particle Ice Detection Probe. This is one of the more elegant detection systems, and I want you to understand exactly how it works because it's a classic example of using physics to solve a practical problem. The basic system consists of two probes, mounted perpendicularly from the forward fuselage. So imagine two small probes sticking straight out from the nose area, at right angles to the fuselage skin. Along with those two probes, you have a relay and a flight deck warning. That's the whole system — two probes, a relay, and a warning. Now, here's the clever part. Under nil ice conditions — that is, when there's no ice — the forward probe acts as an emitter. It emits Beta particles. Beta particles are high-energy, high-speed electrons. These particles stream backward toward the rear probe, which acts as a detector. So you have a continuous stream of Beta particles traveling from the front probe to the rear probe. Here's the key principle: Beta particles are absorbed by ice. So when icing conditions exist, and ice builds up on the probes, that ice absorbs some of the Beta particles. The result is that fewer particles reach the rear detector. The detector senses this reduction. Now, there's a specific threshold built into the system. At a certain Beta particle count rate — and this corresponds to 0.4 mm of ice — a relay in the detector probe operates. When that relay operates, it causes a warning on the flight deck. That warning can take the form of an ECAM system display and a single chime. ECAM stands for Electronic Centralised Aircraft Monitor — it's the system that displays warnings and system status on the flight deck screens. So you'd see a visual display and hear one chime. There's also a system test function. When you run the test, it gives you the same indications — the same ECAM display and the same single chime. That way you can verify the system is working without actually having ice. Now, let me show you what these probes look like. — that's the Teddington ice detector, and is the Smiths ice detector. These are earlier mechanical types. The Beta particle probe is Figure 12.8, which I don't have a scan for, but the principle is what I just described. Now, moving on to a much simpler aid — the Ice Formation Spot Light. Many aircraft have two of these, mounted one on each side of the fuselage. They're positioned so they light up the leading edges of the mainplanes — that's the front edge of the wings. When you switch them on, you can visually examine the wing leading edges for ice formation. This is a purely visual aid. And here's an important note: in some aircraft, this may be the only aid to ice detection at night. So on certain types, if you're flying at night, these spot lights are all you have to check for ice. Now I want to give you the operational rules that tie all this together. An awareness of the in-flight conditions — temperature and moisture — is essential for all aircrew. There's a general rule for engine protection: apply it when the IOAT is +10°C or below, and the air contains visible moisture. IOAT stands for Indicated Outside Air Temperature — that's the temperature reading you get from the aircraft's sensors. So the rule is: if the indicated outside air temperature is 10 degrees Celsius or colder, and you can see moisture in the air — clouds, fog, rain, that sort of thing — then you apply engine anti-icing or de-icing protection. Airframe protection is different. It's generally applied at the onset of indicated icing. That means you start protecting the airframe as soon as you have evidence that ice is actually forming. That evidence can come from visual indications — ice on the leading edges, on aerials, on windscreen wipers, and so on — or it can come from the ice detector systems we've been discussing. Now, here's how the warnings work. Ice warnings usually take the form of an amber caution light. And in some systems, that warning can actually initiate the de-icing or anti-icing systems automatically — but only if those systems have been preselected to 'auto'. So the crew sets the system to auto before flight, and when ice is detected, the system kicks in by itself. But there's a critical limitation I need you to remember. Mechanical de-icing by the 'Boots' method must not be initiated until a specific depth of ice has built up. The Boots method — that's the inflatable rubber boots on the leading edges that expand to crack off ice. You cannot just switch those on the moment you see a trace of ice. You have to wait until a specific depth has accumulated. That's because the boots need a certain thickness of ice to work effectively — if you fire them too early, they won't crack the ice off properly. So there's a deliberate delay built into the procedure. Let me make sure you've got the full picture. The Beta particle probe gives you a precise, quantitative measurement — 0.4 mm of ice triggers the warning. The spot lights give you a visual check. And the operational rules tell you when to act: engine protection at +10°C or below with visible moisture, airframe protection at the onset of indicated icing, and Boots only after a specific ice depth has built up. Each method has its place, and knowing when to use which is part of your job as a professional pilot.