Ice and Rain Protection — Page 247, Lesson 307

I want to walk you through the Sangamo Weston ice detector, because it's a really good example of how we think about ice detection differently on aircraft. Let me start with the core idea, because it shapes everything else. Ice can only form when you have two things at once: moisture in the air and freezing temperatures. Most ice detectors work by letting ice actually build up on them and then sensing that accretion. The Sangamo Weston does something different. It detects the two conditions separately — moisture and temperature — rather than detecting actual ice formation. That's why we call it an inferential method of ice detection. It infers that icing conditions exist, rather than waiting for ice to physically accrete. Every other ice detector you'll study uses the principle of ice accretion; this one is the exception. The system is built from three main components, and I want you to hold all three in your head because they work together as a team. First, the Moisture Detector Controller. This is the brain, and it sits in the base of the unit. Its job is to sense the temperature difference between two sensing bulbs — a "wet" bulb and a "dry" bulb. When that temperature difference reaches a predetermined value, and provided the thermal switch is made, relays operate. Those relays either trigger an ice warning or initiate the anti-icing or de-icing cycles. Notice the two conditions that must both be true: the temperature difference has to hit its set value, and the thermal switch has to be closed. Both gates have to open. Second, the Moisture Sensing Head. This is the part that actually detects water. It consists of two heated metal resistance bulbs, both sitting in the airflow. They're arranged so that the leading bulb screens the rear one — meaning the front bulb shields the back bulb so that no moisture can impinge upon it. So when the detector encounters free water in the airflow, the leading bulb gets wet, while the shielded rear bulb stays dry. Here's the physics that makes it work: the dry rear bulb cools at a slower rate than the wet leading bulb. Why? Because the wet bulb loses heat faster through evaporation. So the two bulbs cool at different rates, and that difference in temperature is exactly what the controller is sensing. Third, the Thermal Switch. This is a contact-operating thermometer, housed in a bulb, exposed to ambient temperature. Its role is to be the gatekeeper for temperature. When the ambient temperature is above freezing, the thermal switch prevents the moisture detector from sending an ice warning signal — even if the moisture detector is sensing water in the airflow. When the temperature is below freezing, the thermal switch allows the warning signal to be sent. So you see, the thermal switch is what confirms the "freezing" half of the icing condition, while the moisture sensing head confirms the "moisture" half. Let me tie the whole sequence together, because this is how the system thinks. The moisture sensing head detects free water in the airflow and produces a temperature difference between its wet and dry bulbs. The controller senses that difference. But the controller won't act unless the thermal switch is made, which only happens when ambient temperature is below freezing. When both conditions are satisfied — water present and temperature below freezing — the relays operate and you get your ice warning, or the anti-icing or de-icing cycle starts. One thing I want to emphasise, because it's a classic exam distinction: this detector detects icing conditions, not ice itself. It's inferential. The moment you see "Sangamo Weston," I want you to think "inferential — detects conditions, not accretion." That's the signature of this unit. Now, let me show you the physical layout, because the arrangement of those bulbs matters. That figure shows you the three components and how they're situated. You can see the moisture sensing head with its two bulbs in the airflow, the controller in the base, and the thermal switch exposed to ambient temperature. The key visual takeaway is the shielding — the leading bulb protecting the rear one from moisture impingement. That single design choice is what creates the wet/dry temperature difference the whole system depends on. So to summarise the logic in one breath: moisture sensing head detects water, thermal switch confirms freezing temperatures, controller combines both signals, and only then do the relays fire the warning or the anti-icing or de-icing cycle. Inferential detection, three components, two conditions, one warning. That's the Sangamo Weston.