Let me pick up right where we left off with the sensitive altimeter — Page 53, Lesson 67

Let me pick up right where we left off with the sensitive altimeter. The key point I want you to lock in is this: the subscale setting only changes when the pilot physically turns the knob. Nothing else changes it. Here's what that means in practice. A change in altitude, or a change in surface pressure, has no direct effect on the reading of the subscale. So if you climb, or if the weather system moves in and the pressure at the ground changes, the subscale does not move on its own. It stays exactly where you set it. Now, here's the clever bit about the mechanism. When the pilot alters the subscale setting, the altimeter pointers move. So turning the knob drives the pointers. But the design is such that the reverse does not apply. During a climb, for example, the pointers rotate because the static pressure changes with height, but the subscale setting remains completely unchanged. The pointers move independently of the subscale, and the subscale moves independently of the pointers. It's a one-way relationship. Let me make sure you understand the terminology here. The subscale is that small window, usually at the bottom of the altimeter face, that shows the pressure setting you've dialled in. On British altimeters, that subscale has a setting range between 800 and 1050 hectopascals. Hectopascal is the unit we use for pressure in aviation — you'll sometimes see it written as hPa. So the lowest you can set is 800, the highest is 1050, and that's the full range of travel on the knob. Why does this matter operationally? Because it means the subscale is your reference, your datum. You set it to the appropriate pressure — QNH, QFE, or standard 1013 — and it stays put while the pointers do all the work of showing your height changes. The instrument is telling you height relative to whatever pressure you've dialled in, and that reference doesn't drift on its own. So to summarise the relationship: pilot turns knob → subscale changes → pointers move. But altitude changes → pointers move → subscale stays fixed. That one-way coupling is fundamental to how the sensitive altimeter works, and it's exactly what Figure 5.3 illustrates.