The subscale is purely pilot-controlled — Page 53, Lesson 67

Let me walk you through this. We're looking at the subscale on a sensitive altimeter — that's the little window with numbers you set on the face of the instrument. The key idea here is that the subscale setting only changes when the pilot turns the knob. That's the only input that moves it. A change in altitude — say you climb from 5,000 to 10,000 feet — has no direct effect on the reading of the subscale. Likewise, a change in surface pressure, like the barometric pressure at the airfield changing, has no direct effect on the subscale either. The subscale is purely pilot-controlled. Now here's the clever part about the mechanism. When the pilot alters the subscale setting, the altimeter pointers move. So if you turn the knob, the hands on the face rotate to reflect the new setting. But the design of the mechanism is such that the reverse does not apply. That means during a climb, for example, the pointers rotate as you gain altitude, but the subscale setting remains unchanged. The pointers move with altitude, but they can't drive the subscale back. It's a one-way relationship — the knob drives the pointers, but the pointers can't drive the knob. Now, a practical detail for British altimeters. The subscale setting range is between 800 and 1050 hectopascals. That's the range of pressure values you can set into the window. So when you're setting QNH or QFE, you're turning that knob within that 800 to 1050 hPa range. Let me tie this to the figure. shows the sensitive altimeter. The principle of operation is similar to the simple altimeter, but there are differences — and this subscale mechanism is one of them. The simple altimeter in Figure 5.2 feeds static pressure into the case of the instrument from the static source. The sensitive altimeter builds on that, but adds this subscale that you, the pilot, set. So the takeaway: the subscale is a pilot-set pressure reference. It only changes when you turn the knob. Altitude changes and surface pressure changes don't touch it. The pointers respond to your knob input, but the pointers can't change the subscale. And on British altimeters, you've got that 800 to 1050 hectopascal range to work within.