If you turn down the sub-scale on an altimeter, what happens to the indicated altitude?

When you adjust the sub-scale on an altimeter to a lower setting, it effectively alters the reference pressure level that the altimeter uses to interpret altitude. The sub-scale is set to indicate the local altimeter setting, typically measured in inches of mercury (inHg) or hectopascals (hPa). By turning it down, you are providing the altimeter with a lower pressure value.

As altitude is determined based on the ambient atmospheric pressure, and lower pressure corresponds to a higher altitude, decreasing the sub-scale means that the altimeter will read a lower value for a given pressure. Thus, as you turn the sub-scale down, the indicated altitude decreases. This adjustment can lead to a situation where the pilot may misinterpret their actual altitude above sea level.

In contrast, if the sub-scale is set accurately or turned up, the indicated altitude would increase, reflecting the need for the altimeter to correlate with the pressure instead of the actual altitude. Keeping the sub-scale correctly set is crucial for accurate altitude readings during flight operations, hence understanding the relationship between the sub-scale setting and the indicated altitude is vital for pilots.