Which instrument is a form of pneumatic altimeter with an adjustable barometric scale?

• A. An electronic altimeter that uses GPS to determine altitude.
• B. A standard single-pointer altimeter with fixed sea level reference.
• C. A form of multipointer pneumatic altimeter with an adjustable barometric scale that allows the reference pressure to be set to any desired level.
• D. An instrument used to measure airspeed.

Answer: (C)

Pneumatic altimeters measure altitude by responding to static air pressure and use a barometric setting to convert that pressure into a readable altitude. The key here is having a barometric scale that can be adjusted to match the local atmospheric pressure, so the instrument can show altitude relative to the current pressure reference rather than a fixed baseline. A multipointer pneumatic altimeter with an adjustable scale fits this description exactly, because its multiple pointers convey altitude and its barometric scale can be set to any desired reference pressure. This allows the pilot to read altitude that corresponds to the local pressure setting (the QNH), which is essential for accurate altitude determination in varying conditions.

The other options don’t fit as well. An electronic altimeter that relies on GPS is not a pneumatic instrument and does not use a pressure-based scale. A standard single-pointer altimeter with a fixed sea level reference lacks the ability to adjust for current pressure, which is necessary for accurate altitude readings in different environments. An instrument used to measure airspeed is a different type of instrument altogether and does not measure altitude.