Which instrument does not require a power source to function properly?

The altimeter is an instrument that does not require a power source to function properly because it operates based on the principles of barometric pressure. It measures the changes in atmospheric pressure surrounding the aircraft, allowing it to determine altitude based on the standard atmospheric pressure at sea level. The mechanical design of the altimeter utilizes aneroid wafers that expand and contract in response to changes in air pressure, which gives a direct visual indication of altitude on the dial.

In contrast, the other instruments listed rely on electrical power to function. The vertical speed indicator (VSI) uses a diaphragm connected to a static port and requires a power source to provide accurate readings of the aircraft's vertical speed. The airspeed indicator depends on a difference between dynamic and static pressure, typically requiring an electronic system to interpret this data effectively. The heading indicator, often a gyroscopic instrument, depends on electrical power to maintain and display the aircraft's heading accurately. Understanding the operational principles of these instruments highlights the distinction between those that are mechanically driven and those that are electronically powered.