Which instruments use static pressure only?

• A. Airspeed indicator.
• B. Pitot heater.
• C. Altimeter and VSI.
• D. Magnetic compass.

Answer: (C)

The key idea is that some cockpit instruments rely on static pressure alone to indicate their readings. The altimeter and the vertical speed indicator are the ones that fit this.

An altimeter measures altitude by sensing the ambient static pressure in the aircraft’s static system. As you climb, the surrounding air pressure drops, and the altimeter’s sealed capsules expand or contract to display your altitude. It uses static pressure only; dynamic pressure from air movement (pitot pressure) isn’t required for its indication.

The vertical speed indicator (VSI) shows how quickly you are climbing or descending by monitoring the rate at which static pressure is changing. It uses a small orifice and a diaphragm connected to static pressure, so the instrument responds to the change in static pressure over time, not the absolute pressure itself. This means it also relies on static pressure rather than dynamic (pitot) pressure.

Airspeed is determined by dynamic pressure, which is derived from both pitot (ram) pressure and static pressure. If you only had static pressure, an airspeed indicator wouldn’t read correctly—the pitot pressure is essential for measuring airspeed.

The magnetic compass reads Earth’s magnetic field and is unaffected by static pressure, and the pitot heater is simply a component used to prevent the pitot tube from freezing; it doesn’t constitute an instrument reading based on pressure.

So, the instruments that use static pressure only are the altimeter and the vertical speed indicator.