How many instruments use dual pressures?

• A. All
• B. Two
• C. One
• D. Three

Answer: (C)

In a pitot-static system, “dual pressures” means an instrument uses both pitot (impact/total) pressure and static pressure as inputs. The instruments that rely on both are the airspeed indicator and the Mach number indicator. The airspeed indicator uses the difference between pitot pressure and static pressure to translate dynamic pressure into indicated airspeed. The Mach meter uses both pressures to determine Mach number, since Mach depends on the ratio of true airspeed to the speed of sound, which is tied to static pressure and temperature.

Other instruments, like the altimeter and the vertical speed indicator, respond to static pressure changes alone and do not use pitot pressure in their readings.

So, there are two instruments that use dual pressures.