An airspeed indicator is one of the main flight instruments in an aircraft cockpit because it tells the pilot how fast the airplane is moving through the surrounding air. This matters for takeoff, landing, climbing, cruising, and avoiding stalls or structural damage. The instrument does not measure ground speed directly, because wind can make the airplane move over the ground faster or slower than it moves through the air.
Instead, it uses air pressure to estimate the aerodynamic speed that affects lift and control.
Key Facts
- Dynamic pressure is the pressure caused by moving air: q = 1/2 rho v^2.
- The airspeed indicator compares pitot pressure and static pressure: q = p_total - p_static.
- For low speed flight, indicated airspeed is based on v = sqrt(2q/rho).
- Pitot pressure, also called total pressure, is measured by a forward-facing pitot tube.
- Static pressure is measured from ports that sense the surrounding atmospheric pressure.
- Color arcs mark important V-speeds, such as white for flap operating range, green for normal range, yellow for caution range, and red for never exceed speed.
Vocabulary
- Airspeed Indicator
- A cockpit instrument that displays aircraft speed through the air using the pressure difference between pitot and static pressure.
- Pitot Pressure
- The total pressure measured by a forward-facing tube that includes both static pressure and pressure from the airplane's motion through the air.
- Static Pressure
- The atmospheric pressure around the aircraft measured by static ports that are not aimed into the airflow.
- Dynamic Pressure
- The pressure difference caused by airflow motion, equal to p_total minus p_static.
- V-speed
- A standard airspeed used in aviation to mark important limits or operating speeds for an aircraft.
Common Mistakes to Avoid
- Confusing airspeed with ground speed is wrong because the airspeed indicator measures motion through the air, not motion over the ground.
- Using only pitot pressure is wrong because the instrument needs the difference between pitot pressure and static pressure to find dynamic pressure.
- Ignoring the color arcs is wrong because each arc marks a different safe or unsafe operating range for the aircraft.
- Thinking a higher indicated airspeed always means a higher true airspeed is wrong because altitude, air density, and instrument corrections affect the relationship.
Practice Questions
- 1 An airspeed indicator senses a pitot pressure of 101800 Pa and a static pressure of 101300 Pa. What is the dynamic pressure?
- 2 Using q = 1/2 rho v^2, find the airspeed when q = 720 Pa and rho = 1.20 kg/m^3.
- 3 A pilot sees the needle enter the yellow arc during bumpy air. Explain why slowing down is safer even if the airplane is still below the red line.