The pitot-static system is a set of small pressure ports and tubes that helps a pilot know how fast and how high an aircraft is flying. It uses air pressure from outside the airplane, not electronics alone, to drive three important cockpit instruments. These instruments are the airspeed indicator, altimeter, and vertical speed indicator.
Understanding this system matters because a blocked tube or port can give dangerously wrong information in flight.
The pitot tube faces into the airflow and measures total pressure, which includes the pressure of the moving air. Static ports sit flush with the aircraft skin and measure the still outside air pressure around the airplane. The airspeed indicator compares total pressure with static pressure, while the altimeter and vertical speed indicator use static pressure to estimate altitude and climb or descent rate.
If ice, insects, dirt, or tape block a port, the pressure signal can become trapped or lost, causing instrument errors that pilots must recognize quickly.
Key Facts
- Pitot pressure measures total pressure: P_total = P_static + q.
- Dynamic pressure is q = 1/2 rho v^2, where rho is air density and v is airspeed.
- The airspeed indicator compares pitot pressure and static pressure to estimate speed.
- The altimeter uses static pressure because air pressure decreases as altitude increases.
- The vertical speed indicator uses the rate of change of static pressure to show climb or descent.
- A blocked pitot tube, blocked static port, or blocked drain hole can each create different instrument errors.
Vocabulary
- Pitot tube
- A forward-facing tube that senses total pressure from air moving into it.
- Static port
- A small flush opening on the aircraft skin that senses outside static air pressure.
- Dynamic pressure
- The pressure caused by moving air, equal to q = 1/2 rho v^2.
- Airspeed indicator
- A cockpit instrument that estimates airspeed by comparing pitot pressure with static pressure.
- Vertical speed indicator
- A cockpit instrument that shows how fast an aircraft is climbing or descending from changes in static pressure.
Common Mistakes to Avoid
- Treating pitot pressure as the same as static pressure is wrong because the pitot tube measures total pressure, which includes the effect of the aircraft moving through the air.
- Thinking the altimeter uses the pitot tube is wrong because the altimeter is connected to the static system and responds to outside air pressure.
- Ignoring the static port during blockage problems is wrong because the airspeed indicator, altimeter, and vertical speed indicator all depend on static pressure.
- Assuming a blocked pitot tube always makes airspeed read zero is wrong because trapped pressure can make the airspeed indicator act like an altimeter during climbs and descents.
Practice Questions
- 1 An airplane has static pressure of 79,500 Pa and total pressure of 81,300 Pa. What is the dynamic pressure sensed by the airspeed indicator?
- 2 Using q = 1/2 rho v^2, find the airspeed if dynamic pressure is 1,800 Pa and air density is 1.25 kg/m^3.
- 3 During preflight, a pilot finds tape covering the static port but the pitot tube is clear. Explain which instruments would be affected and why this is unsafe.