The pitot-static system measures air pressure to drive three of the most important flight instruments: the airspeed indicator, the altimeter, and the vertical speed indicator. It does this with two pressure sources — a pitot tube that faces into the airflow to capture ram (dynamic) pressure, and one or more static ports mounted flush with the airframe to sense the ambient static pressure.
Which instrument uses what
- The airspeed indicator compares ram pressure from the pitot tube against static pressure; the difference is displayed as airspeed.
- The altimeter uses static pressure only, displaying altitude based on the surrounding air pressure and the pilot-set reference in the Kollsman window.
- The vertical speed indicator uses static pressure only, sensing the rate at which static pressure changes to show climb or descent.
Blockages and their symptoms
Because the system is purely pressure-driven, blockages produce predictable errors that are heavily tested:
- Blocked pitot tube (ram inlet only), drain open: the airspeed indicator drops toward zero, because trapped ram air bleeds out and no new ram pressure reaches the instrument.
- Blocked pitot tube and drain hole both blocked: the airspeed indicator behaves like an altimeter — it reads higher as the airplane climbs and lower as it descends, because the trapped pressure stays constant while static pressure changes.
- Blocked static ports: the altimeter freezes at the altitude where the blockage occurred, the vertical speed indicator stays at zero, and the airspeed indicator becomes inaccurate (reading low in a climb and high in a descent).
Backup and protection
Many airplanes provide an alternate static source for use if the primary ports clog, often venting to the cabin; because cabin pressure is slightly lower than outside static pressure, using it typically makes the altimeter and airspeed read a little high. A pitot heat system prevents ice from blocking the pitot tube in visible moisture. Checking pitot heat and confirming the instruments behave correctly during the takeoff roll are standard defenses against pitot-static failures in flight.