Pitot-Static System: Construction, Operation and Errors
Construction and operation
The pitot head faces the airflow and supplies total pressure; drain holes and a heater protect it from moisture and icing. Static ports are flush-mounted on the fuselage, usually in undisturbed pressure regions, and are connected to the ASI, altimeter and VSI. The ASI capsule receives pitot pressure inside and static pressure outside; the altimeter is an aneroid capsule stack connected to static only, linked to pointers via mechanical gearing; the VSI is a capsule with a calibrated choke (leak) that produces a pressure difference proportional to the rate of change of static pressure, driving a pointer through a linkage.
Errors and failure signatures
- Blocked pitot (static clear): ASI behaves like an altimeter - it increases in climb and decreases in descent. The altimeter and VSI remain normal.
- Blocked static (pitot clear): ASI over-reads in climb and under-reads in descent; altimeter freezes at the blockage altitude; VSI reads zero (or a fixed value if the blockage is gradual).
- Blocked pitot with blocked drain: ASI freezes at the value when blocked.
- Leak in static line: altimeter reads high, VSI shows a climb, ASI under-reads.
- Pitot heater failure in icing: ASI falls toward zero while other instruments are normal.
- Position error, compressibility error (near M0.4 and above), density error and instrument/installation error.
Worked example
On climb-out the ASI climbs steadily while the altimeter and VSI behave normally. The static ports are clear, so the only plausible cause is a blocked pitot tube (or pitot line) with the drain blocked. The correct response is to cross-check with the standby instruments, use attitude and power, notify ATC, and treat the ASI as unreliable.
Question traps
Questions often describe a combination of two failures (e.g. static blockage plus a leak). Read which instruments are affected and which are normal - that identifies the blocked source. Also remember that the VSI is not instantaneous: it lags several seconds.