Locations, errors, and how they are tamed
Each sensor's reading is only as good as its location and its mechanism, and the syllabus names three error families:
Position error — comes from where the sensor sits: the airflow around the airframe distorts the local pressure, so the static port never samples perfectly undisturbed air, and the error changes with speed, configuration and attitude. It is tamed by choosing the location carefully (flight testing finds the quietest spots) and by correcting the residue: calibration tables in the flight manual, or, on modern aircraft, a static source error correction applied inside the air-data computer.
Instrument error — the mechanism's own imperfection: manufacturing tolerances, friction, hysteresis (the mechanism lagging behind reversing inputs). It is minimised by build quality and periodic calibration, and the small residue is accepted.
Errors from non-longitudinal flow — arise when the airflow no longer meets the probe along its axis: at high angle of attack, in sideslip, and transiently during manoeuvres (the manoeuvre-induced error, worst during pitching and yawing when the pressure field around the airframe is changing). Manoeuvre-induced errors cannot be corrected: they are transient, and they persist for some seconds after the manoeuvre ends, which is why air data is read with suspicion immediately after aggressive pitching or yawing. Against the steady flow-angle errors the defences are probe mouths shaped to forgive flow angles, and above all paired static ports on both sides of the fuselage, plumbed together so that in a sideslip the port facing the wind over-reads, the sheltered port under-reads, and the average stays close to the truth.
The typical system and its outputs
A basic system is two pipes. The pitot line runs from the pitot tube to the two instruments that need total pressure, the airspeed indicator and the Machmeter, and on to the air-data computers. Only the ASI reads the difference outright, which honours the first module's promise that the ASI is nothing but a differential-pressure gauge, total pressure on one side, static on the other. The static line feeds everyone:
the altimeter (a sealed capsule in a case fed with static pressure).
the vertical speed indicator (which times changes of static pressure).
the reference sides of the airspeed indicator and the Machmeter.
The outputs of the system, then: altitude, vertical speed, airspeed and Mach number, plus the raw static and total pressures passed on to the air-data computers and, through them, to the transponder, the autopilot and the flight management system.
Redundancy on large aircraft
One pitot tube and one static port make a single point of failure, so transport aircraft multiply everything. Typically three pitot probes (captain, first officer, standby) and three independent static systems, each drawing from a pair of ports, one on each side of the fuselage, cross-connected for the sideslip averaging above. The captain's and first officer's sensors feed separate air-data computers; the standby system is routed independently of the main air-data computers: on older aircraft plumbed directly to mechanical standby instruments, on modern aircraft to an integrated standby instrument with its own sensor electronics and protected power. The circuits stay independent end to end: a blockage, leak or icing failure in one leaves two others intact, and disagreement between them is itself information, the trigger for the unreliable-airspeed drills. Interconnections exist only as deliberate, guarded exceptions: selector valves that let a pilot switch a failed source for a healthy one.