Two pressures, three ports.
The atmosphere offers a moving aircraft exactly two measurable pressures. Static pressure is the weight of the air above you — it falls as you climb, and it is sampled at static ports: small holes placed where airflow passes by without piling up. Total pressure is static plus the pressure of motion — sampled by the pitot tube, a forward-facing open pipe that brings the airflow to a stop and feels the extra squeeze.
The difference between the two is dynamic pressure — ½ρv², pure speed. That subtraction is the entire theory of the airspeed indicator: pitot on one side of a diaphragm, static on the other, and the needle shows the difference. No electronics required, though in a modern instrument two digital sensors do the same subtraction a hundred times per second.
The third opening is the TE probe on the fin — covered in depth in the total energy article. It manufactures a special mixture: static minus dynamic, so that a vario plumbed to it ignores your stick inputs. Three holes, three different pressures, one panel.
Who drinks from which line.
The altimeter uses static alone: it is a barometer with an altitude scale. The vario uses the rate of change of its line — raw static in a basic installation, the TE probe in a proper one. The ASI is the only instrument that needs two lines, pitot and static together. And your flight computer reads all three, because everything it computes — true airspeed, wind, netto, final glide — starts from these pressures.
One subtlety earns its own paragraph: position error. A static port is never in perfectly undisturbed air; the fuselage accelerates flow around itself, so the port reads slightly wrong, and differently at different speeds and sideslip angles. Manufacturers place ports where the error is small and stable — which is why a static port position is not a suggestion, and why taping over a port "temporarily" rewrites the calibration of three instruments at once.
Break one line. Read the panel.
Every instrument reads its own pressure honestly. This is the baseline — memorize how it feels, because every fault below announces itself as a deviation from it.
Water, ice and tape.
The enemies of the system are unglamorous. Rain leaves droplets in the pitot that turn into a blockage on the next flight's first climb. Washing the glider pushes water straight into the ports unless they are taped first — and the tape, forgotten, is a complete blockage with a perfect paint match. Ice arrives in wave flights and closes whichever opening freezes first. And in the fuselage, decades-old silicone tubing hardens, cracks at the instrument spigots, and slips off during the winter re-panel — the leak arrives with the new instrument.
This is why gliding clubs preach the same ritual: ports taped for washing, tape off at the daily inspection, drains checked, and after any panel work a proper leak check before the next launch — not a hopeful first winch.
The panel is a plumbing diagram.
Once you see the cockpit this way, troubleshooting becomes mechanical. Every reading is a pressure, every pressure has a pipe, and every pipe has exactly three failure modes: blocked, leaking, or connected to the wrong place. Ask which instruments misbehave together, and the shared line points at itself — the fault simulator above is nothing more than that logic, drawn.
And the payoff for a tight system is everything downstream: the total energy article explains what a healthy TE line buys you, and the vario-lag article explains what the instrument does with the pressure once it finally arrives clean.
The sensor unit — fast digital pressure sensors on pitot, static and TE, one clean plumbing story for the whole panel.
Standalone vario — its compensation is only as good as the three lines feeding it.
The racing computer — airspeed, altitude, netto and speed-to-fly all start at these three ports.