Three Tubes and a Diaphragm: How an Airplane Knows Its Speed and Altitude

Some of the most important instruments in an airplane have no electronics at all. The airspeed indicator, the altimeter, and the vertical speed indicator are driven entirely by air pressure, through the pitot-static system — a small network of tubes and ports that turns the pressure of the surrounding air into the three numbers a pilot lives by. Two kinds of pressure feed the system. The static ports, flush with the fuselage, sample the ambient (static) air pressure, which falls predictably with altitude — that is what the altimeter reads, and what the vertical speed indicator watches for change. The pitot tube, pointed into the airflow, captures the higher "ram" pressure of the air the aircraft is flying into. The airspeed indicator measures the difference between pitot and static pressure; that difference grows with speed, and the instrument converts it into knots. The system's elegance is also its vulnerability: block a tube and the instruments lie. A pitot tube plugged by ice, dust, or an insect's nest can make the airspeed indicator read wildly wrong — sometimes climbing while the aircraft slows. Blocked static ports corrupt altitude and vertical-speed readings. Several airline accidents trace directly to obstructed pitot-static systems feeding false data to the crew, which is why aircraft carry pitot heat, why static systems have alternate sources, and why preflight inspection always includes checking that the pitot tube is clear and uncovered. Pilots are trained to recognize the signatures of a blocked system and to cross-check the suspect instrument against others — pitch attitude, power setting, and GPS groundspeed. The Pilot's Handbook of Aeronautical Knowledge devotes careful attention to it. A pressure instrument is only as honest as the air reaching it, and respecting that fragile chain of tubes is part of what separates a trained pilot from a passenger.