The Russian plane An-148 that crashed on icy terrain, killing all 71 people on board may have been caused by the pilots’ failure to activate heating for pitot-static system (air pressure measuring system), resulting in flawed speed data, says the investigators.
The Interstate Aviation Committee said, after studying the plane’s flight data recorder, the crash occurred after the pilots saw varying data on the plane’s two air speed indicators.
The pilots set the plane on autopilot mode after taking off from Domodedovo Airport in Moscow, but took manual controls back when they saw clashing speed data. The incorrect slow speed reading may have caused the pilots to suddenly speed up to avoid stalling, triggering a dramatic plummet from the sky just six minutes after it took off.
Pitot-Static system works by measuring dynamic and static air pressure and provide data for airspeed indicator, altimeter, and vertical speed indicator. Static lines connect to all three instruments and ram air pressure from the pitot tube connects to only the airspeed indicator.
Ram air pressure from the aircraft’s pitot tube is directed into a diaphragm in an analog airspeed instrument case. Static air pressure from the aircraft static vent(s) is directed into the case surrounding the diaphragm. As the speed of the aircraft varies, the ram air pressure varies, expanding or contracting the diaphragm. Linkage attached to the diaphragm causes a pointer to move over the instrument face, which is calibrated in knots or miles per hour (mph).
The most common problem with the pitot-static system is a blockage of the pitot tube or the static ports, or both. Pitot tubes have an electronic heating element inside of the tube that prevents ice from blocking the air inlet or drain hole. The pilot can send electric current to the element with a switch in the cockpit when ice-forming conditions exist.
If the pitot tube becomes blocked, and its drain hole remains clear, the airspeed will read zero.
If the pitot tube and its drain hole are blocked, the airspeed indicator will act like an altimeter, reading higher airspeeds with an increase in altitude. This situation can be dangerous if not recognized immediately.
If the static port(s) become blocked and the pitot tube remains operable, the airspeed indicator will barely work and indications will be inaccurate.
High performance and jet transport category aircraft pitot-static systems may be more complicated. They utilize digital air data computers (DADC). Essentially, all pressures and temperatures captured by sensors are fed into the ADC. Analog units utilize transducers to convert these to electrical values and manipulate them in various modules containing circuits designed to make the proper compensations for use by different instruments and systems. A DADC usually receives its data in digital format. Systems that do not have digital sensor outputs will convert inputs into digital signals via an analog-to-digital converter.