Pitot-Static Instruments

 



    The Pitot-static instrument system consists of the altimeter, air speed indicator (ASI), and vertical speed indicator (VSI). The displayed values are calculated using a mix of static and ram pressure from the static ports and pitot tube (ERAUSpecialVFR, 2016).

    On December 1st, 1974, a Boeing 727-200 experienced catastrophic system failure resulting from an iced over pitot tube. Following a takeoff from JFK Intl. Airport, the aircraft climbed to 13,500 feet; after levelling off, it accelerated from 264 to 305 knots and began climbing at 2500 fpm. Eventually, the aircraft surpassed 340 knots and 6500 fpm. An overspeed warning horn sounded at 23,000 feet at 405 knots, after which the plane pitched down and began descending at 15,000 fpm. It descended more than 23,000 feet in 83 seconds before striking the ground.

    Common system failures include a blocked static port or pitot tube. In flight, most blockage results from ice though on the ground dirt and insects are common culprits. A failed pitot tube only affects the ASI; it will read an altitude instead of an airspeed. A failed static system will cause the altimeter to not change with altitude, though the ASI will increase with climb and decrease with decent, and the VSI will read the last vertical speed before it is blocked. Partial blockages can still interfere with instrument readings.

    Because of this, there are plenty of redundancies and preventative measures put in place. Multiple pitot tubes, static ports, heaters to guard against ice, covering the openings on the ground, and inspecting them during your preflight are all reliable strategies to avoid false readings (ERAUSpecialVFR, 2016). In case preventative measures fail, it is always good to know what the signs of a failing, or failed, pitot-static system are.


References

ERAUSpecialVFR. (2017, August 1). Pitot-static instruments. YouTube. Retrieved September 26, 2021, from https://www.youtube.com/watch?v=kdFGbUouE_4&index=49&list=PLzW-Ub1FWeZzdOHQhNK0U0Ci1a-VRH8IO.


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