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Thread: Measuring Air Speed

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    Senior Member Speedbird1's Avatar
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    Measuring Air Speed

    I am confused about how aircraft measure air speed. Sometimes, a static port is used but what about a pitot tube? Or do most airliners use a combination. I think an AeroPeru once crashed due to faulty air speed readouts because a maintenance worker left a piece of silver tape which covered the Static Port. Asd a result, a faulty readout ensued and the aircraft was doomed.

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    Moderator Matt Molnar's Avatar
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    Re: Measuring Air Speed

    The pitot tube does not work by itself, it works in conjunction with the static port. Very basic explanation (let me know if it's not correct): the pitot tube measures the pressure of moving air, while the static port measures the overall air pressure, and the difference between the two is used to determine the speed.
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    Re: Measuring Air Speed

    While airliners are a bit different due to the high altitudes/speeds, the basic idea of measuring airspeed is the pitot/static one. The plane has a pitot tube which faces forward and has an opening; as the plane moves through the air, the air pressure in the pitot tube changes, increasing as the plane accelerates. The pressure at the pitot tube is compared with that from the static port, which is usually on the side of the plane, or otherwise positioned so that its pressure is only affected by the ambient outside air pressure. The difference between the two pressures is used to figure out the airspeed.

    If the static port is blocked, the pressure on that end will remain steady at sea level. Since the static pressure is how altitude is indicated, blocking the static port will affect the altimeter readout.

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    Senior Member moose135's Avatar
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    Re: Measuring Air Speed

    Both the pitot tube and static port is used to determine airspeed. The pitot tube has a small opening facing forward, and as the aircraft moves, oncoming air "rams" into the opening. The measure of this ram pressure determines airspeed. This would be the end of it, if aircraft never left the ground, or if air pressure were constant everywhere. Since air pressure decreases as altitude increases, simply measuring the ram pressure would result in lowering airspeed readings as the aircraft climbed, due to the lower pressure (density) of the air. That is where the static port plays its part.

    The static port actually has two roles: it measures the "static" air pressure - that is the air that is not subjected to the ram forces, which is why you will see them positioned on the side of the aircraft, where they are not affected by the air moving past them. The absolute value of this static pressure is used to determine the altitude of the aircraft. The second use of this pressure is for determining air speed. As the altitude increases, and the pressure decreases, the airspeed indicator system measures the difference in pressure between the ram and static air, and displays this as Indicated Air Speed.

    There have been several accidents due to blocked pitot/static ports. The AeroPeru 757 that you mention is one where the static ports were covered with tape while the aircraft was being cleaned, and the tape was not removed. In 1974, a Northwest 727 crashed at Bear Mountain, NY, after the pitot tubes iced over on climbout (pitot heat was not turned on). With a steady reading on the ram pressure side, and a decreasing pressure on the static side, the airspeed indicators gave the crew the erroneous indication of continuously increasing airspeed on the climb. Despite raising the nose to an extreme angle, and ultimately reducing power, they couldn't understand why the airspeed and rate of climb continued to increase. Finally, at low airspeed and high angle of attack, the aircraft stalled, and spiraled into the ground.

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    Re: Measuring Air Speed

    Great explanation Moose.
    If you want to look at another incident involving the Pitot tube research Birgenair 301, which crashed off the cost of the Dominican Republic after one of the Pitot tubes was jammed and gave false readings.

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    Senior Member Idlewild's Avatar
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    Re: Measuring Air Speed

    Is there any reason why GPS can't be used for airspeed?
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    Re: Measuring Air Speed

    GPS measures groundspeed, not airspeed, which is an important difference.
    Phil Gengler - NYCA's "other Phil"

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