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Transcript of Group 1
Ps = Static pressure
p = fluid density
V = fluid velocity A major application for Pitot tubes is in the aviation industry, where they is used to measure airspeed.
This is vital for modern aircraft, because if a plane flies outside its “safety envelope”, it can experience structural damage or stall. Air speed is also a critical parameter to flight takeoff and landing. This is what happened on May 31 2009, when an Airbus A330 was flying from Rio de Janeiro to Paris and vanished during a thunderstorm with 288 people on board. So how can such a small device cause a passenger aircraft to crash?
A GPS is not regularly used on airplanes to check airspeed as it could off by up to 25 percent. This is an enormous error and can create a dangerous situation. Below Is a diagram of a Pitot tube used on an aircraft It is believed that the pitot tube had Ice crystals formed inside, which created a blockage and lead to incorrect readings for the pressure difference and so the airspeed. The incorrect airspeed data was then fed into other instruments creating multiple alarms in the cockpit and the autopilot disengaged.
The pilots likely confused and panicked, not knowing what the root cause of the problem was.
They reacted incorrectly, manually put the plane into a stall situation and could not then recover control of the aircraft. How does it work? Consider a tube blocked off at the end but with a discrete port to the side: When placed in a vessel surrounded by a fluid, it measures the ambient pressure of the fluid = Static Pressure, Ps.
If the liquid is moving, the static pressure remains the same. What is a Pitot Tube? How does a simple device like this cause a plane crash? More on this later... In this Prezi, we will talk about a pitot tube, theory, uses and describe how it has contributed to passenger aircraft disasters. Now consider a simple hollow tube opened at the impact end: Consider when a pitot tube is placed in a pipe with a flowing fluid. The pressure measured is the Stagnation pressure, Pt. As fluid velocity increases, so does stagnation pressure. Now consider a device with both of these features. It comprises of two concentric tubes, the outer tube measuring the Static pressure and the inner tube measuring the stagnation Pressure. When placed in a pipe of moving fluid, the difference between the Stagnation and Static pressures can be measured by the deflection of a diaphragm in a chamber connecting the two. From Bernoulli's equation it can be shown that: Rearranging the above equation for V gives: Therefore we can determine V by measuring the difference between the Static and Stagnation pressures in a system. Averaging Pitot Tube It comprises of two narrow concentric tubes. This device is inserted perpendicular to the flow of a fluid. One side will measure the static pressure, the other will measure the stagnation pressure. These ports are located along the length of the tube, thereby determining the approximate average velocity. Pitot tubes used on airplanes: Now for some maths! Video illustration (Munson et al. 2009) (Munson et al. 2009) It was cause by conflicting pitot tube readings after takeoff, which confused the crew who stalled and the plane and it crashed at sea. Pitot tube faults are believed to be the root cause in many other passenger aircraft disasters (InTech, 2009) 1996, a Boeing 747 crashed in the Dominican Republic, killing 189 people. Investigations indicated that wasp may have nested in the tubes while the plane was in maintenance A pitot tube is slender tube that is used to measure flow speed, by measuring the difference between two pressures. The official investigation concluded that it was caused by a chain of events.
The catalyst for these events was a fault with the plane's Pitot tube! Why can’t pilots use a Global Positioning System (GPS) to measure their airspeed?
Borrell, B 2009, ‘What is a Pitot Tube’, Scientific American, 9 June 2009, viewed 5 October 2012, <https://www.scientificamerican.com/article.cfm?id=what-is-a-pitot-tube>
Endress Hauser 2009, viewed 1 October 2012, <http://www.youtube.com/watch?v=D6sbzkYq3_c>
inTech "Pitot tube theory advances in Air France disaster". InTech (0192-303X), 56 (7), p. 10, viewed 1 October 2012, <http://www.isa.org/InTechTemplate.cfm?Section=Automation_Update1&template=/ContentManagement/ContentDisplay.cfm&ContentID=77990>
Measurement Resources, Averaging Pitot Tube Flowmeter Catalogue, E - Averaging Pitot Tube Flowmeter Cat (APTF-A-07), viewed 9 October 2012, < www.measurement-resources.com.au>
Munson, B, Young, DF Okiishi, T & Huebsch, WW 2009, Fundamentals of Fluid Mechanics, 6th edn, John Wiley and Sons. References Theory (Borrell, B 2009) (Measurement Resources 2012) (Endress Hauser 2009)