The Pitot Static Tube sometimes referred to as a Pitot Probe is a differential pressure device used as a flow meter for gases and liquids. It uses a differential pressure principle for the measurement using the known or measured static pressure and total pressure differences know as the dynamic pressure. 31: hydrant pitot gauge conversion chart - Fast Download by tewcs 2014-04-13. Related searches for pitot gauge conversion chart to gpm Some results have been removed Related searches Pitot to GPM Conversion Chart Pitot to GPM Chart Hydrant Pitot Gauge Conversion Chart Pitot Gauge Chart Pitot Gauge Water Flow Charts Pitot Gauge Flow Chart. But here’s a quick overview of how pitot gauges work: A pitot gauge consists of three components: a blade, handle, and pressure gauge. After it’s inserted into an open fire hydrant’s water discharge, a narrow tube inside the blade directs water toward the gauge to create a pressure reading. Pitot tubes (also called pitot static tubes) are used to measure fluid velocity at a point in a fluid. They are commonly used to measure air velocity, but can be use to measure the velocity of other fluids as well. The pitot tube is used to measure the difference between stagnation pressure and static pressure at a point in the fluid.
Active8 months ago
$begingroup$Reading Why turn off pitot tube heating?, it appears that the pitot tube heat can be switched off to extent its lifetime and also in case of short-circuit.
(Thales pitot-static probe, source BEA)
On airliners the probes seem to be heated during the whole flight. This makes me ask myself different but related questions:
- Accidents occurred due to probes freezing. How can a heated pitot probe freeze?
- Extending the life of the tube seems a questionable choice, I'd rather expect the technology to be improved and be able to sustain permanent heat.
Heating a small metal device from -56°C to 5°C (or maybe more, say 100°C, taking into account comments) looks like feasible (not taking into account air friction)⇢ The problem seems to be how to transfer enough heat from the metal (which is very hot) to the ice accretion. - A short-circuit is easily terminated by a fuse or a circuit breaker. Why does the crew have to worry about switching the heat manually, adding a possibility to have it OFF unintentionally?
Community♦
minsmins43.1k18 gold badges196 silver badges323 bronze badges
$endgroup$2 Answers
$begingroup$Captain Bill Palmer, in his book 'Understanding Air France 447' has a section dedicated to answering that very question as it pertains to AF447. Although there is no way to know for certain he puts forward some possibilities.
He quotes a commenter on a website:
One commenter on the Weather Graphics website’s AF447 article provided this interesting observation: “I'm an aircraft icing specialist and wanted to point out a factor that hasn't been discussed much … high ice crystal concentrations. I've seen flight test data from power rollbacks due to flight in high ice crystal environments … In our case, the crystals collected within heated, aspirated Ram Air Temperature sensors, forming a 0 ° C slush…”
He notes that just before the pilot tubes clogged, the sound of ice crystals hitting the windshield could be heard on the CVR.
Ice crystals bounce off the exterior of an airplane and cause no visible ice accretion, but they can enter the probe inlets. When highly specific climatic conditions exist in combination with certain combinations of altitude, temperature, and Mach, the concentration of ice crystals entering a probe can exceed its capacity to melt and evacuate the moisture through its drain holes. The result is that the ice crystals form a physical barrier within the probe that disrupts the measurement of total pressure.
The particular type of ice that may have been responsible was a substance called graupel.
Graupel forms when tiny supercooled water droplets adhere to snow crystals to the point that they engulf the snow crystal itself.
Graupel. Photo from Wikimedia commons
Capt. Palmer notes factors that make graupel a possible suspect:
- No airframe icing. The supercooled water theory is discounted by the fact that the A330's icing detectors were not triggered.
- Graupel has large enough particles to be audible on the voice recorder. It takes a particle with enough mass and inertia (a given density) to hit the fuselage with a sound, instead of flowing around it with the relative wind, like snow.
- Graupel has enough mass to temporarily overwhelm pitot anti-icing when concentrations are high enough. The pitot tubes are hot. But even if you put a snowball on a hot skillet it does not melt instantaneously. If there is enough mass in the blockage, and in combination with new particles being added to the blockage as the first ones melt, it may exceed the pitot tubes capability to melt the obstruction as fast as it is introduced. Graupel is of significantly higher density than snow.
- Graupel has sufficient blocking properties to prevent efficient transmission of dynamic pressure within the pitot tube.For example, water can flow and transmit pressure within the pitot tube, though it too can alter pitot-static readings, a physical non-fluid blockage could shield the pressure sensing port.
- The likely presence of snow or similar form, as evidenced by the St. Elmo's fire discussed by the crew. The accident report stated that the sound of ice crystals hitting the aircraft can be heard about 20 seconds before the airspeed loss and autopilot disconnect.
It must be noted that the exact cause of icing problems on the A330 was never completely identified but it was specific to the particular brand of pitot tubes originally installed. Airbus was in the process of replacing them all with pilot tubes from a different manufacturer.
FreeMan8,14610 gold badges61 silver badges132 bronze badges
TomMcWTomMcW19k11 gold badges80 silver badges175 bronze badges
$endgroup$$begingroup$To answer your questions in order:
1) Pitot tubes are certified to withstand icing under particular circumstances: Within a given temperature range, precipitation amount, altitude etc. If they are operated outside of those ranges then the heating systems may not be effective. In the case of AF447 it was also important that they experienced ice crystal icing and not supercooled water icing - the two respond to pitot heating differently and the same certification requirements can't be used for both.
2) Automated systems are not, yet, perfect at detecting icing conditions. You don't want a system where the pitot is always on, with no possibility to turn it off, for safety reasons and you don't want an automatic system because it might fail to detect icing conditions and hence not activate the heat. That said, in my experience it's very rare to have the pitot heat off: There's not really much point. The crew has to have the discipline to turn it on at the beginning of the flight but after that an automatic system is not helpful if the heat is supposed to remain on anyway.
FreeMan8,14610 gold badges61 silver badges132 bronze badges
os1os1
$endgroup$Not the answer you're looking for? Browse other questions tagged safetypitot-staticaf447-accidentaircraft-subsystem or ask your own question.
Aircraft use pitot tubes to measure airspeed. This example, from an Airbus A380, combines a pitot tube (right) with a static port and an angle-of-attack vane (left). Air-flow is right to left.
Types of pitot tubes
A pitot-static tube connected to a manometer
Pitot tube on Kamov Ka-26 helicopter
Pitot tube on a Renault Formula One car
Location of pitot tubes on a Boeing 777
A pitot (/ˈpiːtoʊ/PEE-toh) tube, also known as pitot probe, is a flow measurement device used to measure fluidflow velocity. The pitot tube was invented by the French engineer Henri Pitot in the early 18th century[1] and was modified to its modern form in the mid-19th century by French scientist Henry Darcy.[2] It is widely used to determine the airspeed of an aircraft, water speed of a boat, and to measure liquid, air and gas flow velocities in certain industrial applications.
Theory of operation[edit]
The basic pitot tube consists of a tube pointing directly into the fluid flow. As this tube contains fluid, a pressure can be measured; the moving fluid is brought to rest (stagnates) as there is no outlet to allow flow to continue. This pressure is the stagnation pressure of the fluid, also known as the total pressure or (particularly in aviation) the pitot pressure.
The measured stagnation pressure cannot itself be used to determine the fluid flow velocity (airspeed in aviation). However, Bernoulli's equation states:
- Stagnation pressure = static pressure + dynamic pressure
Which can also be written
Solving that for flow velocity gives
where
- is the flow velocity;
- is the stagnation or total pressure;
- is the static pressure;
- and is the fluid density.
NOTE: The above equation applies only to fluids that can be treated as incompressible. Liquids are treated as incompressible under almost all conditions. Gases under certain conditions can be approximated as incompressible. See Compressibility.
The dynamic pressure, then, is the difference between the stagnation pressure and the static pressure. The dynamic pressure is then determined using a diaphragm inside an enclosed container. If the air on one side of the diaphragm is at the static pressure, and the other at the stagnation pressure, then the deflection of the diaphragm is proportional to the dynamic pressure.
In aircraft, the static pressure is generally measured using the static ports on the side of the fuselage. The dynamic pressure measured can be used to determine the indicated airspeed of the aircraft. The diaphragm arrangement described above is typically contained within the airspeed indicator, which converts the dynamic pressure to an airspeed reading by means of mechanical levers.
Pitot Tube Conversion Tool
Instead of separate pitot and static ports, a pitot-static tube (also called a Prandtl tube) may be employed, which has a second tube coaxial with the pitot tube with holes on the sides, outside the direct airflow, to measure the static pressure.[3]
If a liquid column manometer is used to measure the pressure difference ,
where
- is the height difference of the columns;
- is the density of the liquid in the manometer;
- g is the standard acceleration due to gravity.
Therefore,
Aircraft[edit]
A pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a static port, and the pitot-static instruments.[4] Errors in pitot-static system readings can be extremely dangerous as the information obtained from the pitot static system, such as airspeed, is potentially safety-critical.
Several commercial airline incidents and accidents have been traced to a failure of the pitot-static system. Examples include Austral Líneas Aéreas Flight 2553, Northwest Airlines Flight 6231, Birgenair Flight 301 and one of the two X-31s.[5] The French air safety authority BEA said that pitot tube icing was a contributing factor in the crash of Air France Flight 447 into the Atlantic Ocean.[6] In 2008 Air Caraïbes reported two incidents of pitot tube icing malfunctions on its A330s.[7]
Birgenair Flight 301 had a fatal pitot tube failure which investigators suspected was due to insects creating a nest inside the pitot tube; the prime suspect is the black and yellow mud dauber wasp.
Aeroperú Flight 603 had a pitot-static system failure due to the cleaning crew leaving the static port blocked with tape.
Industry applications[edit]
Pitot tube from an F/A-18
Weather instruments at Mount Washington Observatory. Pitot tube static anemometer is on the right.
In industry, the flow velocities being measured are often those flowing in ducts and tubing where measurements by an anemometer would be difficult to obtain. In these kinds of measurements, the most practical instrument to use is the pitot tube. The pitot tube can be inserted through a small hole in the duct with the pitot connected to a U-tube water gauge or some other differential pressure gauge for determining the flow velocity inside the ducted wind tunnel. One use of this technique is to determine the volume of air that is being delivered to a conditioned space.
The fluid flow rate in a duct can then be estimated from:
- Volume flow rate (cubic feet per minute) = duct area (square feet) × flow velocity (feet per minute)
- Volume flow rate (cubic meters per second) = duct area (square meters) × flow velocity (meters per second)
In aviation, airspeed is typically measured in knots.
In weather stations with high wind speeds, the pitot tube is modified to create a special type of anemometer called pitot tube static anemometer.[8]
See also[edit]
References[edit]
Notes
- ^Pitot, Henri (1732). 'Description d'une machine pour mesurer la vitesse des eaux courantes et le sillage des vaisseaux'(PDF). Histoire de l'Académie royale des sciences avec les mémoires de mathématique et de physique tirés des registres de cette Académie: 363–376. Retrieved 2009-06-19.
- ^Darcy, Henry (1858). 'Note relative à quelques modifications à introduire dans le tube de Pitot'(PDF). Annales des Ponts et Chaussées: 351–359. Retrieved 2009-07-31.
- ^'How Aircraft Instruments Work.'Popular Science, March 1944, pp. 116.
- ^Willits, Pat, ed. (2004) [1997]. Guided Flight Discovery - Private Pilot. Abbot, Mike Kailey, Liz. Jeppesen Sanderson. pp. 2–48–2–53. ISBN0-88487-333-1.
- ^NASA Dryden news releases. (1995)
- ^'Training flaws exposed in Rio-Paris crash report'. Reuters. 5 July 2012. Retrieved 5 October 2012.
- ^Daly, Kieran (11 June 2009). 'Air Caraibes Atlantique memo details pitot icing incidents'. Flight International. Retrieved 19 February 2012.
- ^'Instrumentation: Pitot Tube Static Anemometer, Part 1'. Mount Washington Observatory. Retrieved 14 July 2014.
Bibliography
- Kermode, A.C. (1996) [1972]. Mechanics of Flight. Barnard, R.H. (Ed.) and Philpott, D.R. (Ed.) (10th ed.). Prentice Hall. pp. 63–67. ISBN0-582-23740-8.
- Pratt, Jeremy M. (2005) [1997]. The Private Pilot's Licence Course: Principles of Flight, Aircraft General Knowledge, Flight Performance and Planning (3rd ed.). gen108–gen111. ISBN1-874783-23-3.
- Tietjens, O.G. (1934). Applied Hydro- and Aeromechanics, based on lectures of L. Prandtl, Ph.D. Dove Publications, Inc. pp. 226–239. ISBN0-486-60375-X.
- Saleh, J.M. (2002). Fluid Flow Handbook. McGraw-Hill Professional.
External links[edit]
Wikimedia Commons has media related to Pitot tube. |
- How 18th Century Technology Could Down an Airliner (wired.com)
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Pitot_tube&oldid=915051911'