Pitot-static system

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.^[1] This equipment is used to measure the forces acting on a vehicle as a function of the temperature, density, pressure and viscosity of the fluid in which it is operating.^[2] Other instruments that might be connected are air data computers, flight data recorders, altitude encoders, cabin pressurization controllers, and various airspeed switches. Errors in pitot-static system readings can be extremely dangerous as the information obtained from the pitot static system, such as altitude, is often critical to a successful flight. Several commercial airline disasters have been traced to a failure of the pitot-static system.^[3]

Pitot-static pressure[edit]

Examples of pitot tube, static tube, and pitot-static tube.

Static ports fitted to an Airbus A330 passenger airliner.

The pitot-static system of instruments uses the principle of air pressure gradient. It works by measuring pressures or pressure differences and using these values to assess the speed and altitude.^[1] These pressures can be measured either from the static port (static pressure) or the pitot tube (pitot pressure). The static pressure is used in all measurements, while the pitot pressure is only used to determine airspeed.

Pitot pressure[edit]

The pitot pressure is obtained from the pitot tube. The pitot pressure is a measure of ram air pressure (the air pressure created by vehicle motion or the air ramming into the tube), which, under ideal conditions, is equal to stagnation pressure, also called total pressure. The pitot tube is most often located on the wing or front section of an aircraft, facing forward, where its opening is exposed to the relative wind.^[1] By situating the pitot tube in such a location, the ram air pressure is more accurately measured since it will be less distorted by the aircraft's structure. When airspeed increases, the ram air pressure is increased, which can be translated by the airspeed indicator.^[1]

Static pressure[edit]

The static pressure is obtained through a static port. The static port is most often a flush-mounted hole on the fuselage of an aircraft, and is located where it can access the air flow in a relatively undisturbed area.^[1] Some aircraft may have a single static port, while others may have more than one. In situations where an aircraft has more than one static port, there is usually one located on each side of the fuselage. With this positioning, an average pressure can be taken, which allows for more accurate readings in specific flight situations.^[1] An alternative static port may be located inside the cabin of the aircraft as a backup for when the external static port(s) are blocked. A pitot-static tube effectively integrates the static ports into the pitot probe. It incorporates a second coaxial tube (or tubes) with pressure sampling holes on the sides of the probe, outside the direct airflow, to measure the static pressure. When aircraft climbs, static pressure will decrease.

Multiple pressure[edit]

Some pitot-static systems incorporate single probes that contain multiple pressure-transmitting ports that allow for the sensing of air pressure, angle of attack, and angle of sideslip data. Depending on the design, such air data probes may be referred to as 5-hole or 7-hole air data probes. Differential pressure sensing techniques can be used to produce angle of attack and angle of sideslip indications.

Pitot-static instruments[edit]

Airspeed indicator diagram showing pressure sources from both the pitot tube and static port

The pitot-static system obtains pressures for interpretation by the pitot-static instruments. While the explanations below explain traditional, mechanical instruments, many modern aircraft use an air data computer (ADC) to calculate airspeed, rate of climb, altitude and Mach number. In some aircraft, two ADCs receive total and static pressure from independent pitot tubes and static ports, and the aircraft's flight data computer compares the information from both computers and checks one against the other. There are also "standby instruments", which are back-up pneumatic instruments employed in the case of problems with the primary instruments.

Airspeed indicator[edit]

Main article: Airspeed indicator

The airspeed indicator is connected to both the pitot and static pressure sources. The difference between the pitot pressure and the static pressure is called dynamic pressure. The greater the dynamic pressure, the higher the airspeed reported. A traditional mechanical airspeed indicator contains a pressure diaphragm that is connected to the pitot tube. The case around the diaphragm is airtight and is vented to the static port. The higher the speed, the higher the ram pressure, the more pressure exerted on the diaphragm, and the larger the needle movement through the mechanical linkage.^[4]

Diagram of an altimeter

Altimeter[edit]

Main article: Altimeter

The pressure altimeter, also known as the barometric altimeter, is used to determine changes in air pressure that occur as the aircraft's altitude changes.^[4] Pressure altimeters must be calibrated prior to flight to register the pressure as an altitude above sea level. The instrument case of the altimeter is airtight and has a vent to the static port. Inside the instrument, there is a sealed aneroid barometer. As pressure in the case decreases, the internal barometer expands, which is mechanically translated into a determination of altitude. The reverse is true when descending from higher to lower altitudes.^[4]

Machmeter[edit]

Main article: Machmeter

Aircraft designed to operate at transonic or supersonic speeds will incorporate a machmeter. The machmeter is used to show the ratio of true airspeed in relation to the speed of sound. Most supersonic aircraft are limited as to the maximum Mach number they can fly, which is known as the "Mach limit". The Mach number is displayed on a machmeter as a decimal fraction.^[4]

A vertical airspeed indicator

Vertical airspeed indicator[edit]

Main article: Variometer

The variometer, also known as the vertical speed indicator (VSI) or the vertical velocity indicator (VVI), is the pitot-static instrument used to determine whether or not an aircraft is flying in level flight.^[5] The vertical airspeed specifically shows the rate of climb or the rate of descent, which is measured in feet per minute or meters per second.^[5] The vertical airspeed is measured through a mechanical linkage to a diaphragm located within the instrument. The area surrounding the diaphragm is vented to the static port through a calibrated leak (which also may be known as a "restricted diffuser").^[4] When the aircraft begins to increase altitude, the diaphragm will begin to contract at a rate faster than that of the calibrated leak, causing the needle to show a positive vertical speed. The reverse of this situation is true when an aircraft is descending.^[4] The calibrated leak varies from model to model, but the average time for the diaphragm to equalize pressure is between 6 and 9 seconds.^[4]

Pitot-static errors[edit]

There are several situations that can affect the accuracy of the pitot-static instruments. Some of these involve failures of the pitot-static system itself—which may be classified as "system malfunctions"—while others are the result of faulty instrument placement or other environmental factors—which may be classified as "inherent errors".^[6]

System malfunctions[edit]

Blocked pitot tube[edit]

A blocked pitot tube is a pitot-static problem that will only affect airspeed indicators.^[6] A blocked pitot tube will cause the airspeed indicator to register an increase in airspeed when the aircraft climbs, even though actual airspeed is constant. This is caused by the pressure in the pitot system remaining constant when the atmospheric pressure (and static pressure) are decreasing. In reverse, the airspeed indicator will show a decrease in airspeed when the aircraft descends. The pitot tube is susceptible to becoming clogged by ice, water, insects or some other obstruction.^[6] For this reason, aviation regulatory agencies such as the U.S. Federal Aviation Administration (FAA) recommend that the pitot tube be checked for obstructions prior to any flight.^[5] To prevent icing, many pitot tubes are equipped with a heating element. A heated pitot tube is required in all aircraft certificated for instrument flight except aircraft certificated as Experimental Amateur-Built.^[6]

Blocked static port[edit]

A blocked static port is a more serious situation because it affects all pitot-static instruments.^[6] One of the most common causes of a blocked static port is airframe icing. A blocked static port will cause the altimeter to freeze at a constant value, the altitude at which the static port became blocked. The vertical speed indicator will become frozen at zero and will not change at all, even if vertical airspeed increases or decreases. The airspeed indicator will reverse the error that occurs with a clogged pitot tube and cause the airspeed be read less than it actually is as the aircraft climbs. When the aircraft is descending, the airspeed will be over-reported. In most aircraft with unpressurized cabins, an alternative static source is available and can be toggled from within the cockpit of the airplane.^[6]

Inherent errors[edit]

Inherent errors may fall into several categories, each affecting different instruments. Density errors affect instruments metering airspeed and altitude. This type of error is caused by variations of pressure and temperature in the atmosphere. A compressibility error can arise because the impact pressure will cause the air to compress in the pitot tube. At standard sea level pressure altitude the calibration equation (see calibrated airspeed) correctly accounts for the compression so there is no compressibility error at sea level. At higher altitudes the compression is not correctly accounted for and will cause the instrument to read greater thanequivalent airspeed. A correction may be obtained from a chart. Compressibility error becomes significant at altitudes above 10,000 feet (3,000 m) and at airspeeds greater than 200 knots (370 km/h). Hysteresis is an error that is caused by mechanical properties of the aneroid capsules located within the instruments. These capsules, used to determine pressure differences, have physical properties that resist change by retaining a given shape, even though the external forces may have changed. Reversal errors are caused by a false static pressure reading. This false reading may be caused by abnormally large changes in an aircraft's pitch. A large change in pitch will cause a momentary showing of movement in the opposite direction. Reversal errors primarily affect altimeters and vertical speed indicators.^[6]

Position errors[edit]

Another class of inherent errors is that of position error. A position error is produced by the aircraft's static pressure being different from the air pressure remote from the aircraft. This error is caused by the air flowing past the static port at a speed different from the aircraft's true airspeed. Position errors may provide positive or negative errors, depending on one of several factors. These factors include airspeed, angle of attack, aircraft weight, acceleration, aircraft configuration, and in the case of helicopters, rotor downwash.^[6] There are two categories of position errors, which are "fixed errors" and "variable errors". Fixed errors are defined as errors which are specific to a particular model of aircraft. Variable errors are caused by external factors such as deformed panels obstructing the flow of air, or particular situations which may overstress the aircraft.^[6]

Pitot-static related disasters[edit]

• 1 December 1974—Northwest Airlines Flight 6231, a Boeing 727, crashed northwest of John F. Kennedy International Airport during climb en route to Buffalo Niagara International Airport because of blockage of the pitot tubes by atmospheric icing.
• 6 February 1996—Birgenair Flight 301 crashed into the sea shortly after takeoff due to incorrect readings from the airspeed indicator. The suspected cause is a blocked pitot tube (this was never confirmed, as the airplane wreck was not recovered).^[7]
• 2 October 1996—AeroPeru Flight 603 crashed because of blockage of the static ports. The static ports on the left side of the aircraft had been taped over while the aircraft was being waxed and cleaned. After the job was done, the tape was not removed.^[8]
• February 23, 2008—B-2 bomber crash in Guam caused by moisture on sensors.^[9]
• 1 June 2009—The French air safety authority BEA said that pitot tube icing was a contributing factor in the crash of Air France Flight 447.^[10]

Gyrocompass

A gyrocompass is a type of non-magnetic compass which is based on a fast-spinning disc and rotation of the Earth (or another planetary body if used elsewhere in the universe) to automatically find geographical direction. Although one important component of a gyrocompass is a gyroscope, these are not the same devices; a gyrocompass is built to use the effect of gyroscopic precession, which is a distinctive aspect of the general gyroscopic effect.^[1][2] Gyrocompasses are widely used for navigation on ships, because they have two significant advantages over magnetic compasses:^[2]

• they find true north as determined by Earth's rotation, which is different from, and navigationally more useful than,magnetic north, and
• they are unaffected by ferromagnetic materials, such as ship's steel hull, which change the magnetic field

  Operation[edit]

  A gyroscope, not to be confused with gyrocompass, is a spinning wheel mounted on gimbal so that the wheel's axis is free to orient itself in any way.^[2] When it is spun up to speed with its axis pointing in some direction, due to the law of conservation of angular momentum, such a wheel will normally maintain its original orientation to a fixed point in outer space (not to a fixed point on Earth). Since our planet rotates, it appears to a stationary observer on Earth that a gyroscope's axis is completing a full rotation once every 24 hours.^{[note 1]} Such a rotating gyroscope is used for navigation in some cases, for example on aircraft, where it is known asheading indicator, but cannot ordinarily be used for long-term marine navigation. The crucial additional ingredient needed to turn such gyroscope into a gyrocompass, so it would automatically position to true north,^[1][2] is some mechanism that results in an application of torque whenever the compass's axis is not pointing north.
  One method uses friction to apply the needed torque:^[3] the gyroscope in a gyrocompass is not completely free to reorient itself; if for instance a device connected to the axis is immersed in a viscous fluid, then that fluid will resist reorientation of the axis. This friction force caused by the fluid results in a torque acting on the axis, causing the axis to turn in a direction orthogonal to the torque (that is, to precess) along a line of longitude. Once the axis points toward the celestial pole, it will appear to be stationary and won't experience any more frictional forces. This is because true north is the only direction for which the gyroscope can remain on the surface of the earth and not be required to change. This axis orientation is considered to be a point of minimum potential energy.
  Another, more practical, method is to use weights to force the axis of the compass to remain horizontal (perpendicular to the direction of the center of the Earth), but otherwise allow it to rotate freely within the horizontal plane.^[1][2] In this case, gravity will apply a torque forcing the compass's axis toward true north. Because the weights will confine the compass's axis to be horizontal with respect to the Earth's surface, the axis can never align with the Earth's axis (except on the Equator) and must realign itself as the Earth rotates. But with respect to the Earth's surface, the compass will appear to be stationary and pointing along the Earth's surface toward the true North Pole.
  Since the gyrocompass's north-seeking function depends on the rotation around the axis of the Earth that causes torque-induced gyroscopic precession, it will not orient itself correctly to true north if it is moved very fast in an east to west direction, thus negating the Earth's rotation. However, aircraft commonly use heading indicators or directional gyros, which are not gyrocompasses and do not position themselves to north via precession, but are periodically aligned manually to true north.^[4][5]

  Mathematical model of a gyrocompass[edit]

  We will consider here a gyrocompass, as a gyroscope which is free to rotate about one of its symmetry axis, and the whole rotating gyroscope is also free to rotate on the horizontal plane, about the local vertical, the zenith. Therefore there are two independent local rotations. In addition to these rotations we will also consider the rotation of the Earth about its North-South (NS) axis, and we will model the planet as a perfect sphere. We will neglect friction and the rotation of the Earth about the Sun.
  In this case a non-rotating observer located at the center of the Earth can be approximated as being an inertial frame. We can set cartesian coordinates  for such an observer (that we will name as 1-O), and the barycenter of the gyroscope will be located at a distance  from the center of the Earth.

  
  

  History[edit]

  The first, not yet practical,^[6] form of gyrocompass was patented in 1885 by Marinus Gerardus van den Bos.^[6] Usable gyrocompass was invented in 1906 in Germany by Hermann Anschütz-Kaempfe, and after successful tests in 1908 became widely used in German Imperial Navy.^[1][6][7]
  The gyrocompass was an important invention for nautical navigation because it allowed accurate determination of a vessel’s location at all times regardless of the vessel’s motion, the weather and the amount of steel used in the construction of the ship.^[3] In the United States, Elmer Ambrose Sperry produced a workable gyrocompass system (1908: patent #1,242,065), and founded the Sperry Gyroscope Company. The unit was adopted by the U.S. Navy (1911^[2]), and played a major role in World War I. The Navy also began using Sperry's "Metal Mike": the first gyroscope-guided autopilot steering system. In the following decades, these and other Sperry devices were adopted by steamships such as the RMS Queen Mary, airplanes, and the warships of World War II. After his death in 1930, the Navy named theUSS Sperry after him.
  Meanwhile, in 1913, C. Plath (a Hamburg, Germany-based manufacturer of navigational equipment including sextants and magnetic compasses) developed the first gyrocompass to be installed on a commercial vessel. C. Plath sold many gyrocompasses to the Weems’ School for Navigation in Annapolis, MD, and soon the founders of each organization formed an alliance and became Weems & Plath.^[8]
  
  

  

  The 1889 Dumoulin-Krebs gyroscope
  Before the success of gyrocompass, several attempts had been made in Europe to use gyroscope instead. By 1880, William Thomson (lord Kelvin) tried to propose a gyrostat (tope) to the British Navy. In 1889, Arthur Krebs adapted an electric motor to the Dumoulin-Froment marine gyroscope, for the French Navy. Giving the Gymnote submarine the ability to keep a straight line under water during several hours, it allowed her to force a naval block in 1890.

  Errors[edit]

  A gyrocompass is subject to certain errors. These include streaming error, where rapid changes in course, speed andlatitude cause deviation before the gyro can adjust itself.^[9] On most modern ships the GPS or other navigational aids feed data to the gyrocompass allowing a small computer to apply a correction. Alternatively a design based on an orthogonal triad of fibre optic gyroscope or ring laser gyroscopes will eliminate these errors, as they depend upon no mechanical parts, instead using the principles of optical path difference to determine rate of rotation.^[10]