Pitot-static system
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A Pitot-static system consists of a system of pressure-sensitive instruments and the means by which the appropriate pressures are obtained. A Pitot-static system is generally composed of the pitot-static instruments, a Pitot tube and a static port.[1] The system allows a pilot to know an aircraft's airspeed, Mach number, altitude, and altitude trend. Other equipment that might be connected are air data computers, flight data recorders, altitude encoders, cabin pressurization controllers, and various airspeed switches.
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[edit] Pitot-static pressure
The Pitot-static system of instruments works by measuring pressures or pressure differences to give an indication of speed and altitude.[1] These pressures can come 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.
[edit] Pitot pressure
The Pitot pressure is obtained from the Pitot tube, which measures the ram air 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 as it will be less distorted by the aircraft's structure. When airspeed is increased, the ram air pressure is increased, which can be translated by the airspeed indicator.[1]
[edit] Static pressure
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 alternate 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.
[edit] Pitot-static instruments
The Pitot-static system obtains pressures for interpretations by the Pitot-static instruments. While the explanations below explain traditional, mechanical instruments many modern aircraft use air data computers (ADC) to calculate airspeed, rate of climb, altitude and mach number. Two ADCs receive total and static pressure from independent Pitot tubes and static ports, and the aircraft compares the information from both computers and checks one against the other. There are also "standby instruments", which are back-up pneumatic instruments like the ones described above.
[edit] Airspeed indicator
The airspeed indicator is the only instrument that uses the ram pressure from the Pitot tube. The airspeed indicator is connected to both the ram and static pressure sources. The greater the difference between the ram pressure and the static pressure, the higher the airspeed reported.[2] 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.[2]
[edit] Altimeter
The altimeter is used to determine changes in air pressure that occur as altitude changes.[2] 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 altitude. The reverse is true when descending from higher to lower altitudes.[2]
[edit] Machmeter
A Machmeter shows the ratio of the speed of sound to the true airspeed an aircraft is flying. Most high-speed aircraft are limited as to the maximum Mach number they can fly. This is shown on a Machmeter as a decimal fraction.
[edit] Vertical airspeed indicator
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 an aircraft is flying in level flight.[3] The vertical airspeed specifically shows the rate of climb or the rate of descent, which is measured in feet per minute.[3] 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").[2] Through this process, when an 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.[2] The calibrated leak varies from model to model, but the average time for the diaphragm to equalize pressure is between 6 and 9 seconds.[2]
[edit] Pitot-static errors
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".[4]
[edit] System malfunctions
[edit] Blocked pitot tube
A blocked Pitot tube is a Pitot-static problem that will only affect airspeed indicators.[4] A blocked Pitot tube will cause the airspeed indicator to register an increase in airspeed when the aircraft climbs from lower to higher altitudes. In reverse, the airspeed indicator will show a decrease in airspeed when the aircraft descends to lower altitudes. The Pitot tube's location, near the leading edge of the wings, leaves it susceptible to becoming clogged by an obstruction or through icing.[4] For this reason, the FAA recommends that the Pitot tube be checked for obstructions prior to any flight.[3] To prevent icing, many Pitot tubes are equipped with a heating element, and in aircraft certified for instrument flight, a heated pitot tube is required.[4]
[edit] Blocked static port
A blocked static port is a more serious situation in that it affects all pitot-static instruments.[4] 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.[4]
Remedies for a blocked static port may include using an alternate static pressure source, which is located within the cockpit of many light aircraft. Another emergency remedy for a blocked static port in flight is to break the glass on the VSI, which will then supply static air from inside the cockpit. This will only work in an unpressurized cabin, as in most light general aviation aircraft.
[edit] Inherent errors
Inherent errors may fall into several categories, each affecting different instruments. Density errors affect instruments reporting airspeed and altitude. This error is caused by variations of pressure and temperature in the atmosphere. A compressibility error occurs when the air entering the Pitot tube, usually at altitude above 10,000 feet and with airspeeds greater than 200 knots. This error affects the airspeed indicator and causes a reading that is higher than the actual airspeed.[4] 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. This large change in pitch will cause a momentary showing of movement in the opposite direction. Reversal errors primarily affect altimeters and vertical speed indicators.[4]
[edit] Position errors
Another class of inherent errors is that of position errors. A position error is produced by incorrect pressure readings, which are caused by disrupted airflow around the Pitot tube or static port. 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.[4] 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 make 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.[4]
[edit] Pitot-static related disasters
- 6 February 1996 - Birgenair Flight 301 crashes shortly after takeoff due to the Pitot tube being blocked and thus causing the system to provide incorrect airspeed data.[5]
- 2 October 1996 - AeroPeru Flight 603 crashes 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.[6]
[edit] References
- ^ a b c d e f [1997] (2004) in Willits, Pat: Guided Flight Discovery - Private Pilot, Abbot, Mike Kailey, Liz, Jeppesen Sanderson, Inc., 2-48 - 2-53. ISBN 0-88487-333-1.
- ^ a b c d e f g Pitot-Static Instruments - Level 3 - Pitot-Static Instruments. Retrieved on 2007-01-07.
- ^ a b c Pilot Handbook - Chapters 6 through 9. Retrieved on 2007-01-07.
- ^ a b c d e f g h i j Flight Instruments - Level 3 - Pitot-Static System and Instruments. Retrieved on 2007-01-07.
- ^ ASN Aircraft accident description Boeing 757-225 TC-GEN - Puerto Plata, Dominican Republic. Retrieved on 2007-01-07.
- ^ CVR Database - 2 October 1996 - Aeroperu 603. Retrieved on 2007-01-07.
