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. 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. Other instruments that might be connected are
air data computers,
flight data recorders,
altitude encoder,
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.
Diagram of a pitot-static system including the pitot tube,
pitot-static instruments and static port
Pitot-static pressure
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. 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
The pitot pressure is obtained from the
pitot
tube. The pitot pressure is a measure of ram air pressure (the
dynamic air pressure created by vehicle motion or the air ramming
into the tube), which, under ideal conditions, is equal to
stagnation 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. 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.
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. 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. 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.
Multiple pressure
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 used to produce angle of attack and angle of
sideslip indications.
Pitot-static instruments
Airspeed indicator diagram showing pressure sources from both the
pitot tube and static port
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'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
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. 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.
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. 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.
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.
A vertical airspeed indicator
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 or
not an aircraft is flying in level flight. 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. 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"). 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. The calibrated leak varies
from model to model, but the average time for the diaphragm to
equalize pressure is between 6 and 9 seconds.
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".
System malfunctions
Blocked pitot tube
A blocked pitot tube is a pitot-static problem that will only
affect airspeed indicators. A blocked pitot tube will cause the
airspeed indicator to register an increase in airspeed when the
aircraft climbs, even though
indicated 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. 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. 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.
Blocked static port
A blocked static port is a more serious situation because it
affects all pitot-static instruments. 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.
Inherent errors
Inherent errors may fall into several categories, each affecting
different instruments.
Density errors affect instruments
reporting airspeed and altitude. This type of error is caused by
variations of pressure and temperature in the atmosphere. A
compressibility error occurs when the air entering the
pitot tube becomes less able to resist compression. At higher
altitudes, the air is less dense and therefore more easily
compressed; likewise, the aircraft's forward motion itself
compresses the air around the aircraft. Such conditions become
significant at altitudes above and at airspeeds greater than . This
error affects the airspeed indicator and causes a reading that is
lower than the actual true airspeed (TAS) in an environment of
increasing altitude.
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.
Position errors
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. 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.
Pitot-static related disasters
References
-
{{citeweb|url=http://aviation-safety.net/database/record.php?id=19960206-0|title=ASN
Aircraft accident description Boeing 757-225 TC-GEN — Puerto Plata,
Dominican
Republic|accessdate=2007-01-07|}publisher=aviation-safety.net}}
- Lawford. J. A. and Nippress, K. R. (1983). Calibration of
Air-Data Systems and Flow Direction Sensors (AGARD AG-300 -
Vol.1, AGARD Flight Test Techniques Series; R. W. Borek, ed.).
Accessed via Spaceagecontrol.com (PDF). Retrieved on 25
April 2008.
- Kjelgaard, Scott O. (1988), Theoretical Derivation and
Calibration Technique of a Hemispherical-Tipped Five-Hole
Probe (NASA Technical Memorandum 4047).
See also
External links
during climb en route to Buffalo Niagara
International Airport
because of blockage of the pitot tubes by atmospheric icing.
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