Pressure Gauges
Pressure can be defined as the difference in force between two points and can be one of three types-absolute pressure, gauge pressure, or differential pressure. Absolute pressure is pressure relative to a total vacuum. Manifold pressure is an example of an absolute pressure. When the engine is not running, the manifold pressure gauge reads atmospheric pressure, which is the pressure of the atmosphere above a total vacuum. A reading of zero (an impossibility) on a manifold pressure gauge would indicate total vacuum.
Gauge pressure is the difference between atmospheric pressure and the pressure being measured. The cabin differential pressure, which is the difference between atmospheric pressure (static pressure) and the pressure being measured (cabin inside pressure), is an example of this type of pressure.
Differential pressure is the difference between two different pressures. The altimeter is an example of a differential pressure gauge. It indicates the difference between the pressure within a sealed chamber and the pressure outside that sealed chamber (atmospheric pressure).
In an airplane, it's important to monitor the pressures of various fluids, including oil, coolant (yes, there are liquid-cooled aircraft engines flying around out there), and hydraulic pressure. Oil pressure is required to maintain the physical separation between the moving parts inside the engine. If the oil pressure drops too low, metallic parts will meet and seize. The coolant in a liquid-cooled engine must be maintained in a sealed system under pressure to provide for the transfer of heat from the coolant to the air passing through the radiator units. If coolant pressure is lost, the engine will overheat. Hydraulic pressure actuates hydraulically powered units in the aircraft such as wing flaps or landing gear. The power of a hydraulic cylinder comes from the combination of piston area and hydraulic fluid pressure. Any reduction in hydraulic fluid pressure may render hydraulically powered units inoperative.
It is also important to monitor a number of nonfluid pressures, including vacuum, manifold pressure, pitot pressure, and static pressure. Vacuum is a negative pressure provided by an engine-driven vacuum pump or a venturi that provides power for gyro-operated instruments. If the vacuum is too low, the gyros will not operate at the proper speed and the instruments will be unreliable or inoperative. Manifold pressure is the difference between atmospheric pressure and the pressure inside the intake manifold of the engine. Manifold pressure is always less than atmospheric pressure any time that a normally aspirated (non-supercharged) engine is operating and is at its lowest at engine idle. Manifold pressure gauges can provide valuable information regarding the health of the engine and are routinely used to set power levels in conjunction with a variable pitch propeller. Pitot pressure is obtained from the pitot tube and provides part of the information used by the airspeed indicator to show indicated airspeed. The other part of the information needed by the airspeed indicator is obtained from the static system, which reads outside air pressure. The airspeed indicator actually indicates the difference between pitot pressure and static pressure.
There are basically two types of pressure gauge systems available-mechanical and electric.
Mechanical pressure gauge systems require no electrical power to operate and will function fine in an aircraft with no airframe electrical system. These gauges function by reading the pressure directly through a hose or tubing connection, which ducts the pressurized fluid between the pressurized area that is being monitored and the gauge.
Electrical (or electronic) gauge systems operate by using an electrical sending unit mounted on the pressurized area being monitored. This sending unit develops a varying electrical output based on the pressure applied to the unit and sends an electrical signal to a gauge that is mounted in the instrument panel.
Pressure information can easily be obtained in an aircraft with no airframe electrical system by using a device called a Bourdon tube. This tube is a hollow, sealed brass or bronze tube made in a coiled shape. As fluid under pressure is introduced into the tube, the curved tube tends to straighten out. As the end of the tube moves in a small arc, it mechanically actuates a needle and the pressure of the fluid can be read from the face of a properly calibrated gauge.
The Bourdon tube gauge system works only with fairly high pressures and functions better with fluids than gases, making it a good choice for monitoring engine oil or hydraulic pressures. The biggest disadvantage of this is that it requires the introduction of pressurized and possibly hot fluid into the cockpit of the aircraft. If either the fluid line or the gauge fails, the interior of the cockpit could be sprayed with hot fluid.
Pressures that are too low to operate a Bourdon tube indicating system work well with a bellows system. A bellows made with very thin metal can react to very small changes in pressure from low-pressure sources such as the pitot and static systems. Manifold pressure, altimeter, vertical speed, and airspeed indicators are all low-pressure-sensing gauges and are all bellows-operated.
Electrical or electronic indicating systems work well at pressures that are too low to operate a Bourdon tube system. Electrical or electronic systems are lighter and easier to install, do not introduce pressurized fluid into the aircraft, and can be calibrated more accurately than Bourdon tube systems. A failure of these systems will render the indication inoperative but will not release pressurized fluid.
There are also a number of temperatures that require monitoring, including oil, coolant, hydraulic fluid, cylinder head, exhaust gas, and outside air. Oil and coolant temperatures are critical to prevent loss of oil integrity and resulting parts seizure. Hydraulic fluid can also break down and cause loss of hydraulic unit function and seizure of hydraulic cylinders. Excessive cylinder head or exhaust gas temperatures can indicate improper leaning and detonation. Outside air temperature is used in performance calculations relating to density altitude.
A thermocouple-type temperature indicating system is a device that has two dissimilar metals placed together. As these dissimilar metals are heated they generate a minute electrical voltage. The electrical voltage produced rises as the temperature rises. A very sensitive millivolt meter calibrated in degrees in the instrument panel relays temperature data to the pilot.
A thermocouple system has a lag in the reading works only at relatively high temperatures, which makes it useful for cylinder head, exhaust gas, turbocharger, and various turbine temperatures but a poor choice for engine coolant, engine oil, and hydraulic fluid temperature readings.
A Bourdon tube or a bellows gauge also can be used to indicate temperatures, but this is rarely done. If the Bourdon tube or bellows is sealed and the internal temperature is raised, the internal pressure also rises and the tube or bellows expands and gives an indication on the gauge just as with a pressure system.
Some temperatures, such as outside and cabin air temperatures, are still read from old-fashioned mercury thermometers.
Modern higher-technology aircraft use a device called an air data computer that reads all aircraft and engine pressures and temperatures and displays that information on either traditional gauges or the newer cathode ray tube displays. These computerized devices also present various types of integrated displays, operate cockpit resource management systems, and operate pilot warning systems.
It is important to develop an instrument scan that includes all of the aircraft's pressure and temperature gauges. Any drop in pressure or increase in temperature is cause for concern and further investigation.
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