to operate the airplane more precisely, maxi-mum performance and enhanced safety, to gain the essential knowledge about, to separate the pilot and static sources, to measure the height of the airplane, above a given level, the effect of atmospheric pressure and tempera-ture on the altimeter, to fly high enough to clear the highest terrain or obstruction, to re-duce the possibility of a midair collision, to maintain altitudes in accordance with air traffic rules, to take advantage of favorable winds and weather conditions, reference levels from which altitude is measured, to register zero in level flight, the airspeed pointer on the face of the instrument, pressure difference between
pitot impact pressure and
static pressure, to minimize the stress on the air-plane structure, with the remaining engine at ta-keoff power, to provide the power for the head-ing and attitude indicators, to drive the gyroscope of the turn needle, to prevent excessive oscillation of the turn needle, the correct angle of bank for the rate of turn, to sense airplane movement about the yaw and roll axis, to display pictorially the resultant motion, proportional to the roll rate of the airplane, to reset the heading indicator to align it with the magnetic compass, to represent the true horizon, to indicate the attitude of the
airplane relative to the true horizon.
Reading
Ex.73. Read the text.
Text. FLIGHT INSTRUMENTS
The use of instruments as an aid to flight enables the pilot to operate the airplane more precisely, and therefore, obtain maximum performance and enhanced safety. This is particularly true when flying greater dis-tances. Manufacturers have provided necessary flight instruments; however, it is the pilot's re-sponsibility to gain the essential knowledge about how the instruments operate sothat they can be used effectively.
Some flight instruments utilize the pilot-static system for their operation.
The pilot-static system provides the source of air pressure for the operation of the altimeter, vertical speed indicator (vertical ve-locity indicator), and the airspeed indicator.
The installation in newer airplanes sepa-rates the pilot and static sources.
The Altimeter. The altimeter measures the height of the airplane above a given level. Since it is the only instrument that gives altitude information, the altimeter is one of the most im-portant instruments in the airplane. To use the altimeter effectively, the pilot must thoroughly understand its principle of operation and the ef-fect of atmospheric pressure and temperature on the altimeter. The presentation of altitude varies considerably between different types of altimeters. Some have one pointer while others have more.
Types of Altitude
Knowing the aircraft's altitude is vitally impor-tant to the pilot for several reasons. The pilot must be sure that the airplane is flying high enough to clear the highest terrain or obstruc-tion along the intended route; this is especially important when visibility is reduced.
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To keep above mountain peaks, the pi-lot must note the altitude of the aircraft and elevation of the surrounding terrain at all times. To reduce the possibility of a midair collision, the pilot must maintain altitudes in accordance with air traffic rules. Often cer-tain altitudes are selected to take advantage of favorable winds and weather conditions. Also, a knowledge of the altitude is necessary to calculate true airspeeds.
Altitude is vertical distance above some point or level used as a reference. There may be as many kinds of altitude as there are ref-erence levels from which altitude is meas-ured and each may be used for specific rea-sons. Pilots are usually concerned, however, with five types of altitudes:
Absolute altitude – The vertical dis-tance of an aircraft above the terrain.
Indicated altitude – That altitude is read directly from the altimeter (uncorrected) after it is set to the current altimeter setting.
Pressure altitude – The altitude indi-cated when the altimeter setting window is adjusted to 1012,3 HPa. This is the standard datum plane, a theoretical plane where air pressure (corrected to 15° C.) is 760 mm of mercury. Pressure altitude is used for comput-er solutions to determine density altitude, true altitude, true airspeed, etc.
True altitude – The true vertical dis-tance of the aircraft above sea level – the ac-tual altitude. (Often expressed in this man-ner; 10,900 ft. MSL.) Airport, terrain, and obstacle elevations found on aeronautical charts are true altitudes.
Density altitude – This altitude is pressure altitude corrected for nonstandard temperature variations. When conditions are standard, pressure altitude and density alti-tude are the same. Consequently, if the tem-perature is above standard, the density alti-tude will be higher than pressure altitude. If the temperature is below standard, the densi-ty altitude will be lower than pressure alti-tude. This is an important altitude because it is directly related to the aircraft's takeoff and climb performance.
Vertical Speed Indicator. The vertical speed or vertical velocity indicator indicates whether the aircraft is climbing, descending, or in level flight.
The rate of climb or descent is indicated in feet per minute or meters per second. If proper-ly calibrated, this indicator will register zero in level flight.
Although the vertical speed indicator op-erates solely from static pressure, it is a differen-tial pressure instrument.
The Airspeed Indicator
The airspeed indicator is a sensitive, diffe-rential pressure gauge which measures and shows promptly the difference between (1) pi-tot, or impact pressure, and (2) static pressure, the undisturbed atmospheric pressure at flight level. These two pressures will be equal when the aircraft is parked on the ground in calm air. When the aircraft moves through the air, the pressure on the pitot line becomes greater than the pressure in the static lines. This difference in pressure is registered by the airspeed pointer on the face of the instrument, which is calibrated in miles per hour, knots, or kilometers per hour.
There are three kinds of airspeed that the pilot should understand: (1) indicated airspeed; (2) calibrated airspeed; and (3) true airspeed.
Indicated Airspeed (IAS ). The direct instrument reading obtained from the airspeed indicator, uncorrected for variations in atmos-pheric density, installation error, or instrument error.
Calibrated Airspeed (CAS) is indicated airspeed corrected for installation error and in-strument error. Although manufacturers attempt to keep airspeed errors to a minimum, it is not possible to eliminate all errors throughout the airspeed operating range. At certain airspeeds and with certain flap settings, the installation and instrument error may occur. This error is generally greatest at low airspeeds. In the cruis-ing and higher airspeed ranges, indicated air-speed and calibrated airspeed are approximately the same.
The airspeed indicator should be cali-brated periodically because leaks may develop or moisture may collect in the tubing. Dirt, dust, ice, or snow collecting at the mouth of the tube may obstruct air passage and prevent correct indications, and also vibrations may destroy the sensitivity of the diaphragm.
True Airspeed (TAS ). The airspeed in-dicator is calibrated to indicate true airspeed under standard sea level conditions.
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Because air density decreases with an increase in altitude, the airplane has to be flown faster at higher altitudes to cause the same pressure difference between pitot im-pact pressure and static pressure. Therefore, for a given true airspeed, indicated airspeed decreases as altitude increases or for a given indicated airspeed, true airspeed increases with an increase in altitude.
Airspeed Limitations. There are other important airspeed limitations not marked on the face of the airspeed indicator. These speeds are generally found in the Airplane Flight Manual.
One example is the MANEUVERING SPEED. This is the "rough air" speed and the maximum speed for abrupt maneuvers. If during flight, rough air or severe turbulence is encountered, the airspeed should be reduced to maneuvering speed or less to minimize the stress on the airplane structure.
Other important airspeeds include LANDING GEAR OPERATING SPEED, the maximum speed for extending or retract-ing the landing gear if using aircraft equipped with retractable landing gear; the BEST ANGLE OF CLIMB SPEED, important when a short field takeoff to clear an obstacle is required; and the BEST RATE OF CLIMB SPEED, the airspeed that will give the pilot the most altitude in a given period of time. The pilot who flies the increasingly popular light twin-engine aircraft must know the aircraft's MINIMUM CONTROL SPEED, the mini-mum flight speed at which the aircraft is satis-factorily controllable when an engine is sud-denly made inoperative with the remaining en-gine at takeoff power.
Gyroscopic Flight Instruments. Sev-eral flight instruments utilize the properties of a gyroscope for their operation. The most common instruments containing gyroscopes are the turn indicator, turn coordinator, head-ing indicator, and the attitude indicator. The gyroscopic instruments can be operated either by a vacuum or an electrical system. In some airplanes, all the gyros are either vacuum or electrically operated; in others, vacuum systems provide the power for the heading and atti-tude indicators, while the electrical system provides the power to drive the gyroscope of the turn needle.
Turn-and-Slip Indicator. The turn and slip indicator was one of the first instruments used for controlling an airplane without visual reference to the ground or horizon. Its princip-al uses in airplanes are to indicate turn and to serve as an emergency source of bank information in the event the attitude indicatorfails.
The turn and slip indicator is actually a combination of two instruments: the turn needle and the ball or inclinometer. The needle is gyro operated to show rate of turn, and the ball reacts to gravity and/or centrifugal force to in-dicate the need for directional trim.
The turn needle is operated by a gyro, dri-ven by either vacuum or electricity. The turn needle indicates the rate (number of degrees per second) at which the aircraft is turning about its vertical axis. Unlike the attitude indicator, it does not give a direct indication of the banked attitude of the aircraft. A dampening mechanism prevents excessive oscillation of the turn needle.
The ball is actually a balance indicator, and is used as a visual aid to determine coordinated use of the aileron and rudder control. During a turn it indicates the relationship between the an-gle of bank and rate of turn. It indicates the “quality” of the turn or whether the aircraft has the correct angle of bank for the rate of turn.
Turn Coordinator. Recently another type of turn indicator has been developed and is used quite extensively. This instrument is referred to as a "Turn Coordinator". In place of the turn needle indication, this instrument shows the movement of the airplane about the longitudinal axis by displaying a miniature airplane on the instrument. The movement of the miniature air-plane on the instrument is proportional to the roll rate of the airplane. When the roll rate is re-duced to zero, or in other words the bank is held constant, the instrument provides an indi-cation of the rate of turn. This design features a realignment of the gyro in such a manner that it senses airplane movement about the yaw and roll axis and pictorially displays the resultant motion as described above. The conventional inclinome-ter (ball) is alsoincorporated in this instrument. The Heading Indicator (HSI – horizontal situation indicator). The heading indicator (or directional gyro) is fundamentally a me-chanical instrument designed to facilitate the use of the magnetic compass.
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Errors in the magnetic compass are numerous, making straight flight and preci-sion turns to headings difficult to accomplish, particularly in turbulent air. Heading indica-tors, however, are not affected by the forces that make the magnetic compass difficult to interpret.
Because of precession, caused chiefly by bearing friction, the heading indicator will creep or drift from a heading to which it is set. Among other factors the amount of drift de-pends largely upon the condition of the in-strument. Pilots shall bear in mind that the heading indicator is not direction-seeking, as is the magnetic compass. It is important to check the indications frequently and reset the heading indicator to align it with the magnetic compass when required.
The Attitude Indicator (ADI/FDI – attitude/flight direction indicator). The attitude indicator, with its miniature aircraft and horizon bar, displays a picture of the attitude of the airplane. The relationship of the miniature aircraft to the horizon bar is the same as the relationship of the real air-craft to the actual horizon.
The instrument gives an instantaneous in-dication of even the smallest changes in attitude.
The gyro in the attitude indicator is mounted on a horizontal plane and depends upon rigidity in space for its operation. The horizon bar represents the true horizon. This bar is fixed to the gyro and remains in a horizontal plane as the airplane is pitched or banked about its lateral or longitudinal axis, indicating the attitude of the airplane relative to the true horizon.
An adjustment knob is provided with which the pilot may move the miniature air-plane up or down to align the miniature air-plane with the horizon bar to suit the pilot's line of vision. Normally, the miniature airplane is adjusted so that the wings overlap the horizon bar when the airplane is in straight-and-level cruising flight. The pitch and bank limits de-pend upon the make and model of the in-strument. The attitude indicator is reliable and the most realistic flight instrument on the in-strument panel. Its indications are very close approximations of the actual attitude of the air-plane.