AIRPLANE SYSTEMS AIRPLANES THE POWERPLANT AND RELATED SYSTEMS FLIGHT INSTRUMENTS- is the instruments that provide you with information regarding the airplane’s attitude, direction, altitude, and speed.

1. Gyroscopic Instruments – A pictorial view of the airplane’s attitude and rate of turn is provided by the attitude indicator and turn coordinator.

Gyro instruments operate on 2 Fundamental Concepts: 1. Rigidity in Space - the principle that a wheel with a heavily

weighted rim spun rapidly will remain in a fixed position in the plane in which it is spinning. By mounting this wheel, or gyroscope, on a set of Gimbal Rings, the Gyro is able to rotate freely in any direction. Thus, if the Gimbals Rings are tilted, twisted, or otherwise moved, the airplane, in effect, pitches, rolls, and turns around the instrument. The Gyro tends to remain on its axis undisturbed and resists being forced off its axis. Regardless of the twisting forces applied to it, the Gyro remains in the plane in which it was originally spinning. A Gyro will tend to turn at right angles to a force applied to twist it off its axis. It resists movement in the direction of turn and wants to go at right angles to the way it is twisted.

2. Precession – is the tilting or turning of a gyro in response to pressure

or friction in the Gimbals. Gimbals are suspension bearings that allow the gyros’s axes to remain undisturbed while the airplane yaws, pitches and rolls around them. It is not possible to mount a gyro in a frictionless environment. A small force is applied to the gyro whenever the airplane changes direction. The reaction to this force occurs in the direction of rotation, approximately 90 degrees ahead of the point where the force was applied. These causes slow drifting and minor erroneous indications in the gyroscopic instruments.

Gyros have 2 different Vacuum Power Sources:

A. Electrically Powered Vacuum – (Turn Coordinator) B. Engine Driven Vacuum – (Attitude & Heading Indicator) Air is

first drawn into the vacuum system through a filter assembly. It then moves through the attitude and heading indicators where it causes the gyros to spin. After that, it continues to the engine-driven vacuum

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pump where it is expelled. A relief valve prevents the vacuum pressure or suction from exceeding prescribed limits.

It is important for you to monitor vacuum pressure during flight, because the attitude and heading indicators may not provide reliable information when suction pressure is low. The vacuum gauge is usually marked with the normal range. Some airplanes are equipped with a warning light and others may be equipped with an electrically driven, standby vacuum pump.

A. Attitude Indicator (Artificial Horizon) – senses roll as well as pitch, which is the up and down movement of the airplane’s nose. AI uses an artificial horizon and miniature airplane to depict the position of your airplane in relation to the true horizon. This is useful when the natural horizon is obscured by clouds, reduced visibility, or darkness. The Attitude Indicator presents you with a view of the airplane as it would appear from behind. The angle of bank is shown both pictorially by the relationship of the miniature aircraft to the horizon bar and by the alignment of the pointer with the bank scale at the top of the instrument. Pitch is indicated by the position of the “nose,” or center, of the miniature airplane with respect to the horizon bar. The Attitude Indicator is the central element of an instrument scan and is the only instrument that gives direct information about the aircraft’s attitude. It should erect and become stable within 2 minutes (the FAA says 5 minutes.) Align the wings of the symbolic airplane with 90 degree bank indices (corresponding to 0 degree pitch.) This preliminary adjustment should be refined in flight when you determine your level-flight pitch attitude at cruise power. During taxi, after the gyros have reached full speed with normal suction/air pressure, applying the brakes should cause a noticeable pitch down movement in the attitude indicator. Turning should not reveal any indication of bank if the wings are level. If the AI does show bank in a taxi turn, excessive friction in the gimbal bearings is indicated and should be cleaned and over-hauled. 5 degrees of “slop” is maximum permissible amount. The bank index on the face is marked with the first 30 degrees in 10 degree increments and shows one mark each for 60 and 90 degrees. Pitch attitude may be judged as a number of “dots” above or below the horizon. In a bank, the horizon bar remains parallel to the horizon; the symbolic airplane, rigidly attached to the instrument case, rolls with the real airplane showing the bank against the horizon bar in the background. If you pitch up, the horizon bar drops, causing the symbolic airplane to appear “above the horizon.” A bank does not necessarily indicate a turn, and a nose high attitude does not prove you are in a climb. An airplane is a forward slip has a low wing, shown on the attitude indicator as a bank, although it is not turning. An airplane in slow flight has a now high attitude in level flight and even in a descent. The AI merely shows the airplane’s attitude in relation to the horizon.

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AI has limits and if these limits are exceeded, they may tumble and become useless for controlling the airplane. The AI fails gradually without providing much obvious warning signals. The pitch limit is usually 60 to 70 degrees and the bank limit is 100 to 110 degrees. Many newer instruments are “non-tumbling”; they can take 360 degrees of pitch and roll with relatively little error because there are no internal stops to limit travel. If the AI gyro tumbles with a conventional pendulous vane erecting system, the gyro may take an hour or more to re-erect after becoming fully tumbled. The correction rate with the rotor at operating RPM is not more than 3 degrees of pitch or roll per minute, and the system is ineffective at very large error angles. Small precession errors do occur in the attitude indicator. At the end of a 180 degree turn the attitude indicator may show up to 5 degrees of bank. This error will be removed at 2 or 3 degrees per minute by the gyro’s self-erecting system after you return to level flight. In a 360 degree turn, precession error from the second half of the turn cancels that from the first half.

B Heading Indicator (Directional Gyro) – It does not sense the airplane’s heading directly, but has an internal gyroscope whose stability provides accurate heading information once it is initially set to the correct heading. The DG senses the airplane movement and display’s heading based on a 360 degree azimuth, with the final zero omitted. It is your primary source of heading information. The airplane’s heading is shown at the top of the instrument. We need the DG for heading control because of the errors that occur in the magnetic compass during turns, speed changes, and even rough air. The face of the DG is a compass rose divided into 5, 10, & 30 degree segments. Often divisions are marked on the circumference of the instrument to further simplify visualizing the compass rose. To turn right to 45 degrees, just notice the course under the reference mark 45 degrees to the right and turn to that heading. Calculate compass reciprocals by glancing diametrically across the instrument from the heading whose reciprocal you need. Using the DG in this way cuts down on cockpit math. The DG also makes clear which runway goes where as you approach an airport, which can be especially valuable on a circling approach. Check the DG before takeoff. When it has spun up, set it to the magnetic compass prior to taxi and check it against runway heading just before takeoff. This checks the DG for excessive precession and also protects you against the danger and embarrassment of departing from the wrong runway at an unfamiliar airport. Most training airplanes are referred to as “free” gyros. This means they have no automatic, north-seeking system built into them. A normal DG will precess, or drift off its selected heading, and should regularly be checked against the magnetic compass in straight, level, uncelebrated flight. You must regularly align the indicator with magnetic

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compass. 3 degrees of precession in 15 minutes is the maximum permissible error for normal operations. The DG will tend to precess more when several turns are made, as in a holding pattern or during an instrument approach. The DG should always be checked before and during an instrument approach. Like the Attitude Indicator, the DG has limits and will become unreliable if the airplane is banked too steeply. Beyond 55 degrees bank, the DG will lose accuracy and may spin wildly.

C. Turn & Bank (Turn Coordinator) – provides an indication of turn direction and quality as well as a backup source of bank information in the event of an attitude indicator failure. The Turn Coordinator only indicates the rate of turn and does not display a specific angle of bank.

The primary difference between the Turn-and-Slip and the Turn Coordinator is the display of turn, or roll, information. The Turn-and-Slip Indicator or “Needle and Ball” uses a pointer, called a Turn Needle. Turn-and-Slip has little squares with peaked caps, or “Doghouses” to indicate standard rate turns to the left or right. The Turn Coordinator employs a miniature airplane. When you are rolling into or out of a turn, the miniature airplane banks or dips a wing in the direction of the roll and rate of turn. A rapid rate causes the miniature airplane to bank more steeply than a slow roll rate. Rate of Turn is expressed as the number of degrees of heading change per second; it indicates the time required to Turn from one heading to another. In instrument flight, all Turns are made at Standard-Rate Turn. To establish and maintain a Standard-Rate Turn, align the wing of the miniature airplane with the turn index. This equals 3 degrees per second. A complete 360 degree Standard Rate Turn is 2 minutes. For small turns 5 or 10 degrees of heading change- a less than standard rate is used. In both instruments, a gyro wheel is suspended with its axis parallel to the airplane’s lateral axis, positioned by a spring of calibrated strength, in gimbals so the whole assembly can pivot around the airplane’s longitudinal axis. As the wheel spins, any yawing by the airplane causes the wheel to react (precess) by tipping left or right in the gimbals. The more rapid the yaw, the bigger the precessional forces, the further the tipping gyro stretches the spring, and the bigger the displacement of the turn needle or airplane symbol. On Turn Coordinators, the gimbal axis is canted about 30 degrees above horizontal, which causes an exaggerated reaction when the airplane changes bank angle. The damping of stray oscillations in rough air is better in Turn Coordinators than in Turn Indicators. 2 important points about Turn Coordinators: 1.) The exaggerated reaction caused by the canted gimbal axis only occurs when the airplane is changing bank angle, not in a constant angle of bank; and 2.) Although the miniature airplane turn symbol

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suggests a banking airplane, it does not necessarily meant he airplane is in a bank (any more than the turn needle in a turn and bank indicator does); what the symbol really displays over the long term is Rate of Yaw. The Inclinometer is used to depict airplane Yaw, which is the side-to-side movement of the airplane’s nose. Both indicators use a ball in a tube, called the Slip-skid Indicator or and Inclinometer. It provides information relating to the quality of the turn. It is simply a heavy ball in a curved glass tube filled with Kerosene to dampen its movements. In a turn, the ball’s position in the tube is controlled by the balance between gravity (acting downward to toward the center of earth) and centrifugal force (acting toward the outside of the turn). Its purpose is to show whether or not the airplane is in coordinated flight. In any turn, the airplane is affected by its weight and by centrifugal force. The weight pulls the airplane towards the earth, and centrifugal force pulls it towards the outside of the turn. The resultant is a net force pointing downward at an angle to the vertical. A turn is said to be coordinated if the airplane is banked just enough to that the resultant force is directed straight toward the floor of the cockpit. During straight-and-level flight with the ailerons and rudder coordinated, the forces of gravity causes the ball to rest in the lowest part of the tube, centered between the reference lines. You maintain coordinated flight by keeping the ball centered. If aerodynamic forces are unbalanced, the ball moves away from the center of the tube and you can center it by using the rudder. To do this, you apply rudder pressure on the side where the ball is deflected. The simple rule, “Step on the Ball,” may help you remember which rudder pedal to depress. If the ball moves towards the inside of a turn, the airplane is not turning fast enough for its degree of bank and is said to be slipping. This is corrected by applying rudder to increase the rate of turn. If the ball moves towards the outside of the turn, the airplane is turning too fast for the degree of bank and is said to be skidding. The usual cause is too much ruder pressure. A Slip is the rate of turn is too slow for the angle of band and the ball moves to the inside of the turn. A Skid is the rate of turn is too great for the angle of bank, and the ball moves to the outside of the turn. Remember to “Step on the Ball.” You may vary the angle of bank to help restore coordinated flight from a slip or skid. To correct for a slip, you should decrease bank and /or increase the rate of turn. To correct for a skid, increase the band and/or decrease the rate of turn. In slipping and skidding turns, the turn instrument will show the actual rate of turn. Its accuracy is not disturbed by uncoordinated flight. Coordinated flight has several advantages. Most stall-spin accidents result from uncoordinated maneuvers at low airspeed in the turn from base to final. This is similar during departures turns, if the airspeed is allowed to diminish too much, and in strong turbulence. Coordinated flight is more comfortable for the airplane occupants and diminish unnecessary lateral forces.

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Coordinated flight at a given airspeed, the turn rate is directly proportional to the bank angle. The steeper the bank, the faster the airplane will turn. At a given airspeed, a standard rate will always be achieved with a particular angle of bank. The bank angle required for a given turn rate depends on the airspeed. The higher the airspeed, the more bank is necessary for a standard rate turn. The bank angle required for a coordinated standard rate turn is about 15% of the airspeed. In very fast airplanes, it may be necessary to bank more than 30 degrees to achieve a 3-degree per second turn, and this is considered unsafe. In such situations, the bank is limited to 30 degrees and turns are made at less than standard rate. The Attitude Indicator gives direct information about angle of bank, which is not necessarily the same as turn information. The turn instrument gives rate of turn information but says nothing about the angle of bank, except in coordinated flight, where the bank angle associated with a given rate of turn depends upon speed. To check the turn instrument prior to IFR takeoff, observe its indications during taxi turns. As you turn a few degrees left or right, you should see corresponding turn indications on the turn instrument, and the ball should roll toward the outside of the turn. If you turn 90 degrees onto the perpendicular taxiway at a normal taxi speed, you will be turning at considerably more than standard rate; the instrument should show this.

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2. Pressure (Pitot-Static) Instruments – Use pressure-sensitive

devices to convert pressure supplied by the pitot-static system to instrument in the cockpit that show reflect your speed, rate of climb or descent, and altitude while operating on air pressure differentials. The Pitot-Static System consists of: typically uses a single pitot tube and one ore two static ports. Some designs combine the static port with the pitot tube.

1. The Pitot tube – receives Impact/Ram Pressure (air striking the airplane because of its forward motion) from outside the airplane for use by the airspeed indicator. It is usually mounted on the leading edge of the wing or on the nose section, so the opening is aligned or exposed to the relative wind. This provides minimum disturbance caused by the motion of the airplane through the air. This allows ram air pressure to enter the pitot tube before it is affected by the airplane’s structure. Since the pitot tube opening faces forward, an increase in speed increases ram air pressure. As the airplane moves through the air, the impact pressure on the open pitot tube affects the pressure in the pitot chamber. Any change of pressure in the pitot chamber is transmitted through a line connected to the airspeed indicator which utilizes impact pressure for its operations. 2. The Static port or ports – Static pressure (pressure of the still air) enters the pitot-static system through the static port which is normally flush-mounted on the side of the fuselage in an area of relatively undisturbed air. When static ports are used, they are usually located on each side of the fuselage to provide an average static pressure reading. This allows for a more accurate reading and compensates for any possible variation in static pressure due to erratic changes in airplane attitude. 3. Plumbing to transmit pressure to the pressure instruments - 4. An alternate static source.- usually found in the cockpit under the dash near the captain seat. This source is vented inside the cockpit to allow cabin pressure into the static system where ambient air pressure is lower than outside static pressure. Blocked Pitot System - Usually very reliable but do have some limitations. Gross errors almost always indicate blockage of the pitot tube, static port(s) or both. They may occur from:

1. Visible moisture (ice) when temperatures are near the freezing level. Icing is the best known problem of blockage. The pre-flight should therefore always include a check of the pitot heat. Just click on the master switch and pitot heat; the ammeter should immediately show a significant jump. Pitot heat should be used as a preventive measure, not a cure. Switch it on anytime you except to enter visible moisture where the outside air temp. may reach 0 degrees Celsius.

3. The Drain Hole remaining open. A clogged pitot tube, but a clear drain hole will result in an airspeed indication of zero. The pitot system can become blocked completely or only partially if the pitot drain hole remains open. If

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the pitot tube becomes clogged and its associated drain hole remains clear, ram air will no longer be able to enter the pitot system. Air already in the system will vent or escape through the drain hole, and the remaining pressure will drop to ambient (outside) air pressure resulting in a “zero” airspeed reading

EX: The airspeed indicator reading decreases to zero, because the airspeed indicator senses no difference between ram and static air pressure. The airspeed indicator acts as if you airplane is stationary on the ramp. The apparent loss of airspeed is not usually instantaneous. The airspeed generally drops slowly to zero.

The airspeed indicator should be checked for a zero reading on the ramp and a positive reading during the takeoff. 3. Taxiing in snow with a low wing airplane could slightly brush a snow bank. 4. The pitot hole a great hiding place for bugs and insects. One could disable it. 5. Forgetting to remove the protective cover over the pitot tube. A blocked pitot tube can only affect the accuracy of the airspeed indicator. It may not always be detected during preflight, but a quick glance at the airspeed during the takeoff roll can flag it in time to discontinue the takeoff. If the pitot tube, drain hole, and static system all become clogged in-flight, changes in airspeed will not be indicated due to trapped pressures. If the static system remains clear, airspeed will change with altitude. An apparent increase above the level where the pitot tube and drain hole became clogged. This pressure differential causes the airspeed indicator to show an increase in speed. A decrease of indicated airspeed would occur as the airplane descends below the altitude at which the pitot system became obstructed. Static Pressure Chamber & Lines – The static chamber is vented through small holes to the free undisturbed air, and as the atmospheric pressure increases or decreases, the pressure in the static chamber changes accordingly. This pressure change is transmitted through lines to the instruments which utilize static pressure. An alternate source for static pressure is provided in some airplanes in the event of a static port clog. This source usually is vented to the pressure inside the cockpit. Because of the Venturi effect of the flow of air over the cockpit or tiny openings in the cabin. This alternate static pressure is usually lower than the pressure provided by the normal static air source. When the alternate static source is used, the following differences in the instrument indications usually occurs: Slightly higher than normal airspeed (5 to 10 knots) & altimeter (10 to 50 feet) with the VSI displaying a momentary climb immediately after the alternate static source is opened. During preflight for IFR, open the valve and observe a slight jump in the 3 pressure instruments. Not all exhibit a jump.

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Shade your altitudes on the side of safety and use higher than normal approach speeds, especially if your static system has been disabled by ice. If you are carrying structural ice, your stall speed will be higher than usual. Some owner’s manuals say to close vents and windows to minimize the pressure difference between inside and outside the cockpit. Airplanes with static ports on both sides of the fuselage reduce the risk of blocked system by half. It includes tubes to transmit ambient pressure from the ports to the instruments and, usually, an alternate static source for use in the unlikely but dangerous event that the static ports become blocked. Blocked Static System - If the static system becomes blocked but the pitot tube remains clear, the airspeed indicator will continue to operate, however it is inaccurate. Airspeed indications will be slower than the actual speed when the airplane is operated above the altitude where the static ports became clogged because the trapped static pressure is higher than normal for that altitude. Conversely, when you operate at a lower altitude, a faster than actual airspeed will be displayed due to the relatively low static pressure trapped in the system. A blockage of the static system also affects the altimeter and VSI. Trapped static pressure will cause the altimeter to freeze at the altitude at which the blockage occurred. If the VSI, a blocked static system will produce a continuous zero indication. In some airplanes, you can bypass a blocked static system by using an alternate static source. The alternate source is vented inside the cockpit where ambient air pressure is lower than outside static pressure. Minor pitot-static instrument errors may occur… such as slightly higher than normal airspeed and altimeter indications. The VSI may display a momentary climb immediately after the alternate static source is opened.

Pressure (Pitot-Static) Instruments:

A. Airspeed Indicator – displays the speed of your aircraft by comparing ram air pressure with static air pressure—the faster the aircraft moves through the air, the greater the pressure differential measured by this instrument. It measures the dynamic pressure of the airstream rushing against the moving airplane. The pitot tube receives dynamic pressure and static pressure, so the pressure in the pitot system is the sum of the two. The Airspeed Indicator can be thought of as a device that mechanically subtracts static system pressure from pitot pressure, leaving a measure of dynamic pressure only. A mechanical linkage moves the needle on the dial and displays the dynamic pressure sensed as the Indicated Airspeed.

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Ram air pushes against a diaphragm inside the airspeed indicator; the airtight case is vented to the static ports. A mechanical linkage translates the expansion and contraction of the diaphragm into needle movement. Indicated Airspeed (IAS) - Manufacturers use this for the basis for determining aircraft performance. V speeds listed in the POH are indicated airspeeds and do not normally vary with altitude or temperature. This is because changes in air density affect the aerodynamics of the airframe and the airspeed indicator equally. At high angles of attack, the relative wind does not strike the pitot tube straight on. This result in lower-than-normal indicated airspeed. Calibrated Airspeed (CAS) – is indicated airspeed corrected for installation and instrument errors. At high angles of attack, the pitot tube does not point straight into the relative wind. This tends to make the airspeed indicator indicate lower-than-normal at low airspeeds. The difference between indicated and calibrated airspeed is minimal at cruise speeds. You can find the corrections in the POH. You normally use specific indicated airspeeds for various operations, and only concern yourself with CAS when you need to convert to True Airspeed. Equivalent Airspeed (EAS) – is Calibrated Airspeed corrected for adiabatic compressible flow at a particular altitude. At airspeed above 200 KIAS and altitudes above 20,000 ft, air is compressed in front of an aircraft as it passes through the air. Compressibility causes abnormally high airspeed indications, so EAS is lower than CAS. This is significant to pilots of high-speed aircraft, but relatively unimportant to the average light airplane pilot. True Airspeed (TAS) - is the actual speed your airplane move through the undisturbed air. At sea level on a standard day, CAS (or EAS as appropriate) equals TAS. As density altitude increases, true airspeed increases for a given CAS, or for a given amount of power. You can calculate TAS from CAS (or EAS), pressure altitude and temperature using your flight computer. Close to standard temperature, you can get an approximate true airspeed by adding 2% of the indicated airspeed for each 1,000 foot increase in altitude. Ex: To calculate your TAS if you are indicating 150 knots at 5000 ft. 2%

of 150 is 3 knots, so add 3 knots to your indicated airspeed for each 1000 feet of altitude. At 5000 feet this comes to 15 knots, so an estimate of TAS is 165.

At higher altitudes and temperatures, the air pressure is reduced. Since the indicated airspeed reflects dynamic pressure, it drops in relation to the true airspeed as the altitude or temperature increases. TAS should be entered on the flight plan, and you should report to ATC if it varies from the flight plan value by 5% or 10 knots, whichever is greater. The difference between indicated and true airspeed is important on a timed approach to a high altitude airport.

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Suppose the approach has a 10 mile final approach course at an altitude of 8000 feet. If your indicated airspeed is 100 knots, your true airspeed will be about 116, shortening the time to the missed approach point by almost a minute. If you based your groundspeed calculated on the indicated airspeed rather than true, you would be a mile and a half past the airport when the time expired, well beyond the protected obstacle clearance area. Mach – is the ratio of the aircraft’s true airspeed to the speed of sound. A speed of Mach 0.85 means the aircraft is flying at 85% of the speed of sound at that temperature. When computing true airspeed from a conventional airspeed indicator, you must factor in air density, which requires a correction for temperature and altitude. These corrections are unnecessary with a Mach indicator because the temperature determines the speed of sound. Mach is a more valid index to the speed of the aircraft. V Speeds: What is shown on the Airspeed Indicator

1. Vso Stall speed Landing Config. – The lower limit of the white arc corresponds to Vso which is the stalling speed or the minimum steady flight speed in the landing configuration. In small airplanes, this is the power-off stall speed at the maximum landing weight in the landing configurations (gear and flaps down).

2. Vs1 Stall speed Clean Config. – The lower limit of the green arc is Vs1 and is defined as the stalling speed or the minimum steady flight speed obtained in a specified configuration. For small airplanes, this is the power-off stall speed at the maximum takeoff weight in the clean configuration (gear up, if retractable, and flaps up). You should check the POH or specific information on your airplane.

3. White Arc – This arc is commonly referred to as the flap operating range, since its lower limit represents the full flap stall speed and its upper limit provides the maximum flaps speed. You will usually fly your approaches and landings at speeds within the white arc.

4. Vfe Max. Flags Extended Speed – The upper limit of the white arc is Vfe, which indicates the maximum speed with the flaps extended.

5. Green Arc – This is the normal operating range of the airplane. Most of your flying will occur within this range.

6. Vno Max Structural Cruising Speed – The upper limit of the green arc corresponds to Vno, which is the maximum structural cruising speed. You should not exceed it except in smooth air.

7. Yellow Arc – The yellow arc indicates the caution range. You may fly within this range only in smooth air, and then only with caution.

8. Vne Never Exceed Speed (Red Line) – Operating above this speed is prohibited since it may result in damage or structural failure.

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Not all V speeds are shown: Va Maneuvering Speed – The max. speed at which you may apply full and abrupt control movement without the possibility of causing structural damage. It also represents the max. speed that you can safely use during turbulent flight conditions. It is listed in the POH. Va changes when weight changes. Increased weight lowers Va. Vle - Speed not to be exceeded when the gear is extended. Vlo – The max. speed at which you can raise or lower the landing gear.

B. Altimeter – is basically a barometer calibrated in feet instead of inches of mercury. The static system transmits atmospheric pressure from outside the airplane to the altimeter. Sealed chambers called Aneroid Cells inside the instrument expand and contract with pressure changes, and a mechanical linkage moves the hands in response. Inches of Mercury – A barometer measures atmospheric pressure by determining the height of a column of mercury required to produce the same pressure. It is the heaviest liquid known. A lighter liquid such as water would be too awkward. The fifty miles of air above us exerts about the same pressure at the surface of the earth as would a thirty inch column of mercury. The exact value varies with changes in the weather. At sea level, the average, or standard, barometric pressure is 29.92 in Hg. With an increase in altitude, since there is less air above you, the pressure is less and is therefore equivalent to fewer inches of mercury. Atmospheric pressure decreases approx. 1 inch of mercury for each 1000 feet increase in altitude. A Pressure Plane as a level of the atmosphere where the pressure has a certain specified value. Above a particular plane, the pressure is lower; below it, the pressure is higher. The altimeter setting at an airport is the local barometric pressure, adjusted to sea level, not the actual atmospheric pressure at the observation point. Before IFR flight, put the correct setting in the Kollsman window and compare the reading to the field elevation. Field Elevation is the highest point on any usable runway, and the elevations at other places may vary considerably. (Check touchdown zones!) The error should be less than 75 feet by FAA standards. An error that is too low is preferable. If you shoot an approach to minimums and level off when the altimeter shows your minimum descent altitude, you will actually be on the high side of the safe altitude. If the altimeter reads too high, you should add the amount of the error to the minimum altitude on the approach.

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C. Vertical Speed Indicator (VSI) – also called Vertical Velocity Indicator. Uses static pressure to display a rate of climb or descent in feet per minute. As the airplane climbs or descends the VSI determines the vertical speed by measuring how fast the ambient air pressure is increasing or decreasing. The VSI is built around a chamber or aneroid cell which expands and contracts in response to pressure changes. A system of gears and levers moves the needle on the face of the instrument. Both the diaphragm (chamber) and case receive ambient pressure from the static line at existing atmospheric pressure. The case of the instrument is airtight except for a small connection through a restricted passage to the static line of the pitot-static system. Unlike the altimeter, the VSI case also receives static pressure from the static system through a tiny opening often called a Calibrated Leak. When the aircraft is on the ground or in level flight, the pressure outside and within the VSI diaphragm (chamber) are equal and a properly working instrument will read zero. At the start of a climb, the pressure in the VSI diaphragm begins to drop. The pressure outside the chamber and inside the case does not drop as rapidly, because of the small size of calibrated leak. A pressure differential is created, and the chamber contracts, moving the hand on the face of the VSI indicate a climb. The pressure differential exists as long as the climb continues, because the air can never escape through the calibrated leak as fast as through the larger opening to the chamber. At the completion of the climb, the pressure within the chamber stops changing immediately, but it takes several seconds to equalize the pressure in the casing through the calibrated leak, which accounts for the lag in Rate info given by the VSI. VSI provides accurate rate info only in smooth air during level flight or in a sustained constant rate climb or descent. For this reason, the VSI is a backup instrument. The VSI displays 2 different types of information: The first indication of a climb or a descent because the pressure differential occurs as soon as the airplane begins an altitude change. After a short period of time, the VSI will stabilize and display the new rate of climb, which, would be something less than 500 f.p.m.

1. Trend Information – shows you an immediate indication of an increase or decrease in the airplane’s rate of climb or descent.

2. Rate information – shows you a stabilized rate of change. During preflight check, observe the VSI reading on the ramp. After tapping it gently, use whatever it shows as zero rate indication throughout flight. The small adjusting screw on the lower left side of most VSI’s can be used to move the needle to zero on the ground.

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4. Magnetic Compass – is a self-contained unit which does not require

electrical or suction power. To determine direction, the compass uses a simple bar magnet with two poles. The bar magnet in the compass is mounted so the compass can pivot freely and align itself automatically with the earth’s magnetic field. The DG is needed because the magnetic compass bounces around in turbulence and during pitch changes, and also because it exhibits dip errors during speed changes and turns. Because of this, the compass is accurate only when your airplane is in smooth air and in straight-and-level unaccerated flight. Dip errors occur because the compass tries to align itself vertically as well as horizontally with the earth’s magnetic lines of force, which slant downward towards the poles. The geographic north and south poles form the axis for the earth’s rotation. These positions are also referred to as True North and South. Another axis is formed by the magnetic north and south poles. Lines of magnetic force flow out from each pole in all directions, and eventually return to the opposite pole. A freely mounted bar magnet will align itself with the magnetic axis formed by the north/south magnetic field of the earth. The only direction seeking instrument and is a reliable source of heading information. The compass doesn’t work on gyroscopic principles, but you will use it frequently to help correct for gyroscopic precession in the heading indicator. The angular difference between the true and magnetic poles at a given point is referred to as Variation. Most aviation charts are oriented to True North and the aircraft compass is oriented to Magnetic North, you must convert a true direction to a Magnetic Direction by correcting for the variation. The amount of variation you need to apply is dependent upon your location on the earth’s surface. Variation at this point in the Western US is 17 degrees. Since the Magnetic North pole is located to the East of the True North pole in relation to this point, the variation is Easterly. When the Magnetic pole falls to the West of the True pole, variation is westerly. Isongonic lines connect points where the variation is equal, while the agonic line defines the points where the variation is zero. Deviation refers to a compass error which occur due to disturbances from magnetic fields produced by metals and electrical accessories within the airplane itself. It cannot completely eliminated, deviation error can be decreased by manufacturer installed compensating magnets located within the compass housing. The remaining error is recorded on a chart called Compass Correction Card.

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3 Compass errors: 1. Magnetic Dip – When the bar magnet contained in the compass is pulled

by the earth’s magnetic field, it tends to point north and somewhat downward. The downward pull, magnetic dip, is greatest near the poles and diminishes as you approach the equator. Although the compass is not subject to magnetic dip near the equator, as the compass moves closer to the poles errors resulting from magnetic dip increases gradually. Within approx. 300 miles of either magnetic pole, these errors are so great that use of the compass for navigation is impractical. In order to minimize the tilting force on the bar magnet caused by magnetic dip, a weight is placed on the side nearest the equator. For aircraft that fly in the Northern Hemisphere, the weight is placed on the south end of the bar magnet. Unfortunately, the corrective weight, as well as magnetic dip itself, both contribute to acceleration and turning errors.

2. Acceleration Error – If you accelerate or decelerate an airplane on an Easterly or westerly heading, an erroneous indication will occur. As you accelerate an airplane, inertia causes the compass weight on the south end of the bar magnet to lag slightly and turn the compass toward the north. During deceleration, inertia causes the weight to move slightly ahead, which moves the compass toward a southerly heading even though no change of direction has taken place. The compass will return to its previous, and proper, heading once the acceleration or deceleration subsides. Acceleration error is more pronounced as you move closer to due east or west. The error doesn’t occur when you are flying on a directly north or south heading because the bar magnet weight is in line with the direction of travel. These errors are valid in the Northern Hemisphere only. The effects are reverse in the Southern Hemisphere. ANDS (Accelerate North and Decelerate South).

3. Turning Error – is directly related to magnetic dip; the greater the dip, the greater the turning error. It is most pronounced when you are turning to or from headings of north or south. When you begin a turn from a heading of north, the compass initially indicates a turn in the opposite direction. When the turn is established, the compass begins to turn in the correct direction, but it lags behind the actual heading. The amount of lag decreases as the turn continues, then disappears as the airplane reaches a heading of east of west. The compass lags about 30 degree on a north heading in a standard rate turn at mid-latitudes; it leads by about the same amount when heading south during a turn. EX: When a right turn is begun from a heading of 360 degrees, the compass will initially swing 30 degrees in the opposite direction, then begin following the turn, and will finally catch up as the heading reaches 90 degrees. When a turn to the right from a heading of 180 degrees is begun, the compass will swing 30 degrees almost immediately, showing a heading of 210 degrees when the nose of the airplane has barely started to move. The

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compass will turn more slowly than the airplane, so that it reads approx. correctly as the heading passes through west. The South Leads and the North Lags. When turning from a heading of east or west to a heading of north, there is no error as you begin the turn. As the heading approaches north, the compass increasingly lags behind the airplane’s actual heading. When you turn from a heading of south, the compass initially indicates a turn in the proper direction but leads the airplane’s actual heading. This error also cancels out the airplane reaches a heading of east or west. Turning from east or west to a heading of south causes the compass to move correctly at the start of a turn., but then it increasingly leads the actual heading as the airplane nears a southerly direction. The amount of lead or lag is approx. equal to the latitude of the airplane. EX: If you are turning from a heading of south to a heading of west while flying at 40 north latitude, the compass will rapidly turn to a heading of 220 degrees. (180 + 40). At the midpoint of the turn, the lead will decrease to approx. half (20 degrees), and upon reaching a heading of west, it will be zero. As in acceration errors, these lead and lag errors are only valid for flight in the Northern Hemisphere. Lead and lag errors in the Southern Hemisphere act in the opposite directions.

5. Power/Engine Instruments – The engine instruments are part of the pilot’s scan. The instruments that indicate power output are used in controlling the airplane, and the others are important to monitor the equipment’s health. Tachometer in a fixed pitched propeller airplane – measures the speed of the engine rotation. As engine RPM increases, the needle moves clockwise on the face of the tachometer. The reading can be interpreted as a % of sea level brake horsepower by referring to the manufacturer’s power setting chart. The small numbers in the center of the tachometer give the accumulated hours of engine operation. They consistently run slower than the airplane’s “Hobbs” type hour meter. The tachometer gauge counts engine revolutions, registering an “hour” after a certain number of revolutions, regardless of elapsed time. “Tach Time” corresponds to clock time at a certain speed, usually near cruise setting. At slower speeds, “Tach Time” moves more slowly. EX: During instrument approach practice or touch and goes, the tach may show only .6 or .7 after an hour of flying. Manifold pressure in the controllable propeller airplane - is a barometer that records the pressure inside the engine intake manifold. With the engine off, the gauge shows atmospheric pressure, about 30” Hg. at sea level. Where a manifold pressure gauge is used to set engine power, the tachometer registers propeller RPM as set by the prop control.

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When the engine is running, the fuel-air mixture is sucked from the intake manifold into the cylinders. Since the flow of air into the manifold is restricted by the throttle valve in the carburetor, the pressure is lower inside the manifold than outside. The more the throttle is opened, the less restriction exists at the carburetor and the more nearly the manifold pressure approaches the outside pressure. Aspirated (non-turbocharged) engine - the manifold pressure will be slightly lower than ambient pressure at full throttle. At a constant throttle setting, the manifold pressure decreases with altitude about one inch for each thousand feet, just like atmospheric pressure. At higher altitudes, it is impossible to maintain the power settings that are possible near sea level. Sea-level performance can be obtained even at high altitudes using either a supercharging or a turbo charging system. A Supercharger compresses the incoming air using a pump driven by the engine so the manifold pressure may exceed atmospheric pressure. Some engine power must be used to drive the supercharger which decreases the net power increase. A Turbocharger is more efficient because it pressurizes the air using a mechanism driven by engine exhaust gases, which would otherwise be vented overboard. As the altitude increases, the maximum possible manifold pressure decreases. Above the Critical altitude- full throttle (even with a supercharger) will not provide enough manifold pressure to produce the rated horsepower. Both are usually fuel injected. Where a manifold pressure gauge is used to set engine power, the tachometer registers propeller RPM as set by the prop control. The Prop Control sets a governor which controls the propeller pitch, or angle of attack into the airstream. This determines the work each revolution must do and hence the speed at which the engine turns. The ideal propeller setting is in a climb is different from that in level flight and we usually follow the manufacturer’s recommendations in adjusting the propeller settings. Since some warning of engine problems is often given by the temperature and pressure gauges, they should be included in the instrument scan. Oil Pressure Gauge provides a direct indication of the oil system operation. A below-normal pressure may mean an engine leak, lubrication problem, or the oil pump is not putting out enough pressure to circulate oil throughout the engine, while an above-normal pressure may indicate a clogged oil line. Oil Temperature Gauge measures the temperature of oil as it enters the engine. Located next to the oil pressure gauge. The green area is the normal range, red line is the max. temperature. Unlike oil pressure, changes in oil temp. occur more slowly. Check gauge periodically particularly important when using high power settings because this causes increased oil temps. The temperature increases when there is less oil circulating in the engine to absorb the available heat or when bearing failures or similar troubles increase frictional heat.

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Airplanes equipped with cowl flaps may have a Cylinder Head Temperature Gauge that provides a direct temperature reading from one of the cylinders. By monitoring the cylinder head temperature, you can regulate the flow of cooling air by adjusting the position of the cowl flaps using a control in the cockpit. This gauge may be the first sign of an overheating engine do to the warm cylinder head. A high cylinder head temp. in a climb indicates too much load on the engine or insufficient cooling airflow and can be remedied by opening the cowl flaps, increasing speed, using a reduced power setting, or a richer mixture, all of which can improve cooling. An Exhaust Gas Temperature Gauge provides information for mixture control. Cylinder head temperature does react to mixture settings, monitoring it is not the best way of leaning the mixture. The exhaust gas gauge is. The Ammeter is used to monitor the electrical current in amperes within the system. The ammeter almost always heralds electrical failure. Vital to keep in the scan. There are two types:

1. Center Zero – shows whether electricity is flowing into or out of the battery. Positive Reading – indicates that the alternator is charging the battery. Zero Reading - indicates the battery is fully charged and that the alternator is running all electrical equipment currently in use. Negative Reading – means that the load on the electric system is causing a net drain on the battery, which could indicate a complete or partial alternator failure. The electrical load should be reduced until the ammeter shows a positive reading. If this can’t be done, reduce the electrical load to an absolute minimum and terminate flight as soon as possible.

2. Left Zero (Load Meter) – shows the amount of electricity being delivered to the system by the alternator or generator. A healthy reading with a positive load is quite easy to see on the dial. In the event of an alternator failure, the ammeter reading drops for no apparent reason.

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