Private Pilot | Lesson 2 - Aircraft Instruments
TABLE OF CONTENTS:
2.1 THE GYROSCOPIC INSTRUMENTS
2.2 THE PITOT-STATIC SYSTEM
2.3 THE AIRSPEED INDICATOR
2.4 THE ALTIMETER
2.5 THE TYPES OF ALTITUDE
2.6 SETTING THE ALTIMETER
2.7 ALTIMETER ERRORS
2.8 COMPASS TURNING ERRORS
2.1 The Gyroscopic Instruments
There are three main gyroscopic instruments in airplanes:
- The Attitude Indicator,
- The Turn Coordinator, and
- The Heading Indicator.
1. The Attitude Indicator
The attitude indicator, with its miniature airplane and artificial horizon bar, displays a picture of the attitude of airplane with respect to the horizon. The relationship of the miniature aircraft to the horizon bar is the same as the relationship of the actual aircraft to the actual horizon.
The relationship of the miniature airplane to the horizon bar should be used as an indication of pitch and bank attitude. The miniature airplane should convey the sense of whether the airplane is level, nose high, nose low, in a left bank, in a right bank, etc.
The gyro in the attitude indicator is mounted on a horizontal plane and depends upon rigidity in space for its operation.
An adjustment knob is provided with which the pilot may move the miniature airplane up or down to align the miniature airplane with the horizon bar to suit the pilot's line of vision. The proper adjustment to make on the attitude indicator during level flight is to align the miniature airplane to the horizon bar.
2. The Turn Coordinator
The next of the gyroscopic instruments is the turn coordinator.
The turn coordinator provides an indication of the movement of the aircraft about the roll and yaw axes.
It displays a miniature airplane which moves proportionally to the roll rate of the airplane. During a turn, when the bank is held constant, the turn coordinator indicates the rate of turn. The ball indicates whether the angle of bank is coordinated with the rate of turn.
3. The Heading Indicator
The heading indicator (also called the directional gyro) is used as an aid to the magnetic compass in indicating the direction or heading the aircraft is currently flying. The heading indicator is a gyro instrument, so it also depends on the principle of rigidity in space for its operation. Additionally, due to gyroscopic precession, it must be periodically realigned with a magnetic compass.
Realigning the heading indicator with the magnetic compass should only be accomplished during straight-and-level, unaccelerated flight to yield the most accurate reading from the magnetic compass.
Ascent Quick Quiz - 2.1 The Gyroscopic InstrumentsQuestion 1: (Refer to figure 7.) The proper adjustment to make on the attitude indicator during level flight is to align the
Question 2: (Refer to figure 7.) How should a pilot determine the direction of bank from an attitude indicator such as the one illustrated?
Question 3: (Refer to figure 5.) A turn coordinator provides an indication of the
Question 4: (Refer to figure 6.) To receive accurate indications during flight from a heading indicator, the instrument must be
2.2 The Pitot-Static System
There are two major parts of the pitot-static system:
- The impact pressure lines, and
- The static (ambient) pressure lines.
The pitot-static system is a source of impact and ambient pressure for the altimeter, the vertical-speed indicator, and the airspeed indicator.
The pitot tube provides impact (or ram) pressure for the airspeed indicator only.
When the pitot tube and the outside static vents or just the static vents are clogged, all three instruments (altimeter, vertical-speed indicator, and airspeed indicator) will provide inaccurate readings.
If only the pitot tube is clogged, only the airspeed indicator will be inoperative.
Ascent Quick Quiz - 2.2 The Pitot-Static SystemQuestion 1: The pitot system provides impact pressure for which instrument?
Question 2: Which instrument will become inoperative if the pitot tube becomes clogged?
Question 3: If the pitot tube and outside static vents become clogged, which instruments would be affected?
Question 4: Which instrument(s) will become inoperative if the static vents become clogged?
2.3 The Airspeed Indicator
Airspeed indicators have a standard color-coded marking system.
The white arc is the full flap operating range.
- The lower limit of the white arc is the power-off stalling speed (also called VS0) with the flaps and landing gear in their landing positions (that is, flaps fully extended and landing gear down and locked.)
- The upper limit of the white arc is the maximum full flaps-extended speed (VFE). This is the maximum speed that the flaps should be deployed at; any higher an airspeed may place to great a force on the flaps and may result in structure damage.
The green arc is the normal operating range.
- The lower limit of the green arc is the power-off stalling speed in a specified configuration (also called VS1). This "specified configuration" normally consists of, flaps up and landing gear retracted.
- The upper limit of the green arc is the maximum structural cruising speed (VNO) for normal operation.
The yellow arc is the caution range airspeed.
- Flight at airspeeds within this range of airspeeds should only be accomplished in very smooth air.
The red radial line indicates the aircraft airspeed that should never be exceeded (VNE).
- The red radial line is the maximum speed at which the airplane may be operated under any circumstances.
One important airspeed limitation that is not color-coded on the airspeed indicator is the Maneuvering Speed (also called VA).
Maneuvering speed is the the maximum airspeed for flying in "rough" or turbulent air, it is also the maximum speed for executing abrupt maneuvers.
The aircraft's design maneuvering speed is the maximum speed at which full and abrupt deflection of aircraft controls can be made without causing structural damage. This is an important speed to keep in mind when practicing stalls or other maneuvers where the potential need for rapid deflection of the aircraft controls could be made.
When turbulence or "rough" air is encountered, the airplane's airspeed should be reduced to at least maneuvering speed (VA), if not slightly below maneuvering speed. This will ensure that the loads placed on the aircraft due to the turbulence will never exceed the structural load limits of the aircraft - the aircraft may get jostled around a bit, but it will hold together.
Upon encountering severe turbulence, you should attempt to maintain a level flight attitude, and accept variations in altitude and airspeed. Attempting to maintain constant altitude and airspeed may prove to be impossible and could result in abrupt control inputs, and additional control pressure, which add stress to the aircraft's airframe.
Ascent Quick Quiz - 2.3 The Airspeed IndicatorQuestion 1: What does the red line on an airspeed indicator represent?
Question 2: What is an important airspeed limitation that is not color coded on airspeed indicators?
Question 3: (Refer to figure 4.) What is the caution range of the airplane?
Question 4: (Refer to figure 4.) The maximum speed at which the airplane can be operated in smooth air is
Question 5: (Refer to figure 4.) What is the full flap operating range for the airplane?
Question 6: (Refer to figure 4.) Which color identifies the never-exceed speed?
Question 7: (Refer to figure 4.) Which color identifies the power-off stalling speed in a specified configuration?
Question 8: (Refer to figure 4.) What is the maximum flaps-extended speed?
Question 9: (Refer to figure 4.) Which color identifies the normal flap operating range?
Question 10: (Refer to figure 4.) Which color identifies the power-off stalling speed with wing flaps and landing gear in the landing configuration?
Question 11: (Refer to figure 4.) What is the maximum structural cruising speed?
Question 12: Upon encountering severe turbulence, which flight condition should the pilot attempt to maintain?
2.4 The Altimeter
Altimeters usually have three needles or "hands".
Altimeter dials are numbered 0-9.
- The shortest needle is the 10,000 ft interval needle.
- The medium needle is the 1,000 ft interval needle.
- The longest needle is the 100 ft interval needle.
To read an altimeter:
First determine whether the short needle points between 0 and 1 (1-10,000 ft), 1-2 (10,000-20,000 ft), etc.
Second, determine whether the medium needle is between 0 and 1 (0-1,000 ft), 1 and 2 (1,000-2,000 ft), etc.
Third, determine which number the long needle is pointing. So, 1 for 100 ft., 2 for 200 ft., etc.
With practice, determining altitude from the altimeter will probably only require a glance.
Ascent Quick Quiz - 2.4 The AltimeterQuestion 1: (Refer to figure 3.) Altimeter 2 indicates
Question 2: (Refer to figure 3.) Altimeter 1 indicates
Question 3: (Refer to figure 3.) Altimeter 3 indicates
Question 4: (Refer to figure 3.) Which altimeter(s) indicate(s) more than 10,000 feet?
2.5 The Types of Altitude
In aviation there are many different types of altitude.
Absolute altitude is the altitude above the surface, also know as, AGL. Above Ground Level.
True altitude is the actual distance above mean sea level, or, MSL. True altitude is not susceptible to variation with atmospheric conditions.
Pressure altitude is the height above the standard datum plane of 29.92 inches of mercury (also written 29.92" Hg).
Because pressure altitude is based on this standard datum plane, you can always determine pressure altitude by simply adjusting the altimeter setting in the Kolsman window of the altimeter to 29.92. The altitude that is then indicated is pressure altitude.
Conversely, you can obtain an approximate altimeter setting (a setting similar to that which you would receive from ATC) by adjusting the altimeter setting while on the ground so that indicated altitude equals the published airport elevation (MSL).
Density altitude is pressure altitude corrected for nonstandard temperature. (Standard Temperature is defined as 15°C at sea level.) Thus, pressure altitude and density altitude are the same at standard temperature.
Indicated altitude is the same as true altitude when standard conditions exist and the altimeter is calibrated properly.
Pressure altitude and true altitude are the same when standard atmospheric conditions (29.92" Hg and 15°C at sea level) exist.
Ascent Quick Quiz - 2.5 The Types of AltitudeQuestion 1: What is absolute altitude?
Question 2: What is true altitude?
Question 3: What is density altitude?
Question 4: Under what condition is pressure altitude and density altitude the same value?
Question 5: Under what condition is indicated altitude the same as true altitude?
Question 6: Under which condition will pressure altitude be equal to true altitude?
Question 7: What is pressure altitude?
Question 8: Altimeter setting is the value to which the barometric pressure scale of the altimeter is set so the altimeter indicates
2.6 Setting the Altimeter
When adjusting the pressure setting on the altimeter's Kolsman window, the indicated altitude will increases when you change the altimeter setting to a higher pressure and decreases when you change the setting to a lower pressure.
This is actually opposite to the altimeter's reaction due to changes in air pressure. (Usually, as you ascend in the aircraft, the air pressure lowers and the altimeter indicates a higher altitude.)
The indicated altitude will change at a rate of approximately 1,000 ft for each inch of pressure change in the altimeter setting.
When changing the altimeter setting from 29.25 to 29.95, there is a 0.70 in. change in pressure (29.95 - 29.25 = 0.70).
The indicated altitude will increase (due to the higher altimeter setting) by 700 ft. (0.70 x 1,000 = 700).
Ascent Quick Quiz - 2.6 Setting the AltimeterQuestion 1: If it is necessary to set the altimeter from 29.15 to 29.85, what change occurs?
Question 2: If a pilot changes the altimeter setting from 30.11 to 29.96, what is the approximate change in indication?
2.7 Altimeter Errors
Since altimeters can be adjusted for changes in barometric pressure but not for temperature changes, should an airplane fly from an area of warmer than standard temperature to an area of colder than standard temperature, all while maintaining a constant indicated altitude, the airplane's altimeter will indicate lower than actual altitude.
On warm days, the altimeter indicates lower than actual altitude.
Likewise, when pressure lowers en route at a constant indicated altitude, your altimeter will indicate higher than actual altitude until you adjust it.
When flying from high to low (temperature or pressure), look out below.
Low to high (temperature or pressure), clear the sky.
Ascent Quick Quiz - 2.7 Altimeter ErrorsQuestion 1: If a flight is made from an area of low pressure into an area of high pressure without the altimeter setting being adjusted, the altimeter will indicate
Question 2: If a flight is made from an area of high pressure into an area of lower pressure without the altimeter setting being adjusted, the altimeter will indicate
Question 3: Which condition would cause the altimeter to indicate a lower altitude than true altitude?
Question 4: Under what condition will true altitude be lower than indicated altitude?
Question 5: How do variations in temperature affect the altimeter?
2.8 Compass Turning Errors
During flight, magnetic compasses can be considered accurate only during straight-and-level flight at constant airspeed.
The difference between direction indicated by a magnetic compass not installed in an airplane and one installed in an airplane is called deviation - Magnetic fields produced by metals and electrical accessories in an airplane disturb the compass needles.
Compass Acceleration/Deceleration Errors
In the Northern Hemisphere, acceleration/deceleration error occurs when on an east or west heading.
A magnetic compass will indicate a turn toward the north during acceleration when on an east or west heading.
A magnetic compass will indicate a turn toward the south during deceleration when on an east or west heading.
Remember: "ANDS" - Accelerate North, Decelerate South.
Acceleration/deceleration errors do not occur when on a north or south heading.
Compass Turning Errors
In the Northern Hemisphere, compass turning error occurs when turning from a north or south heading. These compass turning errors are caused by a phenomenon known as "magnetic dip."
A magnetic compass will "dip" and tend to lag when turning from a north heading. In fact, at the start of a turn, the compass may even initially indicate a turn in the opposite direction! So:
- If turning to the east (right), the compass will initially indicate a turn to the west and then lag behind the actual heading until your airplane is headed east (at which point there is no error).
- If turning to the west (left), the compass will initially indicate a turn to the east and then lag behind the actual heading until your airplane is headed west (at which point there is no error).
A magnetic compass will lead or precede the turn when turning from a south heading.
Turning errors do not occur when turning from an east or west heading.
These errors diminish as the acceleration/deceleration or turns are completed.
Ascent Quick Quiz - 2.8 Compass Turning ErrorsQuestion 1: In the Northern Hemisphere, a magnetic compass will normally indicate a turn toward the north if
Question 2: During flight, when are the indications of a magnetic compass accurate?
Question 3: Deviation in a magnetic compass is caused by the
Question 4: In the Northern Hemisphere, if an aircraft is accelerated or decelerated, the magnetic compass will normally indicate
Question 5: In the Northern Hemisphere, a magnetic compass will normally indicate initially a turn toward the west if
Question 6: In the Northern Hemisphere, the magnetic compass will normally indicate a turn toward the south when
Question 7: In the Northern Hemisphere, a magnetic compass will normally indicate initially a turn toward the east if