Six Pack (Flight Instruments) (Private Pilot)
The “Six Pack” or the six main basic flight instruments of an aircraft allow the pilot to operate the aircraft at its maximum performance and in a safe manner. All pilots need to become familiar and understand the basic operations with the instruments, systems and the errors associated with those instruments, such as; the pitot-static system and instruments associated, and the gyroscopic instruments along with the compass.
Pitot-Static Instruments & System
Pitot-Static flight instruments are a combination of static air pressure and dynamic pressure when the aircraft is in motion. When these two pressures are working together they operate three flight instruments:
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Altimeter
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Airspeed Indicator (ASI)
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Vertical Speed Indicator (VSI)
Altimeter:
The altimeter simply measures the height or altitude of the aircraft above mean sea level (MSL), which can be corrected for atmospheric pressure differences. Inside of the altimeter are stacked and sealed aneroid wafers that a set to an internal pressure of 29.92 inches of mercury. These wafers are designed to collapse and expand. A high static pressure collapses the wafers showing a decrease in altitude, where as a low static pressure allows the wafers to expand showing an increase in altitude, allowing the needles on the face of the instrument to rotate.
The short needle represents thousands of feet in 1,000 feet increments. The longer needle represents hundreds of feet.
Prior to each flight the pilot should set the altimeter to the proper barometric pressure report at the airport transmitted from either the air traffic control tower, Flight service station (FSS), ATIS, AWOS, or the ASOS.
To set the altimeter to the proper barometric pressure, simple turn the knob at the bottom of the instrument to the proper barometric pressure located in the Kollsman window.
Errors
When the altimeter is set it should read within 75 feet from the surveyed field elevation, if not it needs to be recalibrated by a certified instrument mechanic.
Airspeed Indicator (ASI):
The Airspeed Indicator (ASI) simple measures the speed of the aircraft. As the aircrafts speed increases or decreases the indication will change, similar to the speedometer in your car. The ASI is designed to move air from the pitot tube into the diaphragm, while static pressure is introduced into the airspeed case.
There are different types of airspeed associated with the airspeed indicator.
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Indicated airspeed (IAS) - The airspeed of the aircraft that is directly read by the pilot from the ASI.
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Calibrated airspeed (CAS) - Is indicated airspeed (IAS) corrected for any instrument errors.
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True airspeed (TAS) - Is when Calibrated airspeed is corrected for altitude and nonstandard temperature. This is due to the air density decreasing at higher altitudes. To maintain the same pressure differences between the pitot pressure and static pressure, aircraft needs to fly faster at higher altitudes.
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Ground speed (GS) - Is the actual speed of an aircraft over the ground. It is True airspeed adjusted for wind corrections.
The ASI should be checked prior to takeoff, and read zero. Be aware, the ASI may show an indication higher than zero, if there is a strong wind blowing directly into the pitot tube. On the takeoff roll visually check to see the airspeed is increasing.
Errors
If the pitot system and or the static system becomes blocked by either moister, ice, dirt, bugs, etc. it will affect the airspeed indicator as well as the altimeter and the vertical speed indicator. Some aircraft are equipped with an alternate static source in the flight deck area. By opening the alternate static source, static pressure is drown from the flight deck in to the system.
Vertical Speed Indicator (VSI):
Vertical Speed Indicator, indicates weather the aircraft is climbing, descending or in level flight. The VSI indicates in feet per minute (fpm), and should read zero in level flight if proper calibration. The vertical speed indicator measures static pressure inside the diaphragm. When the aircraft is on the ground or in level flight the pressure inside of the diaphragm are equal. When the aircraft is in a climb or decent the pressure change is immediate and the needle will indicate.
But, if the pitch angle is held constant, the needles will stabilize after a few seconds (approximately 6-9 seconds) and indicate the rate of climb in hundreds of feet.
Errors
During the preflight, visually check to see if the VSI is reading the proper indication on the ground, near zero. During takeoff the VSI should indicate a positive rate climb.
Pitot System:
The pitot tube measures dynamic pressure when the aircraft is in motion, it doesn’t matter if the aircraft is moving in still air at 70 knots or the aircraft is facing into a 70 knot wind.
The pitot tube is designed with a large hole in the front of it which allows total pressure or ram aim to enter the chamber. At the back end of the pitot tube is a small drain hole that allows any moisture to drain out.
During the preflight inspection, pilots must visually check both of the holes in the pitot tube to ensure they are clear of foreign object debris (FOD) and bugs and are not blocked.
The only flight instrument that uses the pitot tube system is the airspeed indicator (ASI).
Static System:
The Static system and pitot system work together. If a static port becomes blocked while the aircraft is in a climb, the airspeed indicator will show a decrease in airspeed. A blocked static port will also affect the altimeter and vertical speed indicator (VSI).
During the preflight inspection, pilots must visually check the holes in the static port to ensure they are clear of foreign object debris (FOD) and bugs and are not blocked.
TIP:
If either the pitot tube or static port appears to be blocked or something is lodged inside i.e. bug, never try to clear it yourself, have a certified mechanic check it.
Gyroscopic Instruments & Systems
Gyroscopic (Gyros) instruments are either operated by a vacuum system, pressure powered or from the aircraft electrical power. There are three instruments, they are:
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Turn Coordinator
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Attitude Indicator
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Heading indicator
Turn Coordinator:
The Turn coordinator is used to maintain a standard rate turn during flight, this is done by aligning the miniature airplane wings to the index marks when rolling the aircraft. The rudder is used to yaw the aircraft, which moves the ball left and right. Apply enough rudder pressure to maintain the ball in the center between the two lines.
The Turn coordinator is electrically powered by direct current (DC).
Instrument check
During the preflight visually check that the ball is centered or resting at the lowest point and there is fluid inside of inclinometer. During a turn while taxing the turn coordinator should indicate a turn in the direction of the tune being made, and the ball moves in the opposite direction.
Attitude Indicator:
The attitude indicator, sometimes referred to as the artificial horizon, which allows a pilot to determine a small changes in the aircrafts attitude such as, a climb, descent, and roll. The attitude indicator is designed with a gyro, which is mounted inside on a horizontal plane and operates depending on the rigidity of space.
The rigidity of space refers to, “the principle that a gyroscope
remains in a fixed position in the plane in which it is spinning”.
The indicator has a miniature airplane in level flight that
corresponds with the aircrafts change in relation to the horizon,
which is adjustable up and down to align with the horizon
bar, for better line of vision. The indicator also shows bank,
in the increments of 10°, 20°, 30°, 45°, 60°, and 90 degrees.
Instrument check
Gyros take a small amount of time to spool up, make sure there is no abnormal sounds being made. During a turn while taxing all gyros should indicate a turn in the direction of the tune being made. Be aware that at idle power the gyros using the vacuum system may not be at operating speeds.
Heading Indicator:
The heading indicator is similar to a magnetic compass to indicate the direction the aircraft is flying. It is designed as a mechanical instrument with a rotating gyro. A small aircraft is centered on the face of instrument, while the rotor turns in a vertical plane when the aircraft is in a turn.
There are numerous errors with heading indicator, which makes precision turns to heading and straight flight difficult. Another error of the indicator, is that it may indicate a heading error as much as 15° every hour. This is due to the gyro being oriented in space, and the Earth rotates in space at a rate of 15° an hour.
Instrument check
During a turn while taxing the heading indicator should indicate a turn in the direction of the tune being made.
Sources of Power:
As mention previously Gyros instruments are either operated by a vacuum system, pressure or from the aircraft electrical power. Some aircraft are built with two sources of vacuum or pressure systems in case one fails.
The vacuum system is one source of that operates the gyros, it is a vane type engine driven pump mounted to the accessory’s case of the aircraft engine. The vacuum spins the gyro by drawing air against the rotor vane, which spins the rotor at a high speed. The vacuum air also operates the gyros for the attitude and heading indicators.
Instrument checks
A gauge is mounted to the aircraft instrument panel, which indicates the amount of pressure. It is important that the pilot monitor the vacuum or suction gauge during flight. Due to the attitude indicator and heading indictor may not be a reliable source to indicate low suction pressure. The normal operating range is indicated on the gauge, which is between 4.5 and 5.5 inches of mercury.
Magnetic Compass
A magnetic compass may be old school and very simple, but is still a required instrument per Title 14 CFR part 91 for both visual flight rules (VFR) and instrument flight rules (IFR) of flight.
A magnet is a piece of metal containing iron that holds lines of magnetic flux, regardless of the size every compass has two poles, the North and South Pole. When two similar poles are placed together they repel from each other.
Compass Errors:
Variation:
Variation is the difference between magnetic north and true north. Magnetic north is what a magnetic points to, unlike true north which is a geographic location. Magnetic north has the ability to shift over time. The distance between magnetic and true north are about 1,300 miles apart.
VFR Sectional charts are oriented in relation to true north, even though we as pilots fly magnetic headings. On VFR charts there are lines drawn on the charts called isogonic lines that connect magnetic variations, whereas IFR En-Route charts are oriented with magnetic north.
Since we are flying in the Southern California area, the variation is 14° east. To be able to fly a true course of 180°, the pilot would have to subtract the variation and fly a magnetic course of 166°.
Another example is if you are flying over Washington, D.C., the variation is 10° west. To be able to fly a true course of 180° (south), the pilot must add the variation to be able to fly a magnetic course of 190°.
Deviation:
Deviation errors are caused by electro-magnetic fields with in the aircraft, from either flowing electrical currents, magnetized parts, as well as Earth magnetic fields. The compass is placed in a location of the aircraft away from any electrical currents that may affect it.
This errors in the aircraft magnetic compass is corrected by the use of Compass Deviation Cards.
Dip Errors:
As you move closer to the magnetics poles the angle created by the vertical pull of the Earth’s magnetic field increases in relation to the Earth’s surface is known as the dip angle. The dip angle increases downward as you move closer to the Magnetic North Pole.
The closer you are to the magnetic pole, it becomes too small to align a compass thus, becomes unusable to use for navigation.
Turning, Acceleration & Deceleration Errors:
When an aircraft turns in flight, the force causes the float assembly to lead and swing resulting in a false northerly turn. To correct this, a northerly turn should be stopped prior to the desired heading. A good rule of thumb is, the pilot he or she should stop a turn 15° prior to a new desired heading.
For turns in a southerly direction, the forces causes the float to lag, resulting in a false turn to the south. This rule of thumb for southerly turns is, that the pilot he or she should turn passed the new designed heading by 15°.
We use a memory aid that may help you remember Northerly Turning Errors, this is “UNOS”.
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U - Undershoot
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N - North
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O - Overshoot
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S – South
When an aircraft accelerates or decelerates, the compass will indicate a slight turn even though the aircraft is on a straight heading. If the aircraft is accelerating on a westerly or easterly heading, the compass will indicate a slight turn to the north. If the aircraft is on a westerly or easterly heading and decelerates, the compass will indicate a turn to the south.
We use a memory aid that may help you to remember acceleration errors, this is “ANDS”.
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A - Accelerate
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N - North
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D - Decelerate
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S - South
Simplified, acceleration compass turns to the north, and decelerates compass turns to the south.
Oscillation:
Is simply the bouncing around of the compass caused by turbulence, since the compass is incased if fluid, it can make it difficult to read.
Outside Air Temperature (OAT) Gauge:
The temperature gauge is a simple device that is mounted so it can sense the outside air temperature. The gauge is calibrated to indicate in either degrees Celsius (°C), Fahrenheit (°F) or both, which provides good information to the pilot, along with lapse rate temperature with altitude changes.
References:
Private Pilot ACS PA.I.G.K1h, PA.VI.A.K2
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