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Pitot-Static Instruments
Gyroscopic Instruments
Magnetic Compass
Digital Flight Instruments
Effects of Atmospheric Conditions
Air exerts 14.7 pounds per square inch at sea level
Changes in temperature affect atmospheric pressure
Warm air rises resulting in lower pressure
Cold air sinks resulting in higher pressure
The standard atmosphere at sea level consists of:
Barometric pressure of 29.92 inches of mercury
Temperature of 59 degrees Fahrenheit
Temperature and pressure normally decrease with an increase in altitude
Standard pressure lapse rate is approximately 1 in. Hg for each 1,000 feet of altitude
Standard temperature lapse rate is approximately 2°C for each 1,000 feet of altitude change
1. Pitot-Static Instruments
Used to determine an aircraft’s speed, altitude, and altitude trend
The system generally consists of:
Pitot Tube
Pitot pressure (impact or ram air pressure) is supplied by the forward facing pitot port
Higher ram air pressure means higher airspeed
Exposed to relative wind also relates to the pitot system
Static Port
Static or atmospheric pressure enters the pitot-static system through a static port in an area of relatively undisturbed air
Static air pressure is used to operate:
Airspeed indicator
Vertical speed indicator
Altimeter
The airspeed indicator is the onlyinstrument to operate using both pitot and static pressure
i. Airspeed Indicator
Operates using both pitot and static pressure
The airspeed indicator compares ram air pressure with static air pressure to determine the airspeed in knots
V-speed: describe the performance limits and characteristics of airplanes
Divided into three color-coded arcs and a red line:
White arcindicates the flap operating range:
The upper end indicates the maximum airspeed you can fly with flaps fully extended (V_FE)
The lower end indicates the speed the airplane stalls with the flaps fully extended (V_SO)
Green arcindicates the normal operating range:
The upper end indicates the maximum structural cruising speed (V_NO)
The lower end indicates the airplane's stall speed when the airplane is at the maximum takeoff weight, the flaps are up, and, if applicable, the landing gear retracted (V_S1)
Yellow arcindicates the caution range:
The range of speed above normal operating range that you should enter only in smooth air and only with caution
Red lineindicates the never exceed speed:
Never operate above this speed because structural damage to the aircraft could occur (V_NE)
Other V-speeds:
V_A (V_O): Design maneuvering speed (listed in the POH; might be greater when the aircraft is heavily loaded and lower when the load is light)
V_LO: Maximum landing gear operating airspeed
V_LE: Maximum landing gear extended airspeed
Types of Aircraft Speeds:
Indicated airspeed (IAS): The reading on the airspeed indicator
Does not reflect variations in air density
Important performance airspeeds are always the same indicated airspeed, regardless of altitude
Calibrated airspeed (CAS): Indicated airspeed corrected for installation error and instrument error
Manufactures try to keep airspeed errors to a minimum
Listed in the POH
True airspeed (TAS): Actual speed through the air
Groundspeed: Actual speed over the surface (equal to TAS in a no wind situation)
A headwind decreases groundspeed
A tailwind increases groundspeed
ii. Altimeter
Measures the aircraft's altitude
Based on mean sea level (MSL)
Three pointers to indicate the altitude:
The longer of the two needles on the altimeter indicates multiples of 100 feet
The shorter of the two needles on the altimeter indicates multiples of 1,000 feet
The pointer indicates multiples of 10,000 feet
The altimeter setting window is sometimes referred to as the Kollsman window
1 inch of change in the altimeter setting equals 1,00 feet of indicated altitude change
The knob on the altimeter adjusts the setting in the Kollsman window to compensate for changes in local barometric pressure
Altimeter settings are accurate onlyin the vicinity of the reporting station on which they are based
Types of Altitude:
Indicated altitude: The altitude measured by the altimeter, and the altitude used most often during the flight
Pressure altitude: The height above the standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 in. Hg as measured by a barometer
Density altitude: Pressure altitude corrected for temperature
True altitude: The vertical distance above mean sea level (MSL)
Absolute altitude: the actual height of the aircraft above the earth's surface, commonly referred to as height AGL
Calibrated altitude: Indicated altitude corrected to compensate for any instrument error
Altimeter Errors:
Failing to keep the altimeter set (most common)
Flying from high pressure to low pressure
When you fly into an area of higher pressure, the true altitude will be higher than the indicated altitude
When you fly into an area of lower pressure, the true altitude will be lower than the indicated altitude
"When flying from high to low, look out below"
Not monitoring the temperature
When atmospheric temperature is higher than standard, true altitude is higher than indicated altitude
When atmospheric temperature is lower than standard, true altitude is lower than indicated altitude
If the temperature is 10°C higher/lower than standard, the true altitude is 4% higher/lower than indicated altitude
iii. Vertical Speed Indicator (VSI):
Measures the rate at which an aircraft gains and loses altitude
The VSI dial is calibrated in 100-foot increments between 0 and 10, and then in increments of 500 feet
The VIS displays two types of information:
Trend information: Shows an immediate indication of an increase or decrease in the aircraft's rate of climb or descent
Rate information: Shows a stablized rate of change in altitude
Blockage of the Pitot-Static System:
Incorrect readings on the pitot-static instruments usually indicate a blockage of:
Blocked pitot tube
When the pitot tube slowly becomes blocked, the indicated airspeed gradually decreases
When the blockage seals the tube completely, the airspeed srops to the lowest value on your airspeed indicator
The blockage traps pressurized air inside the airspeed indicator, so the indicator displays the airspeed at the time of the blockage, regardless of actual airspeed
If the pitot system becomes completely clogged and the static system remains clear, the airspeed indicator acts more like an altimeter
Blocked static ports
If the static system becomes blocked, but the pitot tube remains clear, the following instruments are affected:
Airspeed Indicator: Continues to operate, but the indications are inaccurate
When the aircraft is above the altitude where the blockage occured, the instrument indications are slower than the actual airspeed
When the aircraft is below the altitude where the blockage occured, the instrument indications are faster than the actual airspeed
Altimeter: Air pressure in the system remains unchanged and so does the indicated altitude
VSI: Displays a continuous zero reading
Both Blocked
A clogged pitot tude affects the accuracy of only the airspeed indicator
Blockage of the static system affects all three Pitot-Static Instruments
Turn on the pitot heat to prevent pitot tube icing when flying in a visible moisture
2. Gyroscopic System Components
The Most common instruments containing gyroscopes are:
Turn coordinator
Heading indicator
Attitude indicator
All mechanical gyroscopic instruments enclose a spinning gyro mounted into brackets called gimbals
The spinning gyro:
Senses the aircraft's movement
Displays aircraft movement on the instrument face
Gyroscopic operation rests on two fundamental principles:
Rigidity in space
Fixed position in space
Tends to remain rigid
REsists external forces
Precession
Reaction in the direction of the rotation
Slow drifting and minor error indications
i. Turn Coordinator:
Two components of the turn coordinator:
Turn indicator (roll movement)
Inclinometer (yall movement)
Indicates:
Rate
Coordination
Standard rate turn is a turn of three degrees per second; to perform a standard rate turn, align the wing of the indicator with the turn index in the direction of the turn. During a coordinated turn, the ball remains centered between the reference lines.
Slip: The rate of turn is too slowfor the angle of bank, and the ball moves to the inside of the turn
Skid: The rate of turn is too greatfor the angle of bank, and the ball moves to the outside of the turn
To correct a slip or skid, center the ball by varying the angle of bank or applying rudder pressure in the direction of the deflected ball, or a combination of both actions
ii. Attitude Indicator:
An artifical reference for pitch and roll attitude with respect to the earth's surface
Displays the angle of bank by:
The relationship of the airplane indicator to the deflected horizon bar
The alignment of the pointer with the bank scale
Displays pitch by the position of the nose with respect to the horizon bar
iii. Heading Indicator:
Displays heading information based on a 360-degree compass; the final zero is omitted
The primary source of heading information
It must be set before each flight and periodically adjusted throughout the flight to align it with the magnetic compass
Flight Instrument Sources of Power:
Gyroscopic instruments are powered by either of the following sources:
Electrical Power
Turn Coordinator
Vacuum System
Attitude Indicator
Heading Indicator
3. Magnetic Compass
A simple and reliable source of heading information
Use the magnetic compass to:
Indicate the magnetic heading of the aircraft
Set the gyroscopic heading indicator to correct for precession
Back up the heading indicator
Requires no electrical or vacuum power
Variation:
The angular difference between true north and magnetic north
Isogonic lines: Lines on an aeronautical chart that connect points of equal magnetic variation
Agonic line: The line that connects points where the magnetic variation is zero
To convert a true course to magnetic course, subtract easterly variation and add westerly variation - "East is least, west is best"
Deviation:
A compass error caused by magnetic disturbances from electrical and metal components in the aircraft
Manufacturers can decrease deviation error by installing compensating magnets inside the compass housing
Use the compass correction card to correct for deviation
Compass Errors:
Freedom of movement makes the magnetic compass sensitive to in-flight turbulence
Magnetic dip occurs while turning or changing speed, even in smooth air
Turning
The greater the dip, the greater the turning error
Increases near the poles, where magnetic dip is more apparent
No longer occurs when you fly near the equator
In the northern hemisphere, when making a turn from a northerly heading, the compass gives an inital indication of a turn in the opposite direction. Then it begins to show the turn in the proper direction, but lags behind the actual heading
When turning from an easterly or westerly heading to a northerly heading, no error occurs as the turn begins. However, as the hending approchaes north, the compass increasingly lags behind the aircraft's actual heading
When making a turn from a southerly heading, the compass gives an indication of a turn in the correct direction, but leads the actual heading. Turning error disappears as the aircraft approaches an east or west heading
Accelerating/Decelerating
In the northern hemisphere, when your aircraft accelerates, the compass indicates a turn to the north
In the northern hemisphere, when your aircraft decelerates, the compass indicates a turn to the south
The compass returns to its correct indication when the acceleration/deceleration stops
Are more pronounced as you fly east or west
Do not occur when you fly on a north or south heading
"ANDS: Accelerate North, Decelerate South"
Is accurate only in straight-and-level, unaccelerated flight in smooth air
4. Introduction to Digital Displays
Airspeed Indicator
Altimeter
Horizontal Situation Indicator
Vertical Speed Indicator
Turn Coordinator + Attitude Indicator
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