Lesson 4 Aircraft Instruments

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Basic T Arrangement
Attitude indicator
Heading indicator

Basic T Arrangement
Flying Using Instruments
1. Instrument cross check
2. Interpretation
3. Aircraft control

The Pitot Static Instruments
Airspeed indicator
Vertical speed indicator (VSI)

Inspections And Tests
Pitot static instruments must be inspected and tested every 24 calendar months for IFR
Elt every 12 calendar months
Transponder and encoder every 24 calendar months

Airspeed Indicator
The only true pitot static instrument
Ram air pressure; forced air routed through the pitot tube and lines to the airspeed indicator
Static pressure; ambient air pressure, pressure you would get off a barometer

Airspeed Indicator
Dynamic pressure; the difference between ram and static, read off the face of the instrument
Diaphragm measures pressure forces
Pitot static mast

Airspeed Indicator
Airspeed Indicator
Green arc normal
Yellow arc caution
White arc flaps
Blue line
White triangle

Airspeed Indicator
Vx 65 kts
Vy 72 kts
Best glide 78 kts
Approach 63 kts
1 notch 72 kts
2 notches 67 kts
3 notches 63

Altimeter measures the pressure of the air
It has a sealed case except for static source inlet
Aneroid wafers sense changes in pressure by expanding or contracting

It is hooked directly to the static ports
When going to alternate static source get a rise
Lower pressure inside cabin

How to read it

Approximately 1″ of pressure measured in inches of mercury is equivalent to 1000′ of altitude
The altimeter must be calibrated before takeoff and about once an hour sometimes more and every 100 miles

When flying from high to low look out below
When flying from low to high your high in the sky
Current barometric pressure is set in the Kollsman window, the scale goes from 28.00″ to 31.00″

Sometimes millibars are set in an alternate window (usually in Europe)
Altitude should read + or – 75 feet (for IFR flight)

There are 5 different types of altitudes
Indicated Altitude
Pressure Altitude
Density Altitude
Absolute Altitude
True Altitude

Indicated Altitude
Indicated altitude is the altitude read on your altimeter
Altitudes assigned to aircraft in controlled airspace under instrument flight rules are indicated altitudes except for flights operating in the high altitude route structure. (Above 18,000 MSL)

Pressure Altitude
Pressure altitude is the altitude read on your altimeter when the instrument is adjusted to indicate height above the standard datum plane.
The standard datum plane is a theoretical level where the weight of the atmosphere is 29.92″ of mercury as measured by a barometer.
As atmospheric pressure changes, the standard datum plane may be below, at, or above sea level or station elevation.
Note the examples:
28.92 puts the line at 189 feet MSL
29.92 puts the line at 1189 feet MSL
30.92 puts the line at 2189 feet MSL

Pressure Altitude
Pressure altitude is important as a basis for determining aircraft performance as well as for assigning flight levels to aircraft operating at high altitude.
When we figure pressure altitude we are concerned with what the pressure is at our elevation, not where the 29.92 line is located
Our math computations give us the pressure altitude for our level
When we set 29.92 in our altimeter we are also reading the altitude we are at referenced to 29.92
Note the example:
28.92 gives us a PA of 2189
29.92 gives us a PA of 1189
30.92 gives us a PA of 189
These numbers are more meaningful to us for performance than where the 29.92 line is in the atmosphere

Density Altitude
Density altitude is pressure altitude corrected for nonstandard temperature.
Under standard atmospheric conditions, each level of air in the atmosphere has a specific density, and under standard conditions, pressure altitude and density altitude identify the same level.
Aircraft performance will be effected in a negative way by a higher density altitude.

Absolute Altitude
Absolute altitude is height above the surface.
This height may be indicated directly on a radio/radar altimeter, which measures the time interval of a vertical signal bounced from the aircraft to the ground and back.
Absolute altitude is essential information for flight over mountainous areas and may be approximately computed from indicated altitude and chart elevation data.

True Altitude
True altitude is the height above mean sea level.
This is a mathematical value determined by computer and therefore based upon standard atmospheric conditions assumed in the computer solution.

How Temp Effects Altitude
Make Sure You Study Altimetry Problems In Your Red Book

Vertical Speed Indicator
Measures differential pressure
Is hooked directly to static ports
Has a calibrated leak
Is accurate as a trend instrument immediately
Is accurate as a rate instrument after 6 to 9 seconds

Vertical Speed Indicator

The Gyroscopic Instruments
Turn coordinator
Attitude indicator
Heading indicator (HI)

The Gyroscopic Instruments
The gyro instruments operate on the principle of “rigidity in space”
A spinning gyro resists the movement of the airplane by means of gimbals.
The gyro remains stationary while the airplane moves around it.
Because the gyro is mechanically linked to the plane, there is some friction felt by the gyro, which causes precession.

Vacuum System
Vacuum system diagram

Attitude indicator
Heading indicator
Instrument air gauge (inches of mercury)

Vacuum System
Backup vacuum systems
Venturi type vacuum systems
Pressure systems

Turn Coordinator/Turn And Slip Indicator
In our airplanes, this is an electric gyro instrument
However, some are vacuum
All indicate rate of turn in seconds
A standard rate turn is 3 degrees per second
A 360 degree turn takes 2 minutes

Turn Coordinator/Turn And Slip Indicator
Actually a combination of two instruments
Slip indicator curved glass tube filled with kerosene and a steel ball bearing or black agate
Centrifugal force or lack of it accounts for the ball’s movement

Turn Coordinator/Turn And Slip Indicator
Turn coordinator gyro is mounted on a 30 deg angle
The turn coordinator can sense both roll and yaw
Displays only rate of roll and rate of turn doesn’t directly display bank angle

Turn Coordinator/Turn And Slip Indicator
Turn and slip gyro is horizontal
The turn needle indicates the rate of turn about the vertical axis in degrees per second
Older models required 1 needle with deflection for standard rate turn
Some have dog houses on them and others are calibrated for a 4 minute turn

Attitude Indicator
Pitch and bank vacuum driven gyro; some are electric or pressure driven
Gyro is gimbled on 3 axis rotor spins horizontally, pivots about the lateral axis on a gimbal, which turns about the longitudinal axis
Attitude Indicator

Attitude Indicator
Bank limits 100 to 110 degrees
Pitch limits 60 to 70 degrees
If limits are e
xceeded gyro precesses abruptly causing the gyro to tumble and become unreliable until it precesses back to horizontal at about 8 degrees per minute

Attitude Indicator
Markings are at 10, 20, 30, 60 and 90 degrees bank
Pitch markings are usually on the 5s

Heading Indicator
Directional gyro instrument that is vacuum driven; some are electric; some are pressure driven
10,000 to 18,000 rpm
Pitch and bank limits of about 55 degrees (older ones)

Heading Indicator
Precession error of no more than 3 degrees in 15 minutes
Gyro spins in the vertical axis and remains rigid in space as the azimuth card rotates around it

Magnetic Compass
Two magnetic needles attached to a floating card inside a sealed case filled with acid free white kerosene.
The kerosene supports the card, dampens oscillations, and lubricates the pivot assembly that the card rotates around.
Lubber line inside the glass face.
There is a Deviation Card located on the compass to compensate for errors.

Magnetic Compass Diagram
Magnetic Compass Errors
Variation is the angular difference between true north and magnetic north
Deviation is the pull of the aircraft’s magnetic fields generated by radios and such on the compass card

Magnetic Compass Errors
Magnetic dip points downward closer to the poles causes turning errors.
Usually equal to about the latitude.
Here in the northwest, that amounts to about a 30 degree error.

Magnetic Dip
Heading north, the compass will momentarily indicate a turn in the correct direction then turn opposite.
Heading south, the compass will indicate direction correctly, but at a faster rate.

Magnetic Compass
Northerly turning error when the plane banks the card banks with the plane because of centrifugal force.
This bank causes the vertical component of the earth’s magnetic field to pull the compass off course.
Lead and lag diagram (remember latitude rule).

Magnetic Compass Errors
ANDS Accelerate North, Decelerate South
Ammeter vs Loadmeter
There are two types of electrical measuring gauges out there.
The Ammeter tells exactly how much current the system is using.
In the B-19 we have a version that shows if we have a discharge.
The Loadmeter tells if the alternator is putting out the necessary amount of electricity to meet the demand.

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