Aircraft Instruments, Engines

Commercial Ground School AVF 221

AIRCRAFT INSTRUMENTS

Pitot Static Instruments
Airspeed
Altimeter
Vertical Speed Indicator

Airspeed Indicator
The airspeed indicator is a differential pressure instrument
Static pressure is directed into the case
Ram pressure is directed into the aneroid
As velocity increases, so does Ram pressure causing the aneroid to expand
ram-static=dynamic

AIRSPEED INDICATOR
Position error
static pressure error induced by influence of the flow of air around the airframe
Static pressure errors generally increase with an increase in alpha or yaw angle
Instrument error
Errors caused by the mechanical workings of the instrument

Airspeed Indicator
Types of Airspeed
Indicated
Airspeed read directly off the instrument
Calibrated
IAS corrected for instrument and position error
Equivalent
CAS corrected for compressibility
True
EAS corrected for density effects
Goes up at about 2% per 1000 feet
Some indicators include a true airspeed window
Some are combo Mach/Airspeed

Altimeter
Corrugate bronze aneroid wafers
Expand as altitude is increased
Compress as altitude is decreased
The Sensitive Altimeter has a Kollesman window
The scale goes from 28.00″ to 31.00″

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

Altimeter Errors
Cold weather errors
As temp drops below standard the airmass shrinks
We can calculate how much by figuring True Altitude using the E6b or the ICAO table
High to low, look out below

Altimeter Errors
Knowing the reporting station elevation is needed to find an accurate True Altitude
Remember, you only need to calculate the distance between you and the station
The elevation of the station is unaffected by temperature
This generally isn’t a problem if the reporting station is at the airport your doing approaches to
The station temperature will probably be different than the temperature at your altitude

Altimeter Errors
Nonstandard pressure has a similar effect on altitude
As pressure decreases, so does True Altitude
As pressure goes down, altitude goes up
This looks like a climb on the altimeter, so the pilot adjusts by descending
As a rule of thumb a new pressure should be obtained every 100 miles
During x-country operations, this happens when you’re handed off to another sector under flight following
It’s important to be on the correct setting for the sector you’re operating in to maintain separation
Also so that your mode C matches what is read on the altimeter and ATC mode C data

Vertical Speed Indicator
Aka Vertical Velocity Indicator VVI
Hooked directly into the static line
Is a pressure differential instrument
Small adjustment screw, use a nonmagnetic screw driver
Lag times vary but generally 6-9 seconds
Instantaneous Vertical Speed Indicator IVSI
Uses two accelerometer actuated air pumps to take out the lag

VSI
Both the case and the aneroid are vented to the static source
The case pressure enters and leaves through a pinhole size calibrated leak
This cause the pressure to change more slowly than in the aneroid
As a climb is entered pressure decreases instantaneously in the aneroid but not in the case
This causes the aneroid to be compressed and a climb is indicated on the needle
In a descent the reverse happens

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
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
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
Northerly and southerly turning errors
Acceleration/deceleration errors (ANDS)
Oscillation error

Magnetic Compass Errors
Magnetic Dip
Varies from none at the equator to large amounts the closer to the poles
The correction is roughly equal to 15 + ½ latitude
Magnetic Dip is the culprit behind the
Northerly & Southerly turning errors
Acceleration/deceleration errors

Magnetic Compass Errors
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 Errors
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.
This results in the need for the Lead and lag diagram (remember latitude rule).
Lead/Lag

Magnetic Compass Errors
Accelerate and it turns north
Decelerate and it turns south
This is due to the weights placed under the card on the north pole side of the magnet
Remember the south pole of the magnet is attracted to the earth’s magnetic north pole
The weights help counteract magnetic dip but provide inertia at a right angle to direction of travel when on an east or west heading
Flux Gate Compass

The Gyroscopic Instruments
Attitude Indicator
Heading Indicator
Turn Coordinator

The Gyroscopic Instruments
The attitude and heading indicators 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.
The turn coordinator functions by using precession

RIGIDITY IN SPACE
Once a gyro gets spinning it resists any force to change its path
This is due to Newton’s 1st law
The ability to resist this change is governed by
Angular velocity
Weight or mass of the spinning object
And radius of the weight
Because the spinning gyro has inertia, it resists any attempt to change the inertia
The moment of inertia is radius squared times mass

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

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

Attitude Indicator
Attitude Indicator
Bank limits 100 to 110 degrees
Pitch limits 60 to 70 degrees
If limits are exceeded gyro precesses abruptly causing the gyro to tumble and become unreliable until it precesses back to horizontal at about 8 degrees per minute
Newer attitude indicators don’t have this problem
They are equipped with a
self erection mechanism rather than a caging knob that older ones have

Attitude Indicator Errors
Following a 180 upon rollout:
Indicates a slight climb
Bank in the opposite direction
Following a 360 the errors cancel out
Acceleration error
Shows a slight climb
Deceleration error
Shows a slight descent

Heading Indicator & HSI
Directional gyro instrument that is vacuum driven; some are electric; some are pressure driven
Ours are both electric and vacuum 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

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

TACHOMETER
Most mechanical tachs use a magnetic drag cup
A magnet is turned via a flexible shaft inside an aluminum cup
This creates a magnetic field to be generated within the aluminum which is influenced by the magnet and it spins
Some mechanical tachs use a flyweight and spring arrangement
Electric tachs use a small AC generator mounted on the engine case to transmit current to the cockpit indicator

MANIFOLD PRESSURE GUAGE
Set up is similar to an altimeter
It measures the manifold absolute pressure or MAP
A line runs from the intake manifold directly to the aneroid inside the instrument
A second aneroid compensates for changes in altitude
Since the engine is basically an air pump we can measure the amount of power by taking a reading of the vacuum inside the intake manifold
At idle the highest vacuum exists, at full power the least
So why does map increase when we exercise the prop?

OIL PRESSURE
This is the most important instrument for determining engine health
It works using a Bourdon tube
As pressure is increased the tube straightens
This drives the needle on the gauge
Used for oil pressure, hydraulic pressure, oxygen tank pressure and deicing boot pressure
Some systems use a bourdon tube at the engine and a line filled with a non-flammable, non-freezing fluid into the cockpit

OIL, CYLINDER HEAD TEMPERATURE & EGT
Some use a bourdon tube setup
The expansion of gas due to heat work the tube
Some use a thermocouple
2 dissimilar metals are joined together
As heat is applied, they expand at different rates which causes an electrical current
Metals used include iron and constantan, copper and constantan for CHT gauges
Chromel and alumel are used for EGTs
Also used extensively on turbine engines

FUEL GAUGES
Some are float type
Some are sight gauges
Most are electrical resistance type
A float with an arm is attached to a rheostat
This varies the electrical current to the gauge