Flight Instruments Flashcards
Static Pressure of Atmosphere
- Pressure of atmosphere
- Static Port
Total Pressure
- Pressure created inside the pitot tube as the aircraft moves forward
- Includes both static pressure and pressure increase due to forward motion of aircraft
Dynamic Pressure
- Pressure difference between total pressure and static pressure
- Direct indication of forward speed of aircraft
Density Altitude
Pressure Altitude corrected for non-standard temperature
True Altitude
Exact Altitude above sea level
Absolute Altitude
Exact Altitude above ground level
Altimeter Operation
- When altitude increases, static pressure decreases, capsule expands, gears convert expansion to needle movement up
- sub scale knob used to correct for high and low pressure air (28”Hg - 31”Hg)
- Should not be more than +/- 50 feet of elevation when on ground
Altimeter Position Error
Static port senses dynamic pressure during slips, causing altimeter error
Altimeter Scale Error
- Aneroids not assuming precise size of pressure difference
- Increases with altitude
Altimeter Mechanical Error
Caused by misalignment or slippage in gears
Altimeter Density Error
- Caused by variations in atmospheric pressures and temperatures
- 1’’ Hg error = 1000’ ft error
- High to low, lookout below (altimeter over-reading)
- Low to high, clear blue sky (altimeter under-reading)
Altimeter Hysteresis Error
A lag in indications due to elastic properties of the materials used to construct aneroids
Altimeter Reversal Error
- Caused by rapid or abrupt attitude changes
- only momentary in duration
Vertical Speed Indicator (VSI) Operation
- When altitude increases, static pressure decreases, capsule contracts, gears convert contraction to needle movement up
- VSI returns to 0 when pressure in case matches pressure in diaphragm
- Should be inspected if showing greater than 100 fpm on the ground
VSI Lag Error
- VSI lags 6-9 seconds to settle on a rate due to calibrated leak which is connected to the static source
- Shows trends immediately
- smoother transitions produce less lag
- Instantaneous VSI’s have accelerometers that move the pointer until diaphragm takes over
Calibrated Airspeed
Indicated airspeed corrected for instrument and position error
Equivalent Airspeed
Calibrated Airspeed corrected for compressibility factor
Airspeed Indicator Operation
- Measures difference between total pressure (pitot tube) and static pressure (static port) to get dynamic pressure
- Dynamic pressure is a direct indication of airspeed
Airspeed Density Error
- Since static pressure decreases with altitude, total pressure will also decrease
- This means TAS will be higher than IAS
- Increase IAS 2% for each 1000ft to obtain TAS
Airspeed Instrument Error
Caused by mechanical errors present within the instrument
Hysteresis Error
Lag in indications due to elastic properties of aneroids
Airspeed Position Error
- Airframe disturbing airflow around pitot tube or static port
- Example is high angle of attack
- CAS corrects this
Pitot Source Blocked
- Airspeed Indicator = increasing in climb / decreasing in descent
- Altimeter = unaffected
- VSI = unaffected
One Static Source Blocked
All are inaccurate while slipping and very sensitive in turbulence
Both Static Sources Blocked
- Airspeed Indicator = decreasing in climb / increasing in descent
- Altimeter = Freezes
- VSI = Reads 0
Both Static and Pitot Sources Blocked
- Airspeed Indicator = Freezes
- Altimeter = Freezes
- VSI = Reads 0
Pitot Source Partially Blocked
- Airspeed Indicator =Reads 0
- Altimeter and VSI unaffected
Static Source Partially Blocked
- All will under read in a climb and catch up after levelling off
- All will over read in a descent and catch up after levelling off
Attitude Indicator Operation
- Gyro Mounted Horizontally and spins about a vertical axis
- Usually Vacuum driven
- Functions on principle of rigidity in space
- Linkages transfer movements around gyro to artificial horizon face
- Should be stable while taxiing
- Limitations are 85 degrees of bank and pitch
Heading Indicator Operation
- Gyro Mounted Vertically and Spins Horizontally
- Usually vacuum driven
- Functions on principle of rigidity in space
- Linkages transfer movements around gyro to heading indicator face
- Excessive precession indicates that the gyro is worn
- Limitations are 85 degrees of bank and pitch
Mechanical Precession Heading Indicator Error
- Frictional forces cause gyro to precess
- Causes gyro instruments to drift 3º every 15 minutes
- Reset heading indicator every 15 minutes
Apparent Precession Heading Indicator Error
- Earth rotates under the gyro
- Heading appears to precess depending on latitude
- zero at the equator, 15 degrees an hour at the poles
- Corrected by resetting every 15 minutes
Turn and Slip Operation
- Provides direction and rate of turn, as well as coordination
- Gyro Mounted Vertically and Spins Horizontally
- Electrically Driven
- Functions on principle of gyroscopic precession
- Needle only sensitive to yaw, not roll
Rate One Turn
- On L or R in turn and slip or turn coordinator
- 180º per minute
Turn Coordinator Operation
- Gyro Mounted Vertically and Spins Horizontally
- Canted at 35 degree angle
- Usually electrically driven
- Functions on principle of gyroscopic precession
- Sensitive to roll as well as yaw
Magnetic Compass Operation
- Mounted on a pivot and free to rotate within bowl of kerosene
- Should be free and swinging during turns on the ground
- Limited to 20º of pitch
- Unreliable in NDA due to magnetic dip
Variation
- Difference between magnetic north and true north
- Add westerly track / subtract easterly track to true track to get magnetic track
Acceleration Compass Errors
- Causes a turning error
- ANDS (Accelerate North Decelerate South)
- Error does not exist on north and south headings
Turning Compass Errors
- Causes an acceleration error
- SAND (South Advance North Delay)
- Error does not exist on east and west headings
