Air Speed Indicator (ASI)

Air Speed Indicator

Purpose of Air Speed Indicator

• Air Speed Indicator is designed to detect dynamic pressure
• ASI measures dynamic pressure and displays it as air speed
• Units of measurement are normally Knots or Kilometers per hour

Principle of Air Speed Indicator

• ASI senses Pitot pressure and static pressure
• Pitot pressure is a combination of dynamic and static
• Dynamic pressure is obtained by subtracting static pressure
• Dynamic pressure = Pitot pressure – Static pressure
• Formula for Dynamic pressure = ½ density x velocity2

Construction of ASI

• Static pressure is provided to the air tight case from a static source
• Pitot pressure is made to enter a expandable thin graduated capsule
• Pitot pressure varies with change in air speed
• Static pressures cancels out providing dynamic pressure
• The capsule therefore expands as a factor of dynamic pressure
• This capsule movement is transmitted by a temperature compensated magnifying linkage, to a pointer, indicating air speed of the aircraft

Critical Speed Markings

• V no : Normal Operating Speed Range
• V ne : Red Line Speed: Never Exceed Speed
• V yse : Blue Line Speed: Best Single Engine Climb Speed
• V lo : Maximum Landing Gear Operating Speed

Critical Speed Markings

• V le : Maximum Speed With Landing Gear Extended
• V s0 : Stall Speed
• Minimum Steady Flight Speed With Flaps and Undercarriage Down
• V s1 : Stall Speed
• Minimum Steady Flight Speed in a Specified Configuration

Colour Codes for Speed Range

• White arc refers to flap operating speed range
• Flap operating stall speed (V so ) to Maximum flap operating speed (V fe )
• Green arc refers to normal operating speed range (Vno)
• Yellow arc refers to cautionary speed range
• Normal operating speed (V no) to Never exceed speed (V ne)

Instrument and Position Errors

• Instrument Errors are caused due to manufacturing imperfections
• Position error is caused due to suction due turbulent airflow near static vent
• Instrument and Position errors are tabulated in a correction card

Manoeuvre Induced Errors

• Manoeuvre induced errors are caused due to change in angle of attack
• Manoeuvre induced errors result in Time lag during Air speed indications

Variation of Atmospheric Density

• Density error results from variation of actual and calibrated density of air
• Dynamic Pressure depends on both speed of aircraft as well as air density
• Density varies with temperature & pressure, which in turn varies with Altitude

Density Error

• ASI is calibrated to read airspeeds correctly at air density of 1225 gm/m3
• This density prevails only at ISA conditions
• Dry air (zero humidity)
• MSL pressure of 1013.25 hPa
• MSL temperature of 15 deg C
• In all other conditions, the ASI reads incorrect speeds called density error

Compressibility Error

• Compressibility error occurs due to compression of air at high speeds
• Air is a compressible fluid and compression is significant at high speeds
• ASI is calibrated only for air at its normal state and not when compressed
• Error due to compression are significant at speeds greater than 300 Knots
• Compression errors in air speed have to be corrected above 300 knots

Indicated and Calibrated (Rectified) Air Speed

• Indicated air speed (IAS) is the speed indicated by the Air speed indicator
• It is a factor of dynamic pressure as measured by the instrument
• Calibrated (CAS) is obtained from Indicated air speed (IAS)
• Calibrated (CAS) is also called Rectified air speed (RAS)

Calculating Calibrated (Rectified) Air Speed

• IAS corrected for instrument & position errors provides CAS or RAS
• Correction card table in flight manual is used for applying the corrections
• These corrections could be applied manually or by Air data computer

Equivalent Air Speed

• Equivalent air speed (EAS) is obtained from Calibrated air speed (CAS)
• CAS corrected for compressibility error provides EAS
• Compressibility error correction is mandatory at speeds above 300 knots
• EAS can be calculated using Flight Computer
• EAS = CAS + Compressibility error correction

True Air Speed

• True air speed (TAS) is obtained from Equivalent air speed (EAS)
• TAS is obtained from EAS by applying Density error correction
• Air density reduces with increase in altitude

Variation of True Air Speed

• Density error increases with increase in altitude
• At a constant IAS, TAS increases with increase in altitude
• At a constant TAS, IAS decreases with increase in altitude
• TAS is same as IAS at Mean sea level under ISA conditions

Effect of Pitot Line Blockage in Level Flight

• Change in pitot pressure will not be sensed if pitot line is blocked
• ASI will not register any change of speed
• Indication remains constant for sometime and slowly reduces to zero speed

Effect of Pitot Line Blockage during Climb and Descent

• Considering an aircraft maintaining steady speed
• During descent, ASI under reads due to increase in static pressure
• During Climb, ASI over reads due to decrease in static pressure

Blockage in Static Line during Level Flight

• Change in Static Pressure will not be sensed if static line is blocked
• Static Pressure in the instrument case will remain constant
• ASI would read correctly if there is no change in altitude

Blockage in Static Line in Climb and Descent

• Considering an aircraft maintaining steady speed
• During descent, ASI over reads due to increase in static component
• During climb, ASI under reads due to decrease in static component

Leaks in Pitot System

• Leak in pitot line reduces the pitot pressure sensed by the system
• Naturally, Air speed indicator would under read

Leaks in Static System

• Leaks outside pressurized hull in static line does not cause any error
• Static pressure will be correctly sensed by the instrument
• Leaks inside pressurized hull in static line will sense cabin pressure as static
• Cabin altitude in normally expected to be higher than actual static pressure
• ASI would under read due to lesser capsule expansion

Error Summary

• Considering that the aircraft is maintaining same speed
• If pitot line is blocked, ASI under reads in descent
• IAS reduces as altitude decrease in the altimeter
• If static line is blocked, ASI over reads in descent
• IAS increase as altitude decrease in the altimeter

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