Airspeed Indicator



Introduction:

  • The Airspeed Indicator (ASI) is a Pitot-static instrument used in an aircraft to display the crafts airspeed, typically in knots, to the pilot
  • Airspeed indication is accomplished with the use of a thin, corrugated phosphor bronze aneroid, or diaphragm which measures dynamic pressure of the air between the Pitot tube (ram air) and static port
  • The primary use of the airspeed indicator is to provide guidance during climb, descent, and landing
  • The airspeed indicator is also important for ensuring that structural speeds are not exceeded beyond which the airframe may be stressed and damaged

Indications:

  • Airspeed is generally displayed as a Knot (kt)
    • A knot is non-SI unit for speed measured in nautical miles per hour (NM/Hr)
    • A knot is slower than a mph and a km/hr (1 knot = 1.15 mph = 1.85 kph)
    • Some indicators may be calibrated for Statute Mile (SM) Per Hour or Miles Per Hour (MPH) as well
  • Some aircraft have a separate card to read TAS based on temperature off the OAT gauge
  • IAS will never change despite different atmospherics, so your V-speeds will only vary with weight
  • Va/Vle not depicted
  • True ASI - Temperature compensated to show TAS too

Figure 1: Instrument Flying Handbook, Mechanism of an Airspeed Indicator
Instrument Flying Handbook. Figure 3-11, Mechanism of an Airspeed Indicator

Definitions:

  • Dynamic Pressure: Difference in the ambient static air pressure and the total, or ram, pressure caused by the motion of the aircraft through the air
  • Knot: Nautical Mile Per Hour

Regulation:

  • The airspeed indicator is a required instrument for flight (91.205)

Figure 2: Airspeed Indicator
Airspeed Indicator

V-Speeds:

  • VS0 - VFE:
    • VS0: Stall speed with flaps extended
    • VFE: maximum speed flaps may be extended
    • Range shown in the white band
  • VS1 - VNO:
    • VS1: stall speed with both flaps and landing gear retracted
    • VNO: maximum normal operating speed (in smooth air)
    • Range shown in the green band
  • VNO - VNE:
    • Range shown in the yellow band
  • Multi-engine aircraft display a blue radial line to indicate Vyse
    • This airspeed delivers the best rate of climb if an engine was lost
    • A red line near the lower limit of the airspeed range indicates minimum controllable airspeed (Vmc)
      • This is the lowest speed at which the airplane is controllable when one engine is inoperative and the other engine is operating at full power

Figure 3: Pitot-Static System
Pitot Static System

Components:

  • Static Port: measures static air
  • Pitot Tube: measures "ram air"

How it Works:

  • The static pressure is captured through the static port(s) located on the Pitot tube and on the side of the fuselage near the nose of the aircraft
    • The location is chosen at a location to most accurately detect prevailing atmospheric pressure (parallel to air stream) and avoid dynamic (ram) air pressure
    • Some aircraft will have more than one port to more accurately measure pressure during slips and skids
  • "Ram air" is the air captured through the opening of the Pitot tube by the passage of the aircraft through the air
    • Ram Air can also be termed as total pressure
    • Some Pitot tubes are electrically heated to prevent clogging with ice
    • Most aircraft have an alternate static source intended for instrument meteorological conditions (IMC), however, they are less accurate
  • The ram air is then pushed against the diaphragm, which compares it to the static pressure
  • Conservation of Energy states that total pressure must remain the same and therefore as the Pitot pressure increases or the static pressure decreases, the diaphragm expands
  • This dimensional change is measured by a rocking shaft and a set of gears that drives a pointer across the instrument dial


Airspeeds:

  • Aircrew are primarily concerned with Indicated Airspeed and True Airspeed in flight with regards to performance
  • Ground Speed is also a primary concern for things such as cross-country planning
  • Equivalent airspeed is more designed for engineers and not usually practical for pilots

  • Indicated Airspeed (IAS):
    • The IAS is simply airspeed shown by an airspeed indicator
    • The reading has not been corrected for instrument or system errors
    • As height increases, the indicated airspeed falls below the true airspeed
  • Calibrated Airspeed (CAS):
    • CAS is indicated airspeed of an aircraft, corrected for position and instrument error
    • As discussed above, any errors that interfere with the system reading total and static pressure (which when subtracted give you dynamic pressure) are corrected here
    • This will give the actual speed in which aircraft is moving through the air
    • Calibrated airspeed is equal to true airspeed in standard atmosphere at sea level (High AoA, minimal error at cruise)
    • The POH/AFM has a chart or graph to correct IAS for these errors and provide the correct CAS for the various flap and landing gear configurations
  • Equivalent Airspeed (EAS):
    • The airspeed corrected for compressibility effects above 180-200 knots and 20,000', which is the airspeed the airplane "feels"
    • As the airspeed and pressure altitude increase, the CAS becomes higher than it should be as air molecules begin to stack up against the aircraft and instruments
    • A correction for compression must be subtracted from the CAS
  • True Airspeed (TAS):
    • Because the simple pitot system does not detect air density changes, it is calibrated to standard sea level pressure and any changes in pressure (or altitude) thereby requires a correction
    • TAS indicated airspeed corrected for non-standard temperature and altitude
    • These corrections are approximate increasing with altitude at about 2% per 1000' IAS
    • A calculation example can be found on the performance calculations page
  • Ground Speed (GS):
    • Measurement of the speed across the ground corrected for airmass movement

    Figure 4: Airspeed Conversions
    Airspeed Conversions

    Figure 5: Instrument Flying Handbook, A maximum allowable airspeed indicator has a movable pointer that indicates the never-exceed speed, which changes with altitude to avoid the onset of transonic shock waves
    Instrument Flying Handbook. Figure 3-14, A maximum allowable airspeed indicator has a movable pointer that indicates the never-exceed speed, which changes with altitude to avoid the onset of transonic shock waves
    Figure 7: Airspeed Indicator Markings
    Airspeed Indicator Markings

  • Mach #:
    • Mach number is the ratio of the TAS of the aircraft to the speed of sound in the same atmospheric conditions
    • Some older mechanical Machmeters not driven from an air data computer use an altitude aneroid inside the instrument that converts pitot-static pressure into Mach number
    • Modern electronic Machmeters use information from an air data computer system to correct for temperature errors to display true Mach number
  • Maximum Allowable Airspeed:
  • Figure 6: Instrument Flying Handbook, A Machmeter shows the ratio of the speed of sound to the TAS the aircraft is flying
    Instrument Flying Handbook. Figure 3-13,  A Machmeter shows the ratio of the speed of sound to the TAS the aircraft is flying
    • The maximum airspeed pointer is actuated by an aneroid, or altimeter mechanism, that moves it to a lower value as air density decreases
    • This instrument looks much like a standard air-speed indicator, calibrated in knots, but has an additional pointer colored red, checkered, or striped
    • The maximum airspeed pointer is actuated by an aneroid, or altimeter mechanism, that moves it to a lower value as air density decreases

  • Some aircraft are equipped with true ASIs that have a temperature-compensated aneroid bellows inside the instrument case
  • This bellows modifies the movement of the rocking shaft inside the instrument case so the pointer shows the actual TAS
    • These instruments have the conventional airspeed mechanism, with an added sub-dial visible through cutouts in the regular dial
    • A knob on the instrument allows the pilot to rotate the sub-dial and align an indication of the outside air temperature with the pressure altitude being flown
    • This alignment causes the instrument pointer to indicate the TAS on the sub-dial

Figure 8: Pitot Tube Icing
Pitot Tube Icing

Errors:

  • The pitot-static systems in modern aircraft are reliable, that we are always taught to "believe our instruments"
    • However, when they do fail, the failure may be so insidious that it goes unnoticed until it’s too late
  • Pitot-static failures typically come in three varieties:
    • Icing over the Pitot or static ports
    • Trapped water in the lines (usually after Maintenance fails to cover the ports during a wash)
    • Compromise of system integrity:
      • Leaks due to holes or loose fittings
      • Kinks in the lines
      • Obstructions/blockages
      • Taped or covered ports
  • Blockages in the system can cause a variety of errors
  • To prevent these errors you must complete a thorough pre-flight
  • Blockages can occur from FOD, striking an object (damaging instruments), insects, trapped moisture, loss of system integrity, icing, etc.
  • Pitot tube blockage (static open):
    • Airspeed indicator indicates zero (gradually decreasing)
  • Pitot tube and drain-hole blockage (static open):
    • Airspeed indicator will freeze and read like an altimeter as the total pressure now remains constant and the static pressure changes with climbs and descents
    • The measure of ram air to static air means as altitude increases and pressure decreases, the instrument will read artificially high as it is comparing it to the same dynamic (ram) pressure
    • Likewise, if pressure increases, such as in a descent, it will read artificially low
    • The aircraft will only read the correct airspeed at the altitude where the blockage occured, assuming that static pressure is not changing
  • Static blockage:
    • Airspeed indicator will give erroneous readings (slower readings at altitudes above the blockage, faster below)

Figure 9: Airspeed Indicator Failures
Airspeed Indicator Failures

Preflight Check:

  • The airspeed indicator should read straight up and down, unless a significant wind is being blown into the Pitot tube enough for the aircraft to sense
  • Airspeed should come alive on takeoff roll and as part of your takeoff scan, should be verbalized

Reference:



Written by (mike.manna@cfinotebook.net) || Last modified on