Airspeed Indicator

Airspeed Indicator Function:

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• Airspeed is a measure of differential pressure between the pitot (impact/dynamic pressure) and static pressures
• Simply stated, ram air is pushed against a diaphragm, which is compared to the static pressure
• The static pressure is captured through the static port(s) located on the side of the fuselage
  • 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 use when the primary static source is blocked and is especially important when in instrument meteorological conditions (IMC)
    • Alternate static sources are typically less accurate
• 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

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Airspeed Indications:

• Airspeed is generally displayed as a Knot (kt), but may be displayed in Miles per Hour (MPH) or Kilometers per Hour (KPH)
  • A knot is the unit for speed measured in Nautical Miles per Hour (NM/Hr)
  • A knot is slower than a MPH and a KPH
    • 1 knot = 1.15 MPH = 1.85 KPH

Types of Airspeeds:

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• There are many types of airspeed that pilots read or reference to achieve desired performance or use to plan for navigational purposes

• Indicated Airspeed (IAS):

  • The IAS is the direct airspeed reading shown by an airspeed indicator
  • The reading has not been corrected for variations in atmospheric density, installation error, or instrument errors
  • As height increases, the indicated airspeed falls below the true airspeed
  • Manufacturers use this airspeed as the basis for determining aircraft performance
  • IAS will not normally vary with altitude or temperature and so your V-speeds listed in the AFM/POH will mostly vary due to weight

• Calibrated Airspeed:

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  • Calibrated Airspeed (CAS) is the indicated airspeed of an aircraft, corrected for position and instrument error
    • Errors can include angle of attack, flap configuration, ground proximity, wind direction, to name a few
    • Errors can sometimes equal several knots and are generally greatest at low airspeeds
  • 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 [Figure 5]
  • Note that some aircraft have alternate static sources which may need to be referenced in a separate chart

• Equivalent Airspeed:

  • Equivalent Airspeed (EAS) is not usually practical for pilots, and is more used by engineers to determine performance
  • 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 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
  • Further, as air density decreases with an increase in altitude, an aircraft has to be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure
    • Therefore, for a given CAS, TAS increases as altitude increases; or for a given TAS, CAS decreases as altitude increases
  • TAS is therefore CAS corrected for non-standard temperature, with the help of an Outside Air Temperature (OAT) gauge, and altitude
  • The TAS is the speed that is used for flight planning and is used when filing a flight plan
  • On higher performance aircraft, a true airspeed indicator may be installed

  • Calculating True Airspeed:

    • Flight Computer Method:

      • The most accurate method is to use a flight computer
      • With this method, the CAS is corrected for temperature and pressure variation by using the airspeed correction scale on the computer
      • Extremely accurate electronic flight computers are also available
        • Just enter the CAS, pressure altitude, and temperature, and the computer calculates the TAS

    • Rule of Thumb:

      • A second method, which is a rule of thumb, provides the approximate TAS
      • Simply add 2 percent to the CAS for each 1,000 feet of altitude

      • Formula:

        • 5 (5000 ft) * 0.02 = .1 (correction factor)
        • .1 * 100 KCAS (cruise airspeed) = 10 knots (correction speed)
        • 100 (CAS) + 10 = 110 knots TAS

• Ground Speed (GS):

  • Groundspeed (GS) is the actual speed of the airplane over the ground
  • It is TAS adjusted for wind (airmass movement)
  • GS decreases with a headwind and increases with a tailwind
  • Ground Speed is a primary concern for performance during cross-country planning





• Mach Number:

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  • 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

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Instrument Errors:

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• 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 occurred, assuming that static pressure is not changing

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  Static blockage:

  • If the static system becomes blocked but the pitot tube remains clear, the ASI continues to operate; however, it is inaccurate
  • Airspeed indicator will give erroneous readings (slower readings at altitudes above the blockage, faster below)
    • The airspeed indicates lower than the actual airspeed when the aircraft is operated above the altitude where the static ports became blocked because the trapped static pressure is higher than normal for that altitude
    • When operating at a lower altitude, a faster than actual airspeed is displayed due to the relatively low static pressure trapped in the system
  • If the aircraft descends, the static pressure increases on the pitot side showing an increase on the ASI. This assumes that the aircraft does not actually increase its speed
    • The increase in static pressure on the pitot side is equivalent to an increase in dynamic pressure since the pressure cannot change on the static side
  • If an aircraft begins to climb after a static port becomes blocked, the airspeed begins to show a decrease as the aircraft continues to climb
    • This is due to the decrease in static pressure on the pitot side, while the pressure on the static side is held constant
  • Some aircraft are equipped with an alternate static source in the flight deck
    • In the case of a blocked static source, opening the alternate static source introduces static pressure from the flight deck into the system
    • Flight deck static pressure is lower than outside static pressure
    • Check the aircraft AOM/POH for airspeed corrections when utilizing alternate static pressure
  • A blockage of the static system also affects the altimeter and VSI, as well

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• Airspeed indicator failure demonstrates the importance of relating pitch attitudes to airspeeds
  • An additional tool would be ground-speed readings, like from a GPS, noting that groundspeed does not account for winds-aloft and therefore is not an accurate reading
• Realize too, that errors in the indication can be induced by slipping the aircraft

Preflight Actions:

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

Regulations:

• The airspeed indicator is a required instrument as per Federal Aviation Regulation 91.205 for both day and night visual and instrument flight rules (VFR/IFR)
• According to FAR 91.411, No person may operate an airplane, or helicopter, in controlled airspace under IFR unless:
  • Within the preceding 24 calendar months, each static pressure system has been tested and inspected and found to comply with appendices E and F of part 43 of this chapter;
  • Except for the use of system drain and alternate static pressure valves, following any opening and closing of the static pressure system, that system has been tested and inspected and found to comply with paragraph (a), appendix E, of part 43 of this chapter; and
  • Following installation or maintenance on the automatic pressure altitude reporting system of the ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter

• Airspeed Indicator Testing Regulations:

  • These tests must be conducted by:
    • The manufacturer of the airplane, or helicopter, on which the tests and inspections are to be performed;
    • A certificated repair station properly equipped to perform those functions and holding:
      • An instrument rating, Class I;
      • A limited instrument rating appropriate to the make and model of appliance to be tested;
      • A limited rating appropriate to the test to be performed;
      • An airframe rating appropriate to the airplane, or helicopter, to be tested; or
    • A certificated mechanic with an airframe rating (static pressure system tests and inspections only)

Inertial Reference Unit (IRU), Inertial Navigation System (INS), and Attitude Heading Reference System (AHRS)

• IRUs are self-contained systems comprised of gyros and accelerometers that provide aircraft attitude (pitch, roll, and heading), position, and velocity information in response to signals resulting from inertial effects on system components
  • Once aligned with a known position, IRUs continuously calculate position and velocity. IRU position accuracy decays with time
  • This degradation is known as "drift"
• INSs combine the components of an IRU with an internal navigation computer
  • By programming a series of waypoints, these systems will navigate along a predetermined track
• AHRSs are electronic devices that provide attitude information to aircraft systems such as weather radar and autopilot, but do not directly compute position information
• Aircraft equipped with slaved compass systems may be susceptible to heading errors caused by exposure to magnetic field disturbances (flux fields) found in materials that are commonly located on the surface or buried under taxiways and ramps
  • These materials generate a magnetic flux field that can be sensed by the aircraft's compass system flux detector or "gate", which can cause the aircraft's system to align with the material's magnetic field rather than the earth's natural magnetic field
  • The system's erroneous heading may not self-correct
  • Prior to take off pilots should be aware that a heading misalignment may have occurred during taxi
  • Pilots are encouraged to follow the manufacturer's or other appropriate procedures to correct possible heading misalignment before take off is commenced

V-Speeds:

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• V-speeds are airspeeds of significance to the pilot during flight
• These speeds differ between aircraft and are specified in the Pilot Operating Handbook/Pilot Information Manual (POH/PIM) under performance specifications
• To avoid confusion, this is a list of definitions, should you need them
• These do not apply to every aircraft you fly
• The way you should use this information is to reference the POH and reference unknown speeds against what they mean
  • The V_S0 - V_FE range is indicated by the white band
  • The V_S1 - V_NO range is indicated by the green band
  • The V_NO - V_NE range is indicated by the yellow band
• Do not try to find an unfamiliar speed here and try to find out what it is to your airplane, because it may not exist!
• Your flight training will go over which speeds apply to you based on your training aircraft

• Maximum Allowable Airspeed:

  • 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
• 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

• Maneuvering Speed:

  • Va is defined as the maximum speed at which full control deflection can be abruptly applied without over-stressing the aircraft and depends on aircraft weight
  • As learned from American Airlines 587 (crash in Queens, post 9/11) we learned when you abruptly change the controls multiple times back and forth, you negate Va and you may cause structural failure
  • Formula:
  

Airspeed Indicator Knowledge Quiz:

Conclusion:

• The airspeed indicator is critically important for ensuring that structural speeds are not exceeded
• Exceeding those limits may cause over-stress and damage to the aircraft
• Always keep in mind the effects of parallax error
• Beyond the direct indications from the instrument, think of what else it might be telling you such as Nautical Miles per hour
  • Since Knots = Nautical Miles per Hour, 60 knots (TAS, not IAS!) is 60 NM in an hour, and a NM per minute
• Note that while an approach or landing speed may be specified, the speeds held on approach will differ from final to roundout to flare
• Read more about the Pitot-static system
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References:

• Federal Aviation Administration - Pilot/Controller Glossary
• Aeronautical Information Manual (1-1-15) Inertial Reference Unit (IRU), Inertial Navigation System (INS), and Attitude Heading Reference System (AHRS)
• Bold Method - What's The Difference Between Approach Speed And Threshold Crossing Speed?
• CFI Notebook.net - Performance Calculations
• CFI Notebook.net - Pitot Static System
• Federal Aviation Regulations (91.205) Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements
• Federal Aviation Regulations (91.411) Altimeter system and altitude reporting equipment tests and inspections
• Instrument Flying Handbook (3-8) Airspeed Indicator (ASI)
• NASA - Pitot-Static Tube
• Smart Cockpit.com - Getting to Grips with Aircraft Performance