Airspeed Indicator

Maximum Allowable Airspeed:

• An aneroid or altimeter mechanism actuates the maximum airspeed pointer 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.

Maneuvering Speed:

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• Va is the maximum speed at which abrupt application of full control deflection without over-stressing the aircraft and depends on the aircraft's weight.
• As learned from American Airlines 587 (crash in Queens, post 9/11), abrupt changes in the controls repeatedly negate Va, and you may cause structural failure.
• Formula:

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Indicated Airspeed:

• Indicated Airspeed (IAS) is the direct airspeed reading shown by an airspeed indicator.
• The reading does not correct for atmospheric density variations, installation, 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, so your V-speeds listed in the AFM/POH will mainly 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, and 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 at which the aircraft moves 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 6]
• Some aircraft have alternate static sources, often contained in a separate chart.

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Equivalent Airspeed:

• Equivalent Airspeed (EAS) is calibrated airspeed corrected for compressibility effects above 180-200 knots and 20,000', the airspeed the airplane "feels."
• EAS is not usually practical for pilots and is more used by engineers to determine performance.
• 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:

• True Airspeed (TAS) is CAS corrected for non-standard temperature with the help of an Outside Air Temperature (OAT) gauge and altitude.
• 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 require 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.
• The TAS is the speed used for planning and 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.
    • This method applies temperature and pressure corrections using the computer's airspeed correction scale.
    • 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, a rule of thumb, provides the approximate TAS.
    • 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:

• Ground speed (GS) is the actual speed of the airplane over the ground and is an important performance number for flight planning.
• Groundspeed is TAS adjusted for wind (airmass movement).
  • GS decreases with a headwind and increases with a tailwind.
• In more extreme examples, groundspeed is zero in a straight dive or straight climb despite an airspeed indicator showing rapid acceleration/deceleration.
• Therefore, groundspeed can be the same as (still air) TAS or drastically different (heavy winds, extreme pitch angles) from what is indicated on an airspeed indicator.

• Calculating Groundspeed:

  • Calculating ground speed can be done through complex mathematical equations, but it's more practically done by subtracting headwinds from the TAS and adding tailwinds.

Mach Number:

• 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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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).

Pitot-Tube and Drain Hole Blockage (static open):

• The 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 that as altitude increases and pressure decreases, the instrument will read artificially high compared 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.

Static blockage:

• If the static system becomes blocked but the pitot tube remains clear, the ASI continues to operate; however, it is inaccurate.
• At the altitude where the blockage occurs, airspeed indications would be normal.
  • 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 AFM/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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