Airspeed Indicator (ASI) – Type of Airspeeds, ASI Markings, Limitations

The Airspeed Indicator (ASI) is a crucial instrument in aviation that provides pilots with real-time information about the speed of the aircraft through the surrounding air. It’s an essential tool for maintaining safe flight operations, ensuring optimal performance, and adhering to speed limitations. Following table points will be discussed here.

The Airspeed Indicator (ASI) is a highly sensitive gauge that measures and promptly displays the difference between pitot (impact/dynamic pressure) and static pressure. When the aircraft is parked on the ground in calm air, these two pressures are equal. However, as the aircraft moves through the air, the pressure on the pitot line becomes greater than the pressure in the static lines. This difference in pressure is detected by the airspeed pointer on the instrument’s face, which is calibrated in miles per hour, knots (nautical miles per hour), or both.

The Airspeed Indicator (ASI) is unique because it integrates both the pitot and static systems. It introduces static pressure into the airspeed case while the pitot pressure (dynamic) is directed into the diaphragm. Changes in dynamic pressure cause the diaphragm to expand or contract, affecting the indicating system, which then moves the mechanical linkage and the airspeed needle.

Various Types of Airspeeds.

Similar to altitudes, there are various types of airspeeds, each serving specific purposes.

1. What is Indicated Airspeed (IAS)?

One crucial type is Indicated Airspeed (IAS), which is the direct reading obtained from the ASI. It’s not adjusted for variations in atmospheric density, installation error, or instrument error. Manufacturers use IAS as the basis for determining aircraft performance, and critical speeds such as takeoff, landing, and stall speeds listed in the Aircraft Flight Manual (AFM) or Pilot’s Operating Handbook (POH) are typically indicated airspeeds and remain constant regardless of altitude or temperature changes.

2. What is Calibrated Airspeed (CAS)?

Calibrated Airspeed (CAS) is the Indicated Airspeed (IAS) corrected for installation error and instrument error. Despite manufacturers’ efforts to minimize airspeed errors, it’s impossible to eliminate all errors throughout the airspeed operating range. Installation and instrument errors may accumulate, especially at low airspeeds, and can be significant with certain flap settings. However, in the cruising and higher airspeed ranges, IAS and CAS are nearly identical. Airspeed calibration charts can be referenced to correct for potential airspeed errors.

3. What is True Airspeed (TAS)?

True Airspeed (TAS) is CAS corrected for altitude and nonstandard temperature. TAS increases with altitude due to decreasing air density, requiring faster speeds to maintain the same pressure difference between pitot impact pressure and static pressure. Pilots can determine TAS using flight computers, adjusting CAS for temperature and pressure variation using the airspeed correction scale. Alternatively, a rule of thumb suggests adding 2 percent to CAS for each 1,000 feet of altitude. TAS is crucial for flight planning and filing a flight plan.

4. What is Groundspeed (GS)?

Groundspeed (GS) is the actual speed of the airplane over the ground. It’s TAS adjusted for wind. GS decreases when facing a headwind and increases with a tailwind.

Airspeed Indicator Markings.

Aircraft weighing 12,500 pounds or less, manufactured after 1945, and certified by the FAA are mandated to have Airspeed Indicators (ASIs) marked with a standard color-coded system. This system allows pilots to quickly identify crucial airspeed limitations for safe aircraft operation. For instance, if the airspeed needle enters the yellow arc and is approaching the red line during a maneuver, immediate action should be taken to reduce airspeed.

In single-engine small aircraft, ASIs include the following standard color-coded markings:

• White arc: Often called the flap operating range, it indicates the range for flap speeds. Its lower limit represents the full flap stall speed, and its upper limit indicates the maximum flap speed. Approaches and landings are typically conducted within this range.
• Lower limit of the white arc (V_S0): Represents the stalling speed or minimum steady flight speed in the landing configuration. For small aircraft, it’s the power-off stall speed with gear and flaps down at maximum landing weight.
• Upper limit of the white arc (V_FE): Indicates the maximum speed with flaps extended.
• Green arc: Denotes the normal operating range of the aircraft, where most flying occurs.
• Lower limit of green arc (V_S1): Represents the stalling speed or minimum steady flight speed in a specified configuration. Typically, it’s the power-off stall speed with gear up (if retractable) and flaps up at maximum takeoff weight.
• Upper limit of green arc (V_N0): Indicates the maximum structural cruising speed. This speed should not be exceeded except in smooth air.
• Yellow arc: Indicates the caution range. Aircraft should only operate within this range in smooth air and with caution.
• Red line (V_NE): Represents the never exceed speed. Operating above this speed is prohibited as it may lead to damage or structural failure.

Other Airspeed Limitations.

Other important airspeed limitations, not indicated on the Airspeed Indicator (ASI) face, are typically found on placards and in the Aircraft Flight Manual (AFM) or Pilot’s Operating Handbook (POH). These include:

• Design maneuvering speed (V_A): The maximum speed at which the aircraft can withstand full control deflection or gusts without causing structural damage. VA varies depending on aircraft weight.
• Landing gear operating speed (V_LO): The maximum speed for extending or retracting the landing gear, applicable for aircraft with retractable landing gear.
• Landing gear extended speed (V_LE): The maximum safe speed at which an aircraft can be flown with the landing gear extended.
• Best angle-of-climb speed (V_X): The airspeed at which the aircraft achieves the greatest altitude gain in a given horizontal distance. It’s used for short-field takeoffs to clear obstacles.
• Best rate-of-climb speed (V_Y): The airspeed that results in the greatest altitude gain in a given period of time.
• Single-engine best rate-of-climb (V_YSE): The best rate-of-climb or minimum sink rate in a light twin-engine aircraft with one engine inoperative. It’s indicated on the Airspeed Indicator (ASI) with a blue line and commonly referred to as “Blue Line.”
• Minimum control speed (V_MC): The minimum flight speed at which a light twin-engine aircraft can be adequately controlled with one engine inoperative and the remaining engine at takeoff power.

How to check ASI Instrument?

During the pre-takeoff instrument check, ensure the airspeed indicator (ASI) reads zero. However, if there’s a strong headwind directly impacting the pitot tube, the ASI might register a higher-than-zero reading. As you commence the takeoff roll, verify that the airspeed is progressively increasing at a suitable rate.