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Exploring the Intricacies of the Airspeed Indicator

Aircraft contain all kinds of wonderous indicators and instruments to measure velocity, altitude, weather conditions, etc. However, the single most important aircraft instrument is probably the airspeed indicator. True to its name, the airspeed indicator tells you how fast the plane is going and is important for flight planning and maintaining aircraft performance. With that in mind, the team from Leopard Aviation is here to discuss how an airspeed indicator works and why airspeed indicators matter.

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How Does an Airspeed Indicator Work?

Airspeed indicators work primarily by measuring air pressure differences around the aircraft. The speed indicator itself connects to a series of hollow pipes in the plane that connects to the various instruments on the flight deck, known as the pitot static system. Airspeed indicators have two major “feeds’ that they measure with.

  • The pitot system captures dynamic, moving air. Dynamic pressure is often called ram air because it is ramming into the aircraft.
  • Static pressure, or static air, is the air that is at rest relative to the motion of the plane.

Speed indicators work by measuring pressure differences between static and ram air to find actual airspeed. If you have ever stuck your hand out of the window of a moving vehicle, you know that moving air creates force. In short, an airspeed indicator measures the ram pressure and compares it to static pressure to determine how fast a plane is moving.

Moving air from outside feeds into the pitot tubes and fills a pressure diaphragm. The sensor detects the amount of force from the pitot impact pressure and gives it airspeed indications by converting those values into speed. When properly calibrated, airspeed indicators are a highly accurate means of measuring the actual speed of a modern aircraft.

The higher the ram air pressure difference, the higher the air density, and the faster the airspeed indicator will say the plane is moving. The lower the total pressure difference, air density decreases, and the plane will fly slower. The point of the static ports and static system that measure static pressure is to create a baseline to compare with the dynamic pressure. Static vents that measure static pressures are usually on the lower side of the plane, where the air stream isn’t as turbulent.

True Airspeed vs Indicated Airspeed VS Equivalent Airspeed

Since airspeed indicators use air pressure to measure speed, the indicated airspeed can be less than the actual airspeed at higher altitudes. This is because air pressure, air temperature, and density decrease the higher you get, so the indicator might provide a lower speed. True airspeed (TAS) is the difference between the indicated airspeed and actual speed.

There is also the equivalent airspeed, which is the speed a plane has to fly at sea level to reproduce the same pressure difference at its current altitude. Whether or not a plane’s airspeed is greater than its ground speed depends on whether the air is blowing in the same or opposite direction of the plane.

Why Are Airspeed Indicators Important?

Airspeed indicators are extremely important, as aircraft need to hit a minimum speed to maintain lift. If the plane falls below its ‘stalling speed,’ the pilot can lose control, and the plane can start to lose lift. A plane’s specific stall speed depends on the plane’s size, dimensions, and even weather conditions. As altitude increases, the stall speed decreases as there is less air resistance.

The actual indicator face measures speed in nautical miles per hour (‘knots’) and has a single white needle that is the indicated airspeed (IAS). Indicators also often have bands that tell you when a plane enters the dive speed range, rotation speed range, and maneuvering speed. Airline pilots also need to know airspeed so they can know when to engage the landing gear.

Airspeed Indicator Errors

Although generally reliable, airspeed indicators can malfunction from damage, use, or installation errors, just like all types of aircraft instrumentation. For example, a pitot tube blockage can cause an airspeed indicator malfunction in the static port, incorrectly measuring the static pressure around the plane. Alternatively, instrument errors could be a result of the static line not being installed correctly, causing a bad static pressure reading.

For example, pitot tube freezing is relatively common in cold weather, which closes the static port so static pressure decreases erroneously. Other obstructions or blockages in the pitot tube system can mess up ram airspeed indicator detection because pitot pressure is not calibrated correctly.

As such, a major part of pre-flight safety checks is making sure the pitot tube array and static line are free of blockages. Before a flight, the airspeed indicator should read straight up and down unless a strong wind blows directly into the system. If oriented otherwise, the system has an error.

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Knowing how an airspeed indicator works and various visual flight rules are key parts of being a commercial pilot. Contact Leopard Aviation online or call today at (480) 372-8967 or subscribe to our newsletter to get the latest news on pilot training!

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