Prerequisite Knowledge:
The airspeed indicator is one of the most important instruments when flying, as knowing your airspeed can mean the difference between a safe flight, and 214'ing it into the bay.
Measuring Airspeed
The airspeed indicator works by comparing static pressure to ram air pressure. Ram air pressure is the pressure exerted by the oncoming air; moving faster through the air leads to higher ram air pressure. Ram air pressure is obtained via a pitot tube, which is oriented so that it will face into the oncoming air.
The airspeed indicator is one of the most important instruments when flying, as knowing your airspeed can mean the difference between a safe flight, and 214'ing it into the bay.
Measuring Airspeed
The airspeed indicator works by comparing static pressure to ram air pressure. Ram air pressure is the pressure exerted by the oncoming air; moving faster through the air leads to higher ram air pressure. Ram air pressure is obtained via a pitot tube, which is oriented so that it will face into the oncoming air.
The airspeed indicator works by comparing the ram air pressure and static air pressure. To do this, the ram air pressure is fed into a diaphram, and the static pressure is fed into the airspeed indicator tube. As the ram air pressure increases (or the static pressure decreases), the diaphram expands, turning a series of pinions which move the need clockwise (increasing indicated airspeed). As ram air pressure decreases (or the static pressure increases), the diaphram contracts, turning the needle counter clockwise (decreasing indicated airspeed)
The airspeed indicator is the only instrument in the 6 pack to use ram air pressure and static air pressure. The altimeter/VSI uses only static pressure, and the other instruments use different mechanisms.
Unreliable Airspeed Readings
The airspeed indicator will give incorrect readings if one or both of the ports are blocked, but it will still behave in a predictable manner.
Airspeed Indicator Markings
There are several markings on the airspeed indicator, which usually have a standard set of meanings:
The airspeed indicator is the only instrument in the 6 pack to use ram air pressure and static air pressure. The altimeter/VSI uses only static pressure, and the other instruments use different mechanisms.
Unreliable Airspeed Readings
The airspeed indicator will give incorrect readings if one or both of the ports are blocked, but it will still behave in a predictable manner.
- Pitot tube blocked: With just the pitot tube blocked, ram air pressure is no longer being supplied, and the pressure in the diaphram remains constant. However, the diaphram can still expand and contract with changes in static pressure, which makes the airspeed indicator act like an altimeter. The indicator will show rising airspeed as static pressure decreases (e.g. a climb), and decreasing airspeed as static pressure increases (e.g. descent).
- Static port blocked: The static pressure will no longer be accurate, and all changes in the airspeed indication are due to ram air pressure changes. Normally, the low static pressure at high altitudes results in higher indicated airspeed readings, which is completely normal. As a result, when climbing above the point at which the static port was blocked, the pitot tube will still show airspeed, but it will tend to read lower than normal. When descending below the point at which the static port was blocked, the airspeed indicator will tend to read higher than normal.
- Both pitot tube and static port blocked: This will cause the airspeed indicator to freeze at whatever the current value was, due to none of the pressures inside the airspeed indicator changing. The airspeed indicator will remain completely frozen at that reading no matter what happens.
Airspeed Indicator Markings
There are several markings on the airspeed indicator, which usually have a standard set of meanings:
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That's it for the airspeed indicator! The next article will cover the gyroscopic instruments.