The Static Port
The airplane's static port senses the pressure of static, or non-moving air. It is strategically located on the airplane so that it will provide the most accurate static pressure during all airplane operations.
The pressure from the static port is directed to the altimeter, vertical speed indicator, and airspeed indicator.
The Pitot Tube
The pitot tube is a forward facing device that senses ram air pressure for use with the airplane's airspeed indicator. There are openings in the front and rear of the pitot tube, all of which must be free of debris for the device to function properly.
The Altimeter
Air pressure in the atmosphere drops with increasing altitude. As a result, outside air pressure can be used to determine altitude.
Altimeters are driven by an internal device that expands and contracts as a result of changing air pressure. As it does so, the device mechanically drives the needles of the altimeter, to indicate height above sea level to the pilot.
The Vertical Speed Indicator
The vertical speed indicator tells the pilot the rate at which the airplane is climbing or descending. The instrument operates by measuring the air pressure differential between the static air pressure and the pressure inside the instrument.
The vertical speed indicator is built to be airtight, with the exception of a small, calibrated leak. Since air can only enter or escape the instrument through this hole at a certain rate, a pressure differential is created between the air inside the instrument and the static air outside the instrument anytime the airplane is in a climb or descent.
In level flight, the pressure inside the instrument case and the static pressure are equal. The VSI shows a zero rate of climb or descent to the pilot. Should the pilot enter a climb, for example, the static air pressure would immediately begin to decrease as the airplane's altitude increases. The air inside the vertical speed indicator case is trapped, however. It can only escape at a certain rate through the case's calibrated leak. As a result, the pressure inside the case is greater than the lowered static air pressure outside the case. The VSI indicates a rate of climb.
When the airplane levels, the air leaks out. The pressure inside the case is once again equal with the pressure inside the case, and a zero rate of climb or descent is indicated.
The faster the rate of climb or descent, the greater this pressure differential will be, allowing the instrument to display a rate to the pilot in feet per minute.
The Airspeed Indicator
The airspeed indicator displays the pressure differential between ram air from the pitot tube and static air from the static port. The greater the airspeed differential between the forward facing pitot tube and the static air, the greater the speed of the airplane. So, the airspeed indicator is showing air pressure to the pilot.
The instrument face is calibrated to display the result in knots or miles per hour to the pilot.
Clogged Static Port
The airspeed indicator, altimeter, and vertical speed indicator all use static air pressure. If the static port becomes clogged, these instruments will show erroneous information. The airspeed indicator's operation would be degraded and its displayed information inaccurate. The altimeter may be slow to respond or frozen in place. Either way, it too would display altitude information that is not trustworthy. The vertical speed indicator may display greater than actual rates of climb and descent or other inaccurate information.
Clogged Pitot Tube
The airspeed indicator is the only instrument that uses the pitot tube. If the pitot tube becomes clogged, it may drop to zero, become frozen at a particular airspeed, or provide some other erroneous information to the pilot.
The altimeter and vertical speed indicator are not affected by a pitot tube stoppage.
Alternate Static Source
Some aircraft are equipped with an alternate static source, usually located inside the cabin. This alternate static pressure source is provided in case the primary static port becomes clogged.
The alternate static source should only be used when necessary, because it will result in slightly erroneous readings in the altimeter and airspeed indicator. These errors are a result of the slightly lower than static air pressure that exists inside the airplane's cabin. Whenever the airplane is moving, the air pressure is lowered in the cabin as a result of the cabin's shape. As the air flows around the cabin, it is accelerated, creating a pressure reduction.
The pitot-static instruments are made up of the airspeed indicator, vertical speed indicator (VSI), and altimeter.
The airspeed indicator and vertical speed indicator measure pressure differential, while the altimeter measures pressure.