Pressure altimeters measure altitude in aircraft by subtracting atmospheric pressure from sea level pressure. They are calibrated barometers with a mechanical readout display in the cockpit. The sea level pressure must be set before takeoff and adjusted during flight if necessary. Errors can occur due to mechanical, scaling, hysteresis, reverse, or installation/location issues.
Pressure altimeters are devices used primarily in aircraft to measure altitude. A pressure altimeter is essentially just a barometer that has been calibrated to measure an aircraft’s current altitude rather than barometric pressure. Altimeters usually need to be set by the pilot before takeoff and sometimes adjusted in flight.
Barometers measure air pressure using mercury, air, or sometimes water. Air pressure, also called barometric pressure, is simply the air pressure at a given level. Air particles press down on the air and ground below, creating pressure. This is why the barometric pressure at sea level is higher than in the mountains.
A pressure altimeter works by measuring the atmospheric pressure of your current location and subtracting it from the pressure at sea level. Barometric pressure is not a constant, so the pilot will need to set the sea level pressure on the altimeter before takeoff. There are two ways to set the pressure: the first is in the Kollsman window, which sets inches of mercury, or the second is to manually adjust the dial on the altimeter to set your current sea level altitude to ground level. The air traffic controllers at the tower will generally give the pilot the correct setup before takeoff.
Many times a change in elevation will not create a change in sea level pressure, but occasionally there are times when the sea level pressure changes from takeoff to landing. For example, drastic weather changes will cause a change in barometric pressure. When this occurs, the pilot should adjust the sea level air pressure reading during flight to ensure that the altitude reading is accurate.
A pressure altimeter is mechanical. Its main case is fixed to the outside of an aircraft, while the readout display is inside the cockpit. The readout is similar to an analog tachymeter: a graduated circular dial, usually marked in hundreds or thousands, along with small plastic pointers that indicate actual altitude.
A small air intake valve on the back of the case allows air to enter the altimeter. Mechanical instruments, called aneroid barometers, expand or contract depending on the air pressure. This motion drives a series of gears, springs and pinions, ultimately turning a drive shaft connected to the hands on the readout. The crankshaft moves these pointers around the dial to show the correct altitude.
There are several types of errors that are of concern when using a pressure altimeter. Mechanical errors, scaling errors and hysteresis errors are usually due to a problem with the internal gears or the elasticity of the material. Reverse errors and errors in installation or location cause a false pressure reading due to airflow issues.
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