Prerequisite Knowledge:
The altimeter and vertical speed indicator are two instruments that show the aircraft's height above the ground. This article will cover how they both work in detail.
Altimeter
The altimeter shows the aircraft's height in feet above mean sea level (ASL, or MSL). Almost all heights in aviation are measured ASL, as measuring above the ground itself is generally not an accurate reference due to large variations in terrain elevation.
As you move higher into the atmosphere, the air pressure drops at a measurable rate. An aircraft's altimeter measures the outside air pressure, and then uses that to produce an altitude reading on the altimeter.
On the exterior of the aircraft is static port, which is used to measure the static air pressure (the pressure of the outside air). The static port is generally located on the side of the aircraft, not facing forward or backward (facing forward into the oncoming air would provide a bad reading, since it would not be able to measure the static air pressure). The static port is connected to the back of the altimeter via a single line.
The altimeter and vertical speed indicator are two instruments that show the aircraft's height above the ground. This article will cover how they both work in detail.
Altimeter
The altimeter shows the aircraft's height in feet above mean sea level (ASL, or MSL). Almost all heights in aviation are measured ASL, as measuring above the ground itself is generally not an accurate reference due to large variations in terrain elevation.
As you move higher into the atmosphere, the air pressure drops at a measurable rate. An aircraft's altimeter measures the outside air pressure, and then uses that to produce an altitude reading on the altimeter.
On the exterior of the aircraft is static port, which is used to measure the static air pressure (the pressure of the outside air). The static port is generally located on the side of the aircraft, not facing forward or backward (facing forward into the oncoming air would provide a bad reading, since it would not be able to measure the static air pressure). The static port is connected to the back of the altimeter via a single line.
The static air enters the altimeter tube, which is completely sealed except for the static air line. On the inside of the altimeter tube are aneroid wafers, which are entirely sealed. The pressure of the air inside the wafers is constant, whereas the pressure inside the altimeter tube is equal to the static air pressure. As the aircraft climbs, the static air pressure decreases, which allows the wafers to expand. As the wafer expands, it turns a series of gears, which is shown as the needles turning clockwise on the altimeter (gaining altitude). When descending, the static air pressure increases, which causes the wafers to contract. This turns the gears in the opposite direction, which is displayed as a counter clockwise needle movement (losing altitude).
Usually, an analog altimeter like this one has three needles. The longest needle with the chevron on the end indicates the altitude in tens of thousands of feet, the middle needle indicates thousands of feet, and the long needle without the chevron indicates the altitude in hundreds of feet. The altimeter in this image is indicating an altitude of about 1,400 feet.
Altimeters also have a Kollsman window (named after Paul Kollsman, its creator). The Kollsman window lets the pilot manually calibrate the altimeter, and is located on the right side of the altimeter. The knob to change the actual setting in the window is located on the bottom left.
Static air pressure varies with altitude, but there are also other things that can affect it too. The biggest contributor to altimeter error are temperature variations. When the altimeter is calibrated to the standard altimeter setting (29.92 inches of mercury or 1013 hPa), it will correctly display the altitude when flying in ISA conditions. ISA conditions refer to the international standard atmosphere, which are a set of standard, defined atmospheric conditions. However, real world weather very rarely follows standard conditions, and this can cause altimeter errors. For example, when the temperature is warmer than ISA, the static air pressure is higher, which causes the altimeter to read a lower altitude than which the aircraft is actually located. This introduces the need for a way to calibrate the aircraft's altimeter, hence the Kollsman window.
During a flight, air traffic control will regularly provide the aircraft with local altimeter settings, which the pilot will dial into their Kollsman window. This will ensure accurate altimeter readings throughout the entire flight.
Altimeter Errors
A popular saying regarding altimeter errors is "from high to low, look out below!". This refers to the altimeter error created when flying from an area of high pressure to an area of low pressure (or high temperature to low temperature), in level flight, without recalibrating the altimeter. The aircraft may be flying level in reality (its true altitude is constant), but its indicated altitude will increase, due to the pressure drop. The pilot will see the altimeter showing an increase in altitude, and try to compensate by descending, to maintain a constant indicated altitude. The altimeter may show level flight, but in reality the aircraft is in a descent, with its true altitude reducing.
Altimeters also have a Kollsman window (named after Paul Kollsman, its creator). The Kollsman window lets the pilot manually calibrate the altimeter, and is located on the right side of the altimeter. The knob to change the actual setting in the window is located on the bottom left.
Static air pressure varies with altitude, but there are also other things that can affect it too. The biggest contributor to altimeter error are temperature variations. When the altimeter is calibrated to the standard altimeter setting (29.92 inches of mercury or 1013 hPa), it will correctly display the altitude when flying in ISA conditions. ISA conditions refer to the international standard atmosphere, which are a set of standard, defined atmospheric conditions. However, real world weather very rarely follows standard conditions, and this can cause altimeter errors. For example, when the temperature is warmer than ISA, the static air pressure is higher, which causes the altimeter to read a lower altitude than which the aircraft is actually located. This introduces the need for a way to calibrate the aircraft's altimeter, hence the Kollsman window.
During a flight, air traffic control will regularly provide the aircraft with local altimeter settings, which the pilot will dial into their Kollsman window. This will ensure accurate altimeter readings throughout the entire flight.
Altimeter Errors
A popular saying regarding altimeter errors is "from high to low, look out below!". This refers to the altimeter error created when flying from an area of high pressure to an area of low pressure (or high temperature to low temperature), in level flight, without recalibrating the altimeter. The aircraft may be flying level in reality (its true altitude is constant), but its indicated altitude will increase, due to the pressure drop. The pilot will see the altimeter showing an increase in altitude, and try to compensate by descending, to maintain a constant indicated altitude. The altimeter may show level flight, but in reality the aircraft is in a descent, with its true altitude reducing.
Vertical Speed Indicator (VSI)
The VSI shows the rate of change of the aircraft's altitude (the rate of climb or descent). A VSI works by measuring the difference in pressure between two points, usually a few seconds apart.
The VSI shows the rate of change of the aircraft's altitude (the rate of climb or descent). A VSI works by measuring the difference in pressure between two points, usually a few seconds apart.
Inside the VSI is a diaphram, which, unlike the altimeter, is not sealed. The diaphram is connected directly to the static pressure source, and therefore the air pressure inside the diaphram is equal to the static pressure. The pressure outside the diaphram, inside the VSI tube, is also equal to the static pressure, but from a few seconds prior. A calibrated leak controls the flow of air in and out of the case, and is designed to create a small delay between the actual static pressure change, and the pressure changes in the VSI tube. Basically, the pressure in the diaphram equals the current static pressure, whereas the pressure in the VSI tube is from a few seconds prior. The diaphram is connected to the needle on the gauge via several gears, so expansion/contraction of the diaphram results in changes in the VSI needle.
This allows the VSI to measure the difference between the two pressures, and figure out the rate of climb, or descent. For example, in a climb, the static pressure is constantly dropping. Therefore, the pressure in the VSI tube will always be slightly higher than the pressure in the diaphram, since the pressure in the tube is delayed. The diaphram contracts, which indicates a climb on the gauge. In a descent, the static pressure is increasing, which means the pressure in the case is slightly lower than the pressure in the diaphram. This means the diaphram expands, which is displayed as a descent by the needle on the gauge.
The VSI relies on static air, which means it is prone to the same errors as the altimeter (e.g. it will indicate a climb when flying from high to low pressure, even though the true altitude isn't changing). Another that is unique to the VSI is a delay. Generally, it takes several seconds for the VSI to give an accurate reading, which means it should not be referenced for small altitude changes. In that case, the altimeter should be looked at, and small changes on there should be noted.
If the static port ever becomes blocked, the altimeter will freeze and not change, due to the static pressure no longer changing. The VSI will also show level flight, due to the constant static pressure
That's it for the altimeter and VSI! The next article will cover the airspeed indicator.
This allows the VSI to measure the difference between the two pressures, and figure out the rate of climb, or descent. For example, in a climb, the static pressure is constantly dropping. Therefore, the pressure in the VSI tube will always be slightly higher than the pressure in the diaphram, since the pressure in the tube is delayed. The diaphram contracts, which indicates a climb on the gauge. In a descent, the static pressure is increasing, which means the pressure in the case is slightly lower than the pressure in the diaphram. This means the diaphram expands, which is displayed as a descent by the needle on the gauge.
The VSI relies on static air, which means it is prone to the same errors as the altimeter (e.g. it will indicate a climb when flying from high to low pressure, even though the true altitude isn't changing). Another that is unique to the VSI is a delay. Generally, it takes several seconds for the VSI to give an accurate reading, which means it should not be referenced for small altitude changes. In that case, the altimeter should be looked at, and small changes on there should be noted.
If the static port ever becomes blocked, the altimeter will freeze and not change, due to the static pressure no longer changing. The VSI will also show level flight, due to the constant static pressure
That's it for the altimeter and VSI! The next article will cover the airspeed indicator.