Pressure altitude and density altitude are two terms that often confuse rookie pilots, but understanding them is no easy feat. Nonetheless, familiarizing yourself with the differences between each can help you better understand the forces acting on your aircraft. While an experienced pilot can readily explain their distinctive features, for those that need clarification, this blog will provide a brief overview of pressure altitude and density altitude.
Before diving into the peculiarities of pressure altitude and density altitude, we must outline the importance of an altimeter. Altimeters are one of six primary aircraft instruments, either analog or digital, which can be located in a majority of general aviation cockpits. In addition to the altimeter, the other instruments include the airspeed indicator, attitude indicator, vertical speed indicator, heading indicator, and turn coordinator.
Though all of the instruments provide critical information about the aircraft, the altimeter is tasked with measuring static air pressure which is defined as the pressure of the atmosphere. However, it must not be confused with a barometer which only indicates atmospheric pressure. In fact, altimeters translate atmospheric pressure into altitude, allowing pilots to know how high the aircraft is. Keep in mind that radar and GPS altitude can give this same information, but are not equipped to sense changes in air pressure like an altimeter. More than that, a radar only works at lower altitudes.
On piston-driven aircraft, pilots are required to input new information into the altimeter on a routine basis, with 10 to 15 minutes intervals being the most common. Whether the pilot is using a glass cockpit or traditional “steam gauges,” the pilot must set the altimeter to the altimeter pressure of the nearest airport. With such calibration, navigation errors are quite common. As such, pilots must have clear communication with air traffic controllers in order to acquire this information. Pilots can easily adjust the settings on analogue altimeters by turning a small dial.
Meanwhile, a static port outside of the aircraft’s fuselage measures air pressure. It is generally located at a point on the aircraft where it can read the flow of the air without disturbing other control surfaces. The static port is composed of a port with a tube that connects to a series of aneroid wafers in a small container. These wafers are sealed so that pilots can accurately measure the air reaching them from outside of the aircraft.
As air from the static port makes its way to the aneroid wafers, they either expand or collapse depending on the static pressure they experience. When the pressure is low, the instruments usually expand. In contrast, when the pressure is high, they are pushed down. Other mechanisms within the altimeter are linked to this chamber, and the data is sent to the part of the instrument facing the pilot.
Another important part to consider when measuring pressure altitude and density altitude is the Kollsman window. Usually positioned at a 3 o’clock position on the right side of the altimeter, the Kollsman window consists of a mechanism that provides barometric pressure readings. It is important to note that US-made aircraft report in terms of InHg (inches of mercury), while European Kollsman windows are read in millibars.
Pressure altitude can be defined as the attitude displayed on the altimeter when the Kollsman window is set to 29.92 InHg. As the aircraft’s true altitude is highly dependent on temperature, pilots cannot use pressure altitude below 18,000 feet. This is why pilots are required to check in with a new air traffic controller since local altimeter calibration is needed. The altimeter has the ability to indicate how high the aircraft is above sea level by determining the difference between the pressure in the aneroid wafers and the atmospheric pressure supplied to the static port.
Density altitude, on the other hand, is pressure altitude adjusted for temperature and humidity. It can be described as what the aircraft “feels” like it is experiencing. This is particularly important because all aircraft will face performance declines with increasing density altitude. Additionally, density altitude is calculated as a part of the pre-flight preparation process. In general, most Pilot Operating Handbooks (POHs) will incorporate a chart outlining how to calculate this information. For instance, pilots must refer to this handbook in order to calculate density altitude which reflects air density affecting an aircraft. The less the air density is at high altitudes in hot weather, the less lift is generated over the wings. Such data can prepare the pilot for an aircraft underperforming in such conditions.
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