What is pressure altitude

During the flight, a pilot must update their altimeter setting periodically to ensure the altimeter continues to show an accurate reading. Whenever a new air traffic controller is contacted, the controller gives the current altimeter setting for the area. Certain standards in aviation were created simply for standardization. This could then be used as a baseline for aircraft performance. Of course, in real life, there are few normal days. Air pressure and temperature change with the weather.

But someone had to draw a line somewhere. The line drawn is known as the standard datum plane SDP. The standard conditions at sea level are Pilots consider these numbers far more often than you might imagine.

The pilot must then correct the numbers to figure out how the plane will act in the actual conditions that it will experience on the day of the flight. The easiest way to do this is to use the pressure altitude. What does matter is the density of the air.

Pressure altitude is something of an imaginary number used by pilots. They can calculate it based on the weather forecasts, and then they can take that number to their airplane flight manual and look up the performance charts.

By setting On both glass cockpit as well as analogue altimeters, the pilot adjusts the altimeter using a small dial. These calibrations compensate for local weather conditions so that the altimeter is accurate.

A tube from the port connects it to a series of aneroid wafers in other words, a small metal disk in a small container. These wafers are sealed so that they measure only the air reaching them from outside the airplane.

As the air from the static port reaches the aneroid wafers, they expand and collapse based on the static pressure they experience. When the pressure is low, the instruments can expand; under higher pressure, they are pushed down. Mechanisms within the altimeter are connected to this chamber, and the data is then fed to the part of the instrument facing the pilot.

Airplanes flying at lower altitudes compress the wafers; those located higher up will show increased altitude as the aneroid wafers expand. Part of the altimeter is known as the Kollsman window.

The Kollsman window consists of a mechanism which provides barometric pressure readings. It vanishes when the airplane goes below this level. The barber pole is designed so that pilots make more accurate readings when glancing at the face of the altimeter.

Pressure altitude is the attitude displayed on the altimeter when the Kollsman window is set to That is why every time a pilot checks in with a new air traffic controller, the local altimeter calibration is necessary. The altimeter indicates how high the airplane is above sea level by calculating the difference between the pressure in the aneroid wafers and the atmospheric pressure fed into the static port.

So, what is the difference between pressure altitude vs. Density altitude is pressure altitude corrected for temperature and humidity. For example, an airplane could be taking off from a sea level airport on the coast of Florida, and yet if it is hot and humid, the density altitude could be as high as 5, feet, in which case the airplane, even at sea level, requires a longer takeoff run and has a reduced rate of climb. Density altitude is calculated as part of the pre-flight preparation process.

Pilots need to know density altitude because it reflects air density. The less the air density at high altitudes in hot weather, the less lift is generated over the wings. Having this information can help the pilot prepare for an aircraft underperforming in such conditions.

Pilots might need to fly with fewer bags or people, or even delay the flight if it seems the runway is not long enough to have a sufficiently long takeoff roll. The Importance of Aeronautical Decision Making. Matthew A. Become a better pilot. Subscribe to the Boldmethod email and get real-world flying tips and information direct to your inbox, every week.

To: Separate email addresses with commas. Now What? All Videos. Planes Careers. Thanks to Bose for making this story possible. Check out the full series here. And if you want to know why we fly with Bose, learn more about their headsets here. Live from the Flight Deck. Swayne Martin. Sign Up. Recommended Stories. Here are the best ways to recognize the conditions that might get you stuck on the ground, or worse, in the air and diverting to another airport.