Pressure and Density altitude play a big role in performance calculations on the test, and it’s a consistent weak area, so let’s have a look at it here.
In low pressure air, air molecules are spread out, there isn’t as much air in a given space when the pressure is low. When the air pressure is high, those air molecules become more tightly packed and the air is denser.
Aircraft performance depends on this density. The propeller is more effective when it is pushing more air molecules to produce thrust. The wing generates more lift when it is pushing more air molecules downwards. And the engine produces more power when it has more air molecules going into the cylinders to combust.
The pressure of the air changes as we move up in the atmosphere. At lower altitudes, aircraft experience higher pressure due to the compacting force of all of the air molecules above it pressing down. At higher altitudes, there isn’t as much of this air pressing down, and so the molecules are more spread out and the pressure is lower. Aircraft performance is poorer at higher altitudes.
Heating an airmass causes its molecules to move faster and spread out; higher temperatures create lower pressure air. Aircraft performance is poorer in hotter temperatures.
Changes in weather conditions give rise to high and low pressure air systems on different days. Aircraft performance is poorer on days with low pressure.
Water vapor present in the atmosphere on humid days reduces the density of air. Bigger water molecules push air molecules out of a space of air. Aircraft performance is poorer on days with high humidity.
If we look at all of these factors that are at play on a particular flight and say something about how well the aircraft will perform, we’ll use pressure and density altitude to do it. In other words, after looking at things like altitude, pressure, and temperature, we can say that the aircraft will perform the way it would at a certain altitude, under standard conditions, or under conditions where temperature and pressure are held to a constant.
First of all, if the aircraft climbs, it will perform worse. So the true altitude of the aircraft is a determining factor of performance. Here it is at 8000 feet.
Now, if the pressure falls or is lower than the standard 29.92" at sea level, the aircraft will perform not as though it were at 8000 feet, but as if it were at, say, 11000 feet under that standard model. In fantasy land, where pressure and temp are held constant, this aircraft would behave as if it were at 11000 feet. This is its pressure altitude.
Similarly, if the temperature rises and air molecules spread out. The aircraft will behave as if it is even higher, so raising the temps causes the aircraft to perform not as if it were at its pressure altitude of 11000 feet, but at what’s called its density altitude of 12000 feet.
The definitions are – pressure altitude is true altitude corrected for non standard pressure. Or, the altitude indicated when the altimeter is set to 29.92. Density altitude is pressure altitude corrected for non standard temperature. So it’s a multi step process to get to density altitude.
On the test, a chart like this one will be used to determine pressure altitude and density altitude.
Now, these charts can be very difficult to read even using the straight edge you’ll have with you on test day. So it may be helpful to remember that a 15 degree Fahrenheit increase will lead to about a 1000 foot increase in density altitude. This is helpful if you’re like me and have trouble reading exact figures off these charts!