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's pushing more air molecules to produce thrust, the wing generates more lift when it's 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 in air mass 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 or crowd out air molecules out of a space of air, aircraft performance is poorer on days with high humidity.

If we look at all these factors that are at play on a particular flight and say something about how well the aircraft will perform, we're going to be using pressure and density altitude to say 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'll perform worse, so the true altitude of the aircraft is a determining factor of performance. Here it is at 8,000 feet. Now, if the pressure falls or is lower than the standard 29.92 inches of mercury at sea level, the aircraft will perform not as though it were at 8,000 feet but as if it were at, say, 11,000 feet under that standard model in fantasyland where pressure and temperature held constant. This aircraft would behave as if it were at 11,000 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 11,000 feet but at what's called its density altitude of 12,000 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.

So just to review, an increase in altitude causes an increase in density altitude, a decrease in pressure causes an increase in density altitude, and an increase in temperature causes an increase in density altitude. On the test, a chart like this one will be used to determine pressure altitude and density altitude. Pressure altitude is found by taking the altimeter setting and finding the conversion factor, and adding it to the field elevation.

So if the altimeter is 29.92, it's pretty simple, you find that the conversion factor is zero and add that to the field elevation of, say, 1,000 feet. This means pressure altitude also equals 1,000 feet. What about if you have an altimeter setting of 28.90? You find that conversion factor, which is 957, add it to the field elevation, and get the pressure altitude of 1,957. Let's try a full example like we'd see on the test.

They give us field elevation, temperature, and altimeter setting, we have to find pressure and density altitude. First, we'll start by finding the conversion factor for the altimeter. There's not a line for our altimeter setting of 30.35, so we'll have to interpolate between the two closest ones. This means we'll take the conversion factors for 30.30 and 30.40 and average them, or divide their sum by 2, to get our conversion factor of negative 394. Now if we apply that to the field elevation, subtract 394 from 3,894, we get our pressure altitude of 3,500.

Next, to find density altitude, we use the chart on the left. We find the temperature of 20 degrees Fahrenheit at the bottom and move up until we get to the pressure altitude of three thousand five hundred. This is halfway between the lines for three and four thousand. From there, move to the left and read the density altitude of 2,000 feet. Now these charts can be very difficult to read, even using the straight edge you'll have with you on test day, so it might be helpful to remember that a 15-degree Fahrenheit increase will lead to about a 1,000-foot increase in density altitude. This is helpful if you're like me and have trouble reading exact figures off these charts.