Understanding density altitude is critical for both safe flight operations and success on the FAA knowledge test.
Here’s how you calculate density altitude:
- Find the pressure altitude correction using the FAA provided chart
- Subtract from the field elevation
Need a more detailed explanation? Below, we’ll walk step by step through how the FAA expects you to solve a density altitude problem, what each altitude really represents, and how to work a typical exam-style question.
What Is Density Altitude?
Density altitude describes how the airplane will perform in the current atmospheric conditions.
You can think of it this way: density altitude tells you how the airplane will feel. Put another way, if an airplane has a density altitude of 10,000 feet, it will perform as if it were at 10,000 feet – even if it’s actually sitting on the ground at sea level.
Higher density altitude means:
- Longer takeoff rolls
- Reduced climb performance
- Degraded engine and propeller efficiency
That’s why density altitude is so important to pilots.
The Three Altitudes You Must Understand
To calculate density altitude, the FAA expects you to move through three distinct steps.
1. Field Elevation
Field elevation is simply how high the airport is above mean sea level (MSL). This is your starting point.
2. Pressure Altitude
Pressure altitude is field elevation corrected for non-standard pressure. It tells you how much atmospheric pressure the airplane is experiencing and how that pressure affects performance.
Pressure altitude can be found by setting 29.92 inches of mercury in the altimeter while in the airplane, or by using the FAA’s pressure altitude conversion chart on the written exam.
3. Density Altitude
Density altitude is pressure altitude corrected for non-standard temperature. This final number tells you how well the airplane will actually perform.
Although some pilots call this “performance altitude,” the FAA uses the term density altitude, so that’s the wording you’ll see on the exam.
Using the FAA Density Altitude Chart
The FAA density altitude chart has two main sections.
Pressure Altitude Conversion (Right Side of the Chart)
One column lists altimeter settings, and the adjacent column provides the pressure altitude correction that accounts for non-standard pressure.
For example, at 29.92 the correction is zero because this is standard pressure, while at 30.00 the correction is minus 73 feet.
This correction is applied to the airport elevation to determine pressure altitude.
Temperature Correction (Left Side of the Chart)
Once you have pressure altitude, you find the outside air temperature at the bottom of the chart and move upward until you intersect the appropriate pressure altitude line. From that point, you read left to determine the approximate density altitude in thousands of feet.
FAA Written Exam Example
Let’s work through a typical FAA-style question.
Given:
Altimeter setting: 29.96
Airport elevation: 3,563 feet MSL
Step 1: Find the Pressure Altitude Correction
An altimeter setting of 29.96 is exactly halfway between 29.92, which has a zero correction, and 30.00, which has a correction of minus 73 feet. Half of minus 73 is approximately minus 36.5 feet.
Step 2: Calculate Pressure Altitude
The airport elevation is 3,563 feet, and the pressure correction is minus 36.5 feet.
Subtract the two, and this gives a pressure altitude of approximately 3,526.5 feet (wow! very specific!)
That is the pressure altitude the FAA is looking for in this question.
Why the FAA Tests Density Altitude
The FAA emphasizes density altitude because it directly affects aircraft performance and pilot decision-making. Pilots who do not understand density altitude are more likely to attempt takeoffs in conditions where the airplane cannot perform as expected.
For this reason, density altitude questions frequently appear on private pilot, commercial pilot, and flight instructor knowledge exams.
