One of the topics appearing on the FAA Part 107 exam is high density altitude. This tutorial explains what you need to know on this topic and goes over several practice questions.
These practice questions and material are similar to those in the study guide I used to successfully pass the FAA Part 107 exam with a 90% score. The study guide comes along with 5 free online practice tests and the Airman Knowledge Testing Supplement.
Check out my post on my entire experience of preparing for and passing the FAA Part 107 exam. There, I delve in detail which topics to study most and what to expect on the day of the exam.
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Standard Day Conditions
Manufacturer information on aircraft performance makes reference to certain benchmark conditions. These are standard day conditions and they are:
- at sea level
- temperature of +15°C or +59°F
- atmospheric pressure of 29.92” Hg (inch of mercury) or 1013.2 millibars
Try to remember these numbers as they are often tested on the exam. Deviations from these measurements change the air density and will affect the aircraft performance.
Air molecules are nitrogen (78%) and oxygen (21%). The air density has to do with the amount of air molecules in a given volume. Denser air will have more molecules in a fixed volume.
High air density produces strong aircraft performance because more air molecules will pass through the airplane’s wings. Thus, dense air will produce higher rates of climb, takeoff and thrust for an aircraft.
Conversely, less dense air results in worse aircraft performance. Thin air has a smaller amount of air that will pass through the aircraft’s wings. To achieve the same performance under the thinner air, the aircraft must work harder.
High Density Altitude
Density altitude is altitude corrected for temperature and pressure deviations from the standard day. In layman’s terms, it is the actual altitude at which the plane “feels” it is flying. Density altitude is given as a height above mean sea level (MSL).
The term high density altitude comes from the fact that air density decreases with altitude. Therefore, rising density altitude deteriorates an aircraft’s performance.
Air Density ↓ ⇒ Density Altitude ↑ ⇒ Aircraft Performance ↓
The related term to density altitude is the pressure altitude. Pressure altitude refers to the altitude corrected for the atmospheric pressure deviation from the standard day.
Pressure altitude is like a stepping stone towards defining density altitude. Density altitude controls for both temperature and atmospheric pressure. But, pressure altitude controls only for air pressure. Thus, density altitude is pressure altitude corrected for temperature deviations from the standard day.
Density altitude is pressure altitude corrected for temperature deviations from the standard day conditions.
This means that if the temperature is +15C or +59F, pressure altitude and density altitude are the same. Furthermore, if the temperature deviates from the standard day, density altitude and pressure altitude are different.
Factors Affecting Density Altitude
Let’s go over the 4 main factors that affect the density altitude and aircraft performance. For the FAA part 107 exam, you do not have to know exact formulas. But, you should understand the relationship between density altitude and factors that affect it.
1. Atmospheric Pressure
When the pressure is above the standard day (29.92” Hg or 1013.2mb), there are more air molecules in a given volume of air. This means that the air density increases compared to the standard day. Recall, denser air is more helpful for the flight. Thus, higher atmospheric pressure increases an aircraft’s performance and decreases density altitude.
Air Pressure ↑ ⇒ Air Density ↑ ⇒ Density Altitude ↓ ⇒ Aircraft Performance ↑
2. Ambient Temperature
When the air temperature rises, the air molecules have more energy. As a result, the air molecules get scattered farther apart from each other. This means that the air density goes down and you have less air to pass through the aircraft wings.
Thus, rising ambient temperature leads to worse aircraft performance and higher density altitude.
Temperature ↑ ⇒ Air Density ↓ ⇒ Density Altitude ↑ ⇒ Aircraft Performance ↓
As you may know, air becomes thinner the higher you go. This means that the amount of air molecules in a give volume decreases and the air density decreases. This has a negative effect on the aircraft performance. Thus, higher altitude decreases the aircraft performance and increases density altitude.
Altitude ↑ ⇒ Air Density ↓ ⇒ Density Altitude ↑ ⇒ Aircraft Performance ↓
Humidity refers to the amount of water vapor in the air. Water vapor has less mass than air molecules, but it occupies about the same amount of space. An increase in humidity will produce less dense air, worse aircraft performance and higher density altitude.
Humidity ↑ ⇒ Air Density ↓ ⇒ Density Altitude ↑ ⇒ Aircraft Performance ↓
High Density Altitude Cheat Sheet
Here is the cheat sheet that will help you on the FAA Part 107 exam. If you remember these facts, you will have no problems with high density altitude on your exam.
- Standard day conditions:
- sea level,
- barometric pressure of 29.92” Hg (1013.2mb)
- Density altitude is pressure altitude corrected for temperature deviations from standard
- Air Pressure ↑ ⇒ Air Density ↑ ⇒ Density Altitude ↓ ⇒ Aircraft Performance ↑
- Air Temperature ↑ ⇒ Air Density ↓ ⇒ Density Altitude ↑ ⇒ Aircraft Performance ↓
- Altitude ↑ ⇒ Air Density ↓ ⇒ Density Altitude ↑ ⇒ Aircraft Performance ↓
- Humidity ↑ ⇒ Air Density ↓ ⇒ Density Altitude ↑ ⇒ Aircraft Performance ↓
- Ambient Temperature > +15°C/+59°F ⇒ Density altitude > Pressure altitude
- Ambient Temperature = +15°C/+59°F ⇒ Density altitude = Pressure altitude
- Ambient Temperature < +15°C/+59°F ⇒ Density altitude < Pressure altitude
Note, if the ambient temperature is not +15C/+59F (standard day), density and pressure altitudes are not the same. But, if the ambient temperature is equal to +15C/+59F, pressure and density altitude are the same.
Why is it so? Because ambient temperature readings higher than +15C/+59F will decrease air density. Recall, pressure altitude does not correct for temperatures deviations from the standard day. However, density altitude does. Thus, density altitude will be higher than pressure altitude at temperatures above +15C/+59F.
High Density Altitude Practice Questions
Let’s take a look at several questions that often show up on the exam.
How would high density altitude affect the performance of a small unmanned aircraft?
A. No change in performance
B. Increased performance
C. Decreased performance
The first question tests how high density altitude affects the aircraft performance. We know that the higher the density altitude is, the lower the performance of an aircraft is. Thus, the answer is C.
If the outside air temperature (OAT) at a given altitude is warmer than standard, the density altitude is
A. equal to pressure altitude
B. lower than pressure altitude
C. higher than pressure altitude
Temperatures above standard (+15C/+59F) decrease air density and aircraft performance. This also produce density altitude that is higher than pressure altitude. Thus, the answer is C.
What are the standard temperature and pressure values for sea level?
A. 15°C and 29.92″ Hg
B. 59°C and 1013.2 millibars
C. 59°F and 29.92 millibars
You need to identify standard day conditions for this question. They are a temperature of +15C/+59F and air pressure of 29.92” Hg or 1013.2 millibars at sea level. This makes choice A the answer. Other choices mingle measurement units in an incorrect manner.
Which factor would tend to increase the density altitude at a given airport referenced in the weather briefing?
A. An increase in barometric pressure
B. An increase in ambient temperature
C. A decrease in relative humidity
This question wants to identify a correct factor that increases the density altitude. Remember, barometric pressure is inversely related to the density altitude. Thus, choice A is incorrect.
A decrease in humidity decreases the amount of water vapor in the air. This increases the amount of air molecules per fixed volume, leading to higher air density. Furthermore, higher air density leads to lower density altitude. This makes choice C incorrect too.
But, rising ambient temperature leads to lower air density and higher density altitude. This makes B the answer.
What effect does high density altitude have on the efficiency of a small unmanned aircraft propeller?
A. Propeller efficiency is increased
B. Propeller efficiency is decreased
C. Density altitude does not affect propeller efficiency
Question #5 asks you about the effect of high density altitude on drone performance. The aircraft performance decreases as density altitude rises. Thus, the answer to this question is B.
What effect does humidity have on performance?
A. It has no effect on performance
B. It increases performance
C. lt decreases performance
The last question asks you about the relationship between humidity and aircraft performance. An increase in humidity leads to a decrease in air density. This will produce worse aircraft performance. Thus, the answer is C.
This is it for this tutorial. We made many more YouTube tutorials to help you prepare for the FAA part 107 exam. If you have questions about this material, let us know in the comments section.