How To Calculate Density Altitude

In this article I’m going to teach you how to calculate density altitude.

The earthbound among us have a very black-and-white view of altitude. It’s just how high the airplane is flying, right? 

Well, pilots know there’s a lot more to that story! There’s true altitude, indicated altitude, and absolute altitude! Those tell the pilot how high they are above sea level and above obstacles.

But wait, there’s more! Beyond those ideas of how high the plane is actually flying, there’s also altitude as a measure of how high the airplane “feels” that it’s flying.

You see, air pressure is always changing due to the weather, and it’s the air pressure—not the height above sea level or any objects—that will tell you how well (or how badly!) your airplane will fly.

So let’s look at the two types of air pressure-related altitudes pilots deal with every day.

Step 1: Pressure Altitude

Pressure altitude is the altitude that an airplane flies if they just ignore the physical earth and just look at the air pressure. Instead, they base their altitude on a random, standardized pressure. 

Standard pressure is defined as 29.92 inHg or 1092 mb. 

Setting your altimeter’s Kolsman window (adjustment window) to 29.92 instead of your current field setting will show you your pressure altitude. If you’re sitting on the ground near sea level and the pressure is high (above 29.92), then your altimeter will display a negative number.

Another time you set 29.92 into the window is when you fly above 18,000 feet.

This is known as the “transition level” in the US, and above this altitude, everyone has a 29.92 set, and everyone is flying a pressure altitude.

To show that it’s different than a regular altitude, airplanes are assigned and fly “Flight Levels” in hundreds of feet. For example, instead of saying you’re 30,000 feet above 29.92, you would say you’re at FL300 (“flight level three zero zero”). 

Step 2: Density Altitude

There are other factors affecting the airplane’s performance, however.

Beyond air pressure, temperature makes a huge difference. Hot air is less dense; therefore, the airplane’s wings, engine, and propeller will produce less lift, power, and thrust, respectively. 

If you correct the pressure altitude for non-standard temperature, you’ll get the density altitude. 

Standard temperature is 15º Celsius or 59º Fahrenheit. If cooler than this, the density altitude will be less than the pressure altitude. If it is hotter, the density altitude will be higher. The higher the density altitude, the higher your plane “feels” and the worse its performance will be.

Moisture in the air, measured in humidity, also affects density altitude.

The more water vapor, the less air there is to make lift, power, and thrust. This is most important when calculating engine power, so most pilots disregard it for day-to-day flying.

Most aviation calculators and tables only correct for non-standard temperatures, but you may find more accurate tools online that also consider humidity. NOAA’s weather.com density altitude calculator is one such example. 

Remember, high density altitudes are bad for aircraft performance. 

Calculating Density Altitude

There are three ways pilots might go about finding the present density altitude. All of them require first knowing what your pressure altitude is. 

Electronic Calculators, Apps, and Online Tools

We live in the digital age, so most of us can easily find a calculator to think for us.

Pilots can’t always rely on this trick unless they have an old-school electric pilot calculator in their flight bag.

For flight planning, your apps will work fine, but come checkride day, the examiner might want to see that you’re ready for the inevitable battery failure. 

Manual E6B Flight Calculator

If you have a manual “whiz wheel” E6B, you can calculate an approximate density altitude in a few seconds.

Find the instructions for doing so toward the center of the calculator side. It’s a simple matter of lining up your pressure altitude with the current temperature.

If you want to pick up an E6B check out our article on the best flight computers.

Tables

Some POHs/AFMs include a conversion table at the performance section’s beginning. Like the E6B, this is a quick and fool-proof way to do the math. 

Alternatively, some graph-style performance charts have you start with pressure altitude. Then, the first correction is for temperature.

This relieves you of the need to do the math yourself since it’s built-in to each performance chart in your POH.

Density Altitude Formula

Finally, you can find density altitude on your own with nobody’s help. It just requires a little arithmetic. Here are the formulas for pressure and the density altitude formula.

Pressure Altitude Formula

PA= (29.92 – press) * 1000 + alt

PA=pressure altitude

press=current sea level pressure

alt=present true altitude or field elevation

Density Altitude Formula

DA = PA + 120 * (OAT – 15)

DA=density altitude

PA=pressure altitude

OAT=outside air temperature in degrees Celsius

Example of Calculating Density Altitude

You’re calculating your takeoff performance for a flight out of the Aspen, CO, airport on a hot summer day. Field elevation is 7,837 feet, and the ATIS reports an altimeter of 30.05 and a temperature of 28. 

See if you can calculate the values using the formulas above. Then, take out your POH/AFM and see how much runway your plane would need to depart at MTOW. Can you do it on the airport’s 8,000′ airstrip? If you can, what would your climb rate be upon departure?

Answer for Aspen Example:

PA = 7,707 feet

DA = 9,267 feet
Reference: FAA’s Pilot’s Handbook of Aeronautical Knowledge, Chapter 4: Principles of Flight

Density Altitude’s Impact on Aircraft Performance

Density altitude affects many aspects of how planes fly. It changes takeoff and landing distances, lift generation, and engine output. These factors combine to alter overall aircraft capabilities.

Takeoff and Landing Considerations

High density altitude makes takeoffs and landings trickier. Your plane needs more runway to get airborne. The thin air provides less lift under the wings. This means you’ll need a faster ground speed to take off.

Landing distances increase too. Your plane carries more speed on approach due to the thinner air. Brakes and reverse thrust are less effective. Plan for longer landing rolls.

Be extra careful at high elevation airports on hot days. Denver’s airport, for example, can have a density altitude over 8,000 feet in summer. This greatly impacts your plane’s performance.

Aerodynamic Performance

As density altitude rises, your plane’s aerodynamic performance suffers. The thinner air reduces lift. Your wings can’t grab as much air to create upward force.

Climb rates decrease. You’ll notice slower acceleration and reduced maneuverability. Stall speeds increase, shrinking your safe flight envelope.

At very high density altitudes, some aircraft may struggle to climb at all. Be aware of your plane’s limits. High density altitude conditions increase risks for pilots and passengers.

Engine Power and Efficiency

Your engine loses power as density altitude increases. Less oxygen means weaker combustion. Fuel-injected engines handle this better than carbureted ones.

Propeller efficiency drops in thinner air. The prop can’t “bite” as much air. This further reduces your thrust.

These factors combine to lower your climb performance and cruise speed. Your fuel efficiency may suffer too. Plan your flights carefully in high density altitude conditions. You may need to reduce payload or make fuel stops on longer trips.

Practical Aviation Applications

Density altitude affects aircraft performance in crucial ways. Pilots must account for it when planning flights and operating aircraft.

Flight Planning and Safety

You need to consider density altitude when planning flights. It impacts takeoff distance, climb performance, and landing distance. On hot days, you may need a longer runway to take off safely.

As mentioned above, high density altitude reduces engine power and propeller efficiency. This means your aircraft climbs slower. You should plan for lower climb rates and may need to use different departure procedures.

When landing, your true airspeed will be higher than normal. This requires longer landing distances. Be prepared to go around if you can’t land safely in the available runway length.

Calculating density altitude before each flight is key for safety. Use charts in your aircraft manual or an electronic flight computer to determine density altitude effects.

Rate of Climb Adjustments

Your aircraft’s rate of climb decreases as density altitude increases. You need to adjust your expectations and procedures accordingly.

At high density altitudes, plan for:

• Slower climb rates
• Lower service ceiling
• Reduced aircraft performance

To compensate:

• Use full throttle for climbs
• Maintain best rate of climb airspeed precisely
• Lean the mixture for maximum power

Be aware that your aircraft may not be able to outclimb terrain or obstacles in high density altitude conditions. Plan alternate routes if needed.

Use your flight computer to calculate expected climb performance. This helps you make safe decisions about terrain clearance and obstacle avoidance.

Tools and Technologies for Measurement

Pilots use several key tools to measure and calculate density altitude. These instruments help ensure safe flight operations by providing accurate data on atmospheric conditions.

Understanding Altimeter Settings

Your altimeter is crucial for determining density altitude. Set it to the local barometric pressure to get pressure altitude. This reading forms the basis for density altitude calculations.

Altimeter settings change with weather. Always get the current setting before takeoff. A 1-inch change in pressure equals about 1,000 feet of altitude.

Remember: As pressure drops, your true altitude increases even if your altimeter shows no change. This affects your plane’s performance, especially at high elevations or on hot days.

Using a Flight Computer

We’ve already addressed flight computers above but as a reminder, a flight computer, or E6B, is essential for quick density altitude calculations. It combines pressure altitude and outside air temperature (OAT) to give you density altitude.

To use it:

1. Set pressure altitude on the outer scale
2. Align OAT on the inner scale
3. Read density altitude where the two meet

Electronic E6Bs make this process even simpler. Just input the data and get instant results.

Flight computers also help convert indicated airspeed (IAS) to true airspeed (TAS). This matters because your plane performs based on TAS, not IAS.

For a quick estimate, you can use a density altitude chart. These visual aids give you a rough idea without complex calculations.

Adapting to Atmospheric Conditions

Pilots need to adjust their flying techniques based on changing air density and weather patterns. These adaptations ensure safe and efficient flight operations in various atmospheric conditions.

Lapse Rates and Atmospheric Layers

The temperature changes with altitude affect air density and aircraft performance. You’ll encounter different lapse rates in each atmospheric layer.

In the troposphere, temperature typically decreases by 2°C per 1,000 feet of altitude gain.

This cooling affects engine power and lift. Your aircraft will perform differently at various altitudes due to these changes. Be prepared to adjust your throttle and pitch to maintain desired performance.

The stratosphere has a more stable temperature profile. When flying at higher altitudes, you’ll experience less temperature variation. This can lead to more consistent aircraft performance over long distances.

Real-Time Weather Adaptations

Weather conditions play a crucial role in flight planning and execution. You must constantly monitor and adapt to changing atmospheric pressure, temperature, and humidity.

High temperatures and humidity increase density altitude, reducing aircraft performance. In these conditions, you may need longer takeoff runs and climb at shallower angles. Plan for reduced climb rates and lower cruise speeds.

Wind patterns also require adaptation. Headwinds will slow your ground speed, while tailwinds can increase it. Adjust your flight plan and fuel calculations accordingly.

Crosswinds may necessitate crab angles or slip techniques during landing.

Geographical and Seasonal Variations

Density altitude changes based on where you are and what time of year it is. These factors affect air density and can impact aircraft performance.

Elevation and High Terrain

Your altitude above sea level greatly affects density altitude. As you climb higher, the air gets thinner. At 18,000 feet, the air is about half as dense as at sea level. This means your plane needs more runway to take off and climb.

High terrain poses unique challenges. Mountain ranges create complex weather patterns that change air density. Valleys can trap warm air, increasing density altitude.

Remember, your altimeter shows height above mean sea level (MSL). But density altitude is what your plane “feels.” In high terrain, true altitude (height above ground) may be low while density altitude is high.

Seasonal Fluctuations in Air Density

Air density changes with the seasons. Hot summer days mean lower air density and higher density altitude. Cold winter air is denser, lowering density altitude.

Seasonal temperature patterns vary by region. Coastal areas often have milder shifts due to ocean influence. Inland areas can see bigger swings between summer and winter.

Humidity also plays a role. Moist air is less dense than dry air. So on a humid summer day, density altitude can be even higher than temperature alone suggests.

Always check density altitude before flight. It’s key to safe operations, especially in summer or at high-elevation airports.