"Density Altitude" represents the altitude in the International Standard Atmosphere (ISA) where the air density would be the same as at the actual location, considering temperature and pressure variations. In simpler terms, density altitude is the altitude at which the aircraft "feels" as if it's flying, concerning its performance, rather than its actual physical altitude above sea level.
What does it affect? Think CALT: Climb Rate, Airspeed, Landing distance, Take-off distance. Climb rate will decrease as the temperature increases. The true airspeed will be higher due to increased temperature as well. Landing distance & Take-off distance will also increase incrementally with temperature. These combined is the perfect recipe for a disaster, but there is a way to avoid this:

Now, this is how I teach students in the 14 DAY ACCELERATED PRIVATE PILOT TRAINING PROGRAM by 14DAYPILOT Flight Academy on how to calculate density altitude and avoid disaster:

The density altitude (DA) is calculated using the following formula:

DA = PA + [120 x (OAT - ISA)]

Where:
- PA is the pressure altitude (the altitude corrected for non-standard pressure).
- OAT is the outside air temperature in degrees Celsius.
- ISA is the International Standard Atmosphere temperature for the given altitude.

To calculate density altitude, we first need to determine the pressure altitude. This can be done using a standard altimeter setting or by using actual pressure readings and a conversion chart.
Let's take Lake Tahoe as an example. The elevation of Lake Tahoe Airport (KTVL) is approximately 6,264 feet (1,909 meters) above sea level.

1. Determine the pressure altitude (PA):

- Check the current altimeter setting (local barometric pressure) at Lake Tahoe Airport or nearby. Let's say it's 28.50 inches of mercury (Hg).
- Convert this setting to inches of mercury: 28.50 inHg.
- Use this value to find the pressure altitude using a conversion chart or formula. For simplicity, you can use an online calculator or chart. Let's assume the pressure altitude is calculated to be 7,000 feet.

2. Determine the temperature:

- Find the current outside air temperature (OAT) at Lake Tahoe. Let's say it's 15°C.

3. Determine the International Standard Atmosphere (ISA) temperature for 7,000 feet:

- The ISA temperature decreases by approximately 2°C for every 1,000 feet increase in altitude. At 7,000 feet, the ISA temperature would be around 1°C.

4. Calculate the density altitude (DA):

- DA = 7,000 + [120 x (15 - 1)]
- DA = 7,000 + [120 x 14]
- DA = 7,000 + 1680
- DA ≈ 8,680 feet

In this example, this means that despite being at a physical altitude of 6,264 feet, the aircraft will perform as if it were at an altitude of 8,680 feet due to the combination of temperature and pressure effects on air density. Pilots must consider density altitude carefully to ensure safe take-offs, landings, and overall flight performance, especially in areas with high elevations like Lake Tahoe.

Now that we know the density altitude, let’s do the performance calculations for your flight back from Lake Tahoe with 2 friends. You’ll be flying the Piper Warrior II, we’ll do the Weight & Balance first. With 2 passengers, and 100 lbs of combined luggage & camp gear this is how it looks:

As you can see, you’ll be very close to your Maximum Takeoff Weight & on the limits of the Center of Gravity for the aircraft. Combined with the density altitude, this will affect the performance of the aircraft significantly.

Now, let’s do your takeoff calculations. You’re going to be on max takeoff weight, with a temperature of 15 degrees C, pressure altitude of 7000 ft, and density altitude close to 8680 feet. The performance charts for the Piper Warrior II are quite easy to use, you have to draw lines based on the information that you currently have, let’s take a look:

First, you’ll take the OAT of Lake Tahoe which is 15 degrees C, draw a line upwards to the 7000 ft pressure altitude, then draw a line upwards from your takeoff weight of 2438 lbs, then another line horizontally to connect both lines. At the point they are connected, you then draw another horizontal line all the way towards the right to find out takeoff ground roll is 2550 ft. And this is being optimistic, assuming there are no wind activities in Lake Tahoe! Next, you’ll have to do the climb rate calculations to 10,000 Ft to clear the mountains around Lake Tahoe.

Same concept here, you’ll first draw a line upwards from 15 degrees C to our pressure altitude of 7000 Ft. Then horizontally to meet the reference line, and we get a climb rate of 220 Feet per Minute (FPM). Would you look at that! While we are able to takeoff from Lake Tahoe, it would certainly be hazardous to try clearing the mountains if we’re climbing at 220 FPM. This would immediately set off red flags for me to not do this flight & wait for better weather to get better performance before flying back. As the Pilot-in-Command (PIC) of this flight, I’ve made a decision that this flight is a NO GO due to poor climb performance. But let's take a look at the Time, Fuel and Distance to Climb chart:

Same concept, but here we need to do 2 colored lines: First line (Red) is the departure airport calculation, Second line (Green) is for the cruising altitude. Let's do the departure airport first. I would first draw a line vertically from the 15 degrees C line since that is our temperature for Lake Tahoe, then meet the pressure altitude line of 7,000 Ft. After that, draw a horizontal line until the line intersects the Fuel, Time & Distance lines respectively. Mark the points at which our horizontal line intersects with the Fuel, Time & Distance lines, draw a line straight down to get the results. Do the same process for our cruising altitude of 10,000 Ft.

Now we got 6 numbers:

Departure Airport:

Fuel : 4 Gallons
Time : 20 Minutes
Distance : 28 NM (Nautical Miles)

Cruising Altitude:

Fuel : 6 Gallons
Time : 31 Minutes
Distance : 44 NM

Now we have to subtract the Departure Airport numbers from the Cruising Altitude numbers.

Fuel : 6 Gallons - 4 Gallons = 2 Gallons
Time : 31 Minutes - 20 Minutes = 11 Minutes
Distance : 44 NM - 28 NM = 16 NM (Nautical Miles)

Based on this chart, it will take us 11 minutes to reach our cruising altitude of 10,000 Ft from the departure airport pressure altitude 7,000 Ft, spend 2 Gallons of fuel during the climb, and take 16 NM of distance.

While there are many other performance calculations that needed to be done before the flight, looking at the climb performance of 220 FPM would already make me stop right there and wait for better weather. It would be hazardous to the safety of this flight, and jeopardizing the safety of me & my friends would be the last thing I’d like to do.

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