What is Density Altitude?

Density altitude is pressure altitude corrected for non-standard temperature. In simpler terms, it is the altitude at which the airplane "feels" like it is flying.

If the density altitude is high, the air is thin, and your aircraft performs as if it were at a higher physical altitude, even if you are at sea level.

The Formula

The standard formula used in aviation is:

DA = Pressure Altitude + [120 x (OAT - ISA Temp)]

• DA: Density Altitude
• OAT: Outside Air Temperature
• ISA: International Standard Atmosphere (15°C at sea level, decreasing 2°C per 1,000 ft).

Too complex? Use the free Pilot One Density Altitude Calculator to get an instant result based on your current METAR.

Why is it Dangerous?

High density altitude drastically reduces aerodynamic performance in three ways:

1. Engine: The engine takes in less air, producing less power (for non-turbocharged engines).
2. Propeller: The propeller has less air to "bite" into, producing less thrust.
3. Wings: The wings generate less lift due to fewer air molecules moving over the surface.

Performance Impact

• Takeoff Roll: Increases significantly. A 10% increase in DA can increase takeoff distance by 10-12%.
• Climb Rate: Drastically reduced. You might struggle to clear obstacles.
• Landing Speed: While your Indicated Airspeed (IAS) remains the same for approach, your True Airspeed (TAS) and Ground Speed will be much faster, requiring more runway to stop.

When to Be Careful ("High, Hot, and Heavy")

You should always check density altitude, but it is critical when:

• High: You are at an airport with high elevation.
• Hot: The temperature is significantly above standard (ISA).
• Heavy: Your aircraft is near maximum gross weight.

Real World Example

Imagine taking off from Madrid (LEMD) which is at ~2,000 ft elevation.

• Winter (5°C): The air is dense. Performance is good.
• Summer (35°C): The high temperature makes the air thin. The Density Altitude might effectively be 4,500 ft! Your Cessna 172 will perform as if it's taking off from a mountain, not a plateau.