Density Altitude

AltimeterTable of Contents

Importance to Members

Overview

Technical Information

Additional Resources

From the AOPA Archives

Table of Contents

Importance to Members

Throughout a pilot's flight training, there is instruction, and likely some experience of the detrimental effect high density altitude has on aircraft performance. But when the sky is blue and the summer sun is hot, even seasoned pilots can forget to carefully calculate takeoff, climb, cruise, and landing performance during preflight planning, sometimes resulting in accidents.

Density altitude is often not understood and its effects on flight can be unanticipated, resulting in takeoff and landing accidents. This subject report explains what density altitude is and briefly discusses how it affects flight. Further information, including the Air Safety Institute's free online interactive Mountain Flying course, is listed at the end of the report.

Please call AOPA’s Pilot Information Center with questions – 800-USA-AOPA (872-2672)  Monday through Friday, 8:30 to 6:00 ET.

Overview

In order to protect ourselves from the effects of density altitude, we must first understand what it is and how it impacts flight. This subject report defines and discusses density altitude, includes the formula used in calculating density altitude and provides a list of flying tips for safer operations in high density altitude conditions.

What Is Density Altitude?

Density altitude is pressure altitude corrected for nonstandard temperature. As temperature and altitude increase, air density decreases. In a sense, it's the altitude at which the airplane "feels" its flying.

How Will High Density Altitude Affect Flight?

On a hot and humid day, the aircraft will accelerate more slowly down the runway, will need to move faster to attain the same lift, and will climb more slowly. The less dense the air, the less lift, the more lackluster the climb, and the longer the distance needed for takeoff and landing. Fewer air molecules in a given volume of air also result in reduced propeller efficiency and therefore reduced net thrust. All of these factors can lead to an accident if the poor performance has not been anticipated.

Technical Information

T ips for Flying in High Density Altitude Areas            

  • Fly in the evening or early in the morning when temperatures are lower.
  • Call a local instructor at your destination airport to discuss density altitude procedures at that airport.
  • Before flying to a high-elevation airport, know whether your aircraft climbs more efficiently with the first increment of flaps. Many aircraft do, but results vary and that first notch of flaps may add more drag than lift.
  • Be sure the aircraft's weight is below 90 percent of maximum gross weight.
  • Don't fill the tanks to the top (see previous tip).
  • Fly shorter legs and make extra fuel stops (tough suggestion to accept, but it results in less exciting takeoffs).
  • Be ready to ferry one passenger to an airport with a lower density altitude, then come back for the other. If you are unsure of conditions, fly around the pattern once alone without baggage to test your aircraft's performance.
  • Have 80 percent of your takeoff speed at the runway's halfway point, or abort. That means having 48 knots IAS in a Cessna 172 at the halfway point.

Calculating Density Altitude

Density altitude in feet = pressure altitude in feet + (120 x (OAT - ISA temperature))

  • AltimeterPressure altitude is determined by setting the altimeter to 29.92 and reading the altitude indicated on the altimeter.
  • OAT stands for outside air temperature (in degrees Celsius).
  • ISA stands for standard temperature (in degrees Celsius).

Keep in mind the standard temperature is 15 degrees C but only at sea level. It decreases about 2 degrees C (or 3.5 degrees F) per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C (or 34 degrees F).
For example, the density altitude at an airport 7000 feet above sea level, with a temperature of 18 degrees Celsius and a pressure altitude of 7000 (assuming standard pressure) would be calculated as follows.

  • 18 – 1 = 17
  • 17 x 120 = 2040  
  • 2040 + 7000 = 9040 feet Density Altitude

This means the aircraft will perform as if it were at 9,040 feet.

Additional Resources

Mountain Flying Course
Air Safety Institute's interactive online course on mountain flying

Density altitude high? Know before you fly 
July 2007

Flying Seasons: Density Altitude
It isn't just for mountains anymore

Proficient Pilot
Density-altitude discussions

Answers for Pilots
Don't sweat it: Keep high density altitude from bringing you down

From the AOPA Archives

Ups and Downs of Takeoffs and Landings

Air Safety Institute Quiz on Density Altitude

Safety Hot Spot: Summer Weather
July 2006

Don't fight the charts
On paper, this flight was a bad idea
AOPA Flight Training, December 2005

The Weather Never Sleeps
The air up there: The basics of gas density
AOPA Flight Training, July 2005

Never Again: Incident at Sullivan Lake
AOPA Pilot, August 2004

Instructor Report: Simulated Mountains
AOPA Flight Training, February 2003

Answers for Pilots: High, hot, and humid
Flying in high density altitude
AOPA Pilot, July 2002

Flying Smart: Aviation Speak
AOPA Flight Training, July 2002

Hazards of Hot and High: Density Altitude and High Terrain Don't Mix
AOPA Flight Training, April 2002

Flying Safe: Accident Analysis — Density Altitude
AOPA Flight Training, August 2001

Into Thin Air: Mountains Are Not a Requirement for High Density Altitude
AOPA Pilot, July 2000

Flying Safe: Instructors Tips — Teaching High Density Altitude
AOPA Flight Training, June 1999

Flying Safe: The Weather Never Sleeps — Airplane Heat Exhaustion
AOPA Flight Training, June 1999

Dealing With Density: Performance Takes a Pounding with Summer's Heat
AOPA Pilot, June 1997

Flying Safe: The Weather Never Sleeps — Density Altitude
AOPA Flight Training, September 1996

Flying Safe: The Weather Never Sleeps: DA Surprise
AOPA Flight Training, September 1995

Mountain Flying: A Primer for High Country Aviation
AOPA Flight Training, January 1993