Answers for Pilots

High, hot, and humid

July 1, 2002

Flying in high density altitude

It's summertime and the living is easy — but your flight might not be. That picture-perfect blue sky and shimmering sun are beguiling. They harbor a surprising hidden danger.

"I tell my students that high density altitude is an invisible hazard," says David Schumacher, AOPA aviation technical specialist and a CFII. "A day with no clouds and high temperatures presents an unseen hazard. If a pilot is not careful, this kind of day could present a serious situation. Hot, high, and humid weather conditions can change a routine takeoff or landing into an accident in no time."

So what exactly is high density altitude? Essentially it means that air pressure decreases as temperature and/or altitude increases, says Schumacher, a former engineer. As air molecules are heated they tend to become excited, producing a lot of kinetic energy resulting in less dense conditions. High density altitude should actually be called high-altitude density.

The result then is lowered aircraft performance. Heat fools aircraft into thinking they are higher than they really are, causing takeoff and landing distances to be longer, reducing climb rates, and lowering service and absolute ceilings. Humidity is not generally considered a major factor in density altitude computations because the effect of humidity is related to engine power rather than aerodynamic efficiency.

"Pilots flying on hot days should consult the airplane's POH to calculate affected landing and takeoff distances," advises Schumacher. Clues that you should look for when considering the effect of high density altitude include: the temperature is above standard temperature for the elevation of your airport; the airplane is performing sluggishly and doesn't "feel right"; you are sweating; none of the locals are flying.

So while Leadville, Colorado, may have the nation's highest public-use airport at 9,927 feet msl, the effects of high density altitude can be felt at your local airport — whether in pancake-flat Florida or bucolic West Virginia if the weather conditions are just so.

As an AOPA member, you have access to the best resource anywhere for information and answers for pilots. AOPA provides information for its members through a vast array of communications technologies. You can reach experts in all fields of aviation via AOPA Online ( www.aopa.org/members/), the AOPA Pilot Information Center (800/USA-AOPA), and e-mail ( inforequest@aopa.org). Aviation technical specialists respond promptly to member requests while AOPA Online provides members with access to information and resources 24 hours a day, seven days a week. The toll-free AOPA Pilot Information Center gives you direct access to specialists in every area of aviation. The center is available to members from 8:30 a.m. to 6 p.m. Eastern time, Monday through Friday.

AOPA Web resources

These articles, written by AOPA Pilot magazine writers, provide answers to frequently asked questions. The AOPA Aviation Services department (800/872-2672) answers more than 100,000 member calls a year from members needing assistance for a variety of aviation-related issues.
www.aopa.org/members/files/pilot/

"Into Thin Air: Mountains Are Not a Requirement for High Density Altitude," by Michael Maya Charles. Explores the reasons accidents happen in high density altitude conditions.
www.aopa.org/members/files/pilot/2000/thin0007.html

"Hot Times: The Good, Bad, and Ugly of Hot-Weather Flying," by Thomas A. Horne. Discusses the effects of high temperatures on summer flying.
www.aopa.org/members/files/pilot/1994/hot9407.html

"Dealing With Density: Performance Takes a Pounding With Summer's Heat," by Marc E. Cook. Explores how aircraft performance is affected by high density altitude.
www.aopa.org/members/files/pilot/1997/dens9706.html

"Ounce of Prevention Stay Focused: Plan the Takeoff — and Take Off According to the Plan," by Julie K. Boatman. Addresses how high density altitude conditions affect the takeoff roll.
www.aopa.org/members/files/pilot/2001/ounce0106.html

"Proficient Pilot: Landing Expectancy," by Barry Schiff. In his monthly column, Schiff discusses the effects of high density altitude on landings.
www.aopa.org/members/files/pilot/1998/prof9808.html

"Safety Corner: Hazy Crazy Days of Summer," by Richard L. Collins. Suggestions on how to plan for flying trips in high density altitude conditions.
www.aopa.org/members/files/pilot/1991/sc9106.html