3 Dangers; High / Hot / Heavy

C172 — N608DL — Santa Ana > (practice area) — Performance T/O, Stalls, Landing — 1.1hrs

One of my ex-military thousands-of-hours pilot friends told me in discussion: "Oh yeah, high, hot and heavy are the 3 dangers in aviation."

It's a question of density altitude, which is directly related to aircraft performance. If you're too high, or it's too hot, the plane won't fly. If you're heavy, it's worse.

I found an article to backup what he was mentioning.
(Revised for length.)

Ref: http://www.nws.noaa.gov/om/aviation/front/12jul-front.pdf
NOAA’s National Weather Service July 2012
By Jim Reynolds, Meteorologist

Hot, High and Heavy—The Deadly Cocktail of Density Altitude

Density altitude really can only be seen through aircraft performance.

Unfortunately, density altitude can have deadly consequences before a pilot has a chance to sense its presence.

By definition, density altitude is “the pressure altitude adjusted for non-standard temperature.” Simply put, increasing temperature at a given atmospheric pressure will cause the air density at that pressure to appear as though it resides at a higher physical altitude.

Pilots fly through an atmosphere of air made up of invisible gases.

Only when there is an excess of particulate matter or water vapor in the air can anything be seen in the flight environment. Because air is otherwise invisible, it is not possible to tell when it becomes thinner.

Thinning happens due to increased space between molecules when an air mass is either warmed, has water vapor added to it, or is raised in elevation. Increased spacing between air molecules, which causes air to become thinner, has the following three effects on aircraft performance:

  1. Slower acceleration on takeoff because of a power production reduction. 
  2. Higher true airspeed required to produce the same lift associated with a lower temperature, which generally requires a longer takeoff roll to achieve.
  3. Slower climb because of the reduction in power production and lift.

Any mix of the atmospheric conditions that cause aircraft to perform at the minimum range of their operating envelope are “high density altitude” situations, and are usually quite dangerous.

(Worse...) Keep in mind that the greater loading of an aircraft by either people or gear will eat away further at the diminished capabilities of the aircraft in high density altitude situations.

It's easy to miss developing high density altitude situations. You might sense an increase in humidity, or mugginess, in your surroundings or observe hazy conditions. You might also get a sense of being at an increased altitude because you find yourself catching your breath when doing tasks that don’t usually cause you to breathe deeply.

These physical signs, however, are not enough for pilots to gain a true understanding of how their aircraft will perform given current conditions.

The only way to truly ascertain how an aircraft will perform is to first compute density altitude according to a chart or a calculator and then correlate this information with aircraft performance data in the aircraft’s operating handbook.

While most density altitude effects are experienced at higher elevations, extremely high temperatures at lower elevations can lead to equally negative aircraft performance problems.

Case in point: high temperatures across portions of the desert southwest can easily exceed 115oF in the summer months. ...a temperature of 120oF will cause most pilots to choose not to fly into or out of the airport until temperatures cool down some.

While the effects of density altitude can be insidious, there are ways to beat this hidden foe.

  • If at all possible, fly early or late in the day. Any reduction in temperature may add hundreds of feet to the elevation the airplane thinks it is operating at.
  • Fly as light as possible. If you don’t need full fuel tanks to reach your destination, take on a lesser amount of fuel, much less if possible. Leave behind all of the baggage you don’t really need.
  • Bring along only those passengers necessary for the trip. High density altitude situations are bad for those individuals that just want to tag along.

In short, don’t let the cocktail of “hot, high and heavy” be hazardous to your health!

Putting the above into a practical example, if you're in a C172, your max Service Ceiling = 13,500ft. That's not very much margin if you're attempting cross any decent mountains.

By the E6B, let's say your mountain to hop over is 8500ft MSL (not so high really), and it's normal pressure day Altimeter 29.92 (which can drift widely), and it's a pretty hot day of 28°C (about 83° F).
Your Density Altitude is already almost 12,000ft, and your plane is struggling as it is, leaving you with only 1,500ft margin before you got nothing to pull you up or forward.

If you need to get any higher, or if that gets any warmer, or the barometer changes much, you're done for.

Or perhaps you gotta jettison your girlfriend out the window.