Density Altitude: The Secret Killer

Now that the warm season is approaching, pilots need to start planning for the secret killer, namely, density altitude. In fact, density altitude (DA) is perhaps just as hazardous as airframe icing. In an NTSB study, density altitude contributed to just as many accidents as icing shown in the pie chart below. To be fair, some of the accidents in the NTSB study that were attributed to density altitude were caused by pilots departing in an over-gross weight aircraft or using improper procedures (e.g., improper flap usage). However, every pilot needs to be aware that gross mistakes such as this are not forgiving when the density altitude is high.

NTSB-Wx-Accident-Study

What is Density Altitude?

In simple terms, density altitude is pressure altitude corrected for non-standard temperature. Therefore, if the pressure and temperature throughout the atmosphere matches the standard, then pressure altitude and density altitude are equal. Of course, during the warm season, the temperature is generally above standard in most locations throughout the U.S. creating an invisible hazard if ignored.

If you’ve ever flown out of a high-elevation airport such as Santa Fe, New Mexico (6,348 feet MSL), you have experienced the effect of density altitude. At these airports, the performance of the aircraft decreases. For fixed-wing aircraft you will experience a longer takeoff distance, longer landing rollout and reduced rate of climb. The higher you are above sea level, the lower the pressure and that means the air is less dense. This reduction in air density reduces the wing’s lift and also lowers the efficiency of the propellor or rotor.

What About Temperature?

The temperature of the air can have the same effect even at sea-level airports. As daytime temperatures begin to creep up into the upper 80s and 90s, the air becomes much less dense similar to being at that high-elevation airport. That means there are fewer air molecules in a given volume of airspace. Less air, means poor aircraft performance similar to what was described above. And of course, the combination of high temperatures at high-elevation airports can make for a serious hazard waiting to happen if unchecked.

While a pilot can determine pressure altitude in the cockpit by setting the altimeter to 29.92″, there’s no handy-dandy instrument that you can use to directly measure the density altitude. It must be calculated based on the station’s pressure (not the altimeter setting), temperature and dewpoint temperature. Not to worry, the ForeFlight Mobile app does all of the work for you.

Finding Density Altitude in ForeFlight

To find the density altitude simply select an airport from the Maps view. In the pop-over window, tap the METAR tab at the bottom and scroll down a bit to see the current density altitude for the chosen airport.

Density-AltitudeJust like any other aspect of weather, pilots need to prepare in advance. Before you close the door to the cockpit, double-check the density altitude in ForeFlight to be sure it won’t adversely affect your flight. To learn more about density altitude, please refer to this FAA safety publication.

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