WX BRIEF

Density altitude

Hot and high factors shorten runways with summer and mountain flying.


Anticipating and adapting

It is a pretty safe bet that density altitude corrections will need to be taken into account any time you find yourself flying into or out of a high-altitude airport in the daylight of a hot summer day. But density altitude affects just about all airports, including coastal ones right at sea level.

If you normally eat up 80% of the short runway at your local coastal strip and a heat wave strikes, you may not get off the ground without first reducing your takeoff weight. If you fly out of certain airports regularly, committing the ISA temperature for the field elevation to memory is not a bad idea.

It will come in handy time and again because you can quickly compare the heat you see in the terminal aerodrome forecast (TAF) with that ISA temperature to forecast a likely density altitude scenario.

If your memory or math skills are not as keen as you’d like, you can also plug the TAF temperature and pressure information into a number of online calculators (such as eustis. army.mil/weather/weather_products/wxconversions.htm#dpa) via your webphone or FBO computer terminal.

For most pilots, density altitude translates into increased takeoff lengths. But at large airports with seemingly infinite runway lengths, is density altitude still a factor? The answer is yes, but in a different way. Aircraft that operate at major airports, such as large cargo planes, are often loaded to MTOW.

As density altitude increases, MTOW must be reduced accordingly to ensure enough lift at the prescribed V speeds. Without that lift, pilots must guess at what higher speed is appropriate for rotation, essentially becoming test pilots.

It happens that high altitudes tend also to have relatively dry air, so things normally cool down quickly as night falls. This is at least one of the reasons that many of the world’s large high-altitude airports—such as UIO (Quito, Ecuador) at 9228 ft, DEN (Intl, Denver CO, USA) at 5431 ft and NBO (Jomo Kenyatta, Nairobi, Kenya) at 5330 ft MSL—have significant night-time operations.

Finally, just because you’ve taken off successfully does not mean you’re out of the woods. Remember that density altitude describes the aircraft’s performance. If your aircraft’s service ceiling is 14,000 ft, you will not clear that 12,000-ft ridge with a 3000-ft density altitude addition.

As a pilot, the best way to deal with unfavorable density altitudes is to truly understand the operational limitations of your aircraft and not allow your situation to force you outside that envelope as “Tom” did. In most instances, a high density altitude will just mean a longer takeoff and landing roll.

Less frequently, it may mean carrying less fuel or restricting cargo weight. Occasionally you may have to delay your departure until things cool off, or perhaps make 2 trips to carry everyone and their cargo. In the rarest situations, you may not have enough runway to take off, regardless of weight, even if you’re just carrying minimum fuel to get you to the next airport.

In these cases, the only option is to relax, find a cold drink or air conditioned FBO and tend to your passengers until the sun goes down.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg Uni­versity. Shein holds a commercial license with instrument rating.

 

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