Density Altitude: Beware of the Heat
Leadville, Colorado. Phoenix, Arizona. Furnace Creek, California.
These are three distinctly different places. Leadville has a harsh winter climate with weather in the negatives and teens for a significant part of the year, while Phoenix is much more temperate in the winter, while Leadville rarely escapes 90-plus-degree weather save for a few months. Their elevations are also vastly different. Leadville is notably the highest-altitude airport in the country at nearly 2 miles from sea level at a staggering 9,934 feet, while Phoenix-Mesa Gateway sits at 1384 feet. Furnace Creek, conversely, sits at negative 210 feet.
What brings these airports together is a phenomenon that plagues nearly every airport in the United States come the summer season: extreme density altitudes. Density altitude is pressure altitude corrected for variations in nonstandard temperature. Just as a soda bottle has pressure variations whether it is closed and shaken or open to ambient pressure, the air molecules in a given amount of space changes too, with the largest variation occurring due to heat.
Hot air rises. When the surface of the earth heats up, so does the air above it, leading to a significantly increased density altitude. For pilots, this incredibly vital information can be an easy-to-skip bit of information tagged onto the end of an automated weather broadcast or a single line in an app like Foreflight or Garmin Pilot. However, its importance cannot be understated on its effect on your airplane. As AOPA puts it, it’s the altitude an airplane “feels” it is flying at.
Performance degradation is a known phenomenon as an airplane is climbing. On a nice, cold day, initial vertical speed can be as high as 1,500 in the average training aircraft, only for a climb to 8,000 feet to take 15-20 minutes as air density decreases and, with it, comes a decrease in vertical speed. The danger is in density altitude, however, when an airplane will climb significantly slower due to heat.
Performance charts take care of this issue. As pilots, we have a legal obligation to ensure our airplane is loaded properly, we have a long enough runway and have considered issues such as obstacle clearance, and that the plane has the performance capabilities for it to do what we need. As one flies their airplane, those book numbers take a real-life hold in our minds, and we know what to expect in certain conditions at certain airfields.
On a high temperature day, especially when flying a fully loaded airplane, take a close second look at the performance charts to ensure you are aware what you are getting into. In the case of Phoenix, Arizona, average density altitudes sit around 2,000 feet in the wintertime, meaning the average Piper Cherokee 180 will climb at a published rate of around 700 feet per minute. On the hottest days of summer, where temperatures can reach as high as 120 degrees or more, density altitudes may reach up to 6,000 feet or more, reducing the published climb rate to less than 400 feet per minute.
In the case of a 70-degree day in Leadville, the density altitude can reach a staggering 13,000 feet. The POH for the Cherokee advises that climbs at this density altitude can be around 100 feet per minute, which is not a great look for an airport surrounded by some of the harshest terrain in the United States.
Takeoff and landing distances and fuel flow are just as affected by density altitude, and for these high-elevation or extreme weather airports, caution must be taken to calculate the numbers properly and set the fuel-air mixture to squeeze the maximum performance out of your engine.
This summer, be mindful when you fly and always conduct a thorough preflight briefing, familiarizing yourself with the numbers you expect your airplane to fly at. For instrument pilots, ensure you can meet required climb gradients for the different departure procedures and missed approaches you may encounter. You can never have enough knowledge before a flight, and if something gives you pause or makes you think you can reconsider, you should double-check your numbers or even delay or cancel the flight. Ensure your aircraft loading is proper for the flight and be aware of the effect weight has on your aircraft and plan accordingly for fuel and passenger loading needs.
As always, err on the side of caution and fly safe, and give these performance thoughts some extra attention this summer flying season.
The numbers in this article are published for example only. This is designed to be an informative article and all preflight action falls on you, as the pilot, to ensure your compliance with regulations and for flight safety.