Density Altitude Revisited

I first wrote about density altitude in my June 16, 2017 post. If you’re not familiar with the concept, I highly recommend reading that earlier post before proceeding. It’ll be easier to understand the graphic. I only learned about this phenomenon after moving to Arizona back in 1978. It was a hot day in June and Sky Harbor International Airport was considering cancelling some flights because of something called density altitude.

Here’s a brief synopsis: When air gets hotter it also gets less dense, just like if you were at a higher altitude. The graphic show the numbers. Less dense air, all things being equal, generates less lift when flowing over an aircraft’s wings. That means you need more speed to take off. And that means you need a longer runway to reach that speed.

The same problem occurs when trying to land. The aircraft has to come in “hotter” (at higher speed) to maintain lift, and that means you need a longer runway to slow down. You can’t just stomp on the brakes. The landing gear “might” take the stress, but the passengers wouldn’t like it.

Therein lies the problem. Airports already at high altitude need longer runways. Denver International Airport at 5430 feet has five runways that are 12,000 feet long and one measuring 16,000 feet — the longest commercial runway in North America. Average runway lengths are 8,000–13,000 feet. Of course, when airports are planned and built, things like altitude and average temperature are taken into account. But as the climate warms more airports are starting to see density altitude issues for flights.

Not that the aviation industry needs more problems, but I’ve already heard of passengers being bumped from flights that were too heavy given the density altitude. I’ve also heard of flights being rescheduled to cooler parts of the day, yet another inconvenience for travelers. FYI, scheduling flights for cooler parts of day has been routine in the Middle East for years.

There are no easy solutions. Most commercial airports are located in cities where extension of the runway is either impossible or cost-prohibitive. Sure, you could make the aircraft wings larger, or increase the power of the engines, or lighten the craft by replacing aluminum components with carbon fiber composites, but none of those are easy retrofits. It may well take a new generation of aircraft to compensate for the global increase in density altitude.

Hot tip for readers: If you intend to travel by air during the summer, schedule your flights for earlier or later in the day. You’ll be less likely to suffer the consequences of getting bumped or rescheduled.

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