Questions from the Cockpit: The lower the better?

Jasmine, a private pilot from Maine, asks: What’s the lowest density altitude ever recorded? And just how well would a plane perform in those conditions? 

What is density altitude? It’s slang for how thin the air is. As we go up in altitude, air molecules socially distance, reducing the air pressure. This has the practical effect that there’s “less” air to breathe in Denver than in Boca Raton.

This thin air affects more than just the pilot. The airplane’s engine needs to breathe, too, so when the air is thinner it develops less power. At the same time, the prop generates less thrust because there is less air to move, and the wings produce less lift.

A glance at any aircraft’s takeoff chart shows that performance drops with altitude — and the reason for this is the air density. The higher you go, the more runway you need to takeoff and the slower you climb. That’s why airports at higher altitudes have longer runways.

But here’s the cool thing: The air can also get thinner without leaving Boca Raton. Just add heat.

Actually, I guess I should have said, here’s a hot tip, rather than “here’s the cool thing.”

Moving on… Increasing air temperature also increases the social distancing of the air molecules. Hot air expands. It thins. Being warmer in any given place is the same as being at a higher altitude, at least when it comes to aircraft performance.

And that’s what density altitude is: It’s a way of quantifying temperature-induced air thinness in an easy to understand way. We simply report density altitude to a comparable real-world altitude on a standard day.

For example, Boca Raton’s airport has a field elevation of 13 feet above sea level. On a hot day, however, the air can be as thin as an airport at 2,235 feet on a “standard day.” So the density altitude would be reported at 2,235 feet, and the plane — although physically located at 13 feet — will perform as if it really was at 2,235 feet. While that’s no big deal when it comes to aircraft performance, consider what adding a couple of thousand feet of altitude in Denver could do. 

Of course, density altitude is actually a bit more complicated than that, and I’m sure that the Aeronautical Science Police will roast me in comments, but simplicity has its beauty, too.

Just know that, among other things, humidity has an important role to play in air density as well, so much so, in fact, that student pilots are taught to be alert to “hot, high, and humid.” Any of the three will reduce performance. Start mixing them, and things get hot in the cockpit, too. 

Of course, Jasmine is wondering about the other side of the coin, when density altitude is low, not high.

Vaccinated by cold, dry conditions, the air molecules abandon social distancing and pack in for a massive group hug. Now the air is “thicker.” It becomes a dense blanket supporting the wings. The engine has more air to drink, while the prop really has something to bite into. Cold air is a free —and delightful — performance upgrade package for your airplane.

As a standard day is defined as 59° Fahrenheit, it really doesn’t take much to effectively lower the elevation of an airport. Northern seacoast airports often have negative density altitudes in the winter.

So, what’s the all-time lowest negative density altitude ever recorded? 

Wilbur Wright, co-father of the airplane, when looking for a spot to test fly “the machine,” wrote to the National Weather Bureau to find out where the windiest places in the country were located. I figured I could do the same for your question. But I didn’t have Wilbur’s luck.

Scott E. Stephens, a meteorologist with the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information, Climatic Science and Services Division, responded that they have no frickin’ idea. 

OK. I might have paraphrased that a bit. He actually wrote back, “This stat is not archived, I’m afraid.” 

Well…Damn.

Turning to a Pilots of America Hangar Talk discussion board, a member asked the group what the lowest density altitude any of them had ever seen. One flier in Alaska — where pilots drain their oil and take it inside overnight to keep it liquid — said he’d seen negative 5,000 “a few” times, and noted he got “really, really good performance.” Another said he once saw negative 7,500, but it was on a day too cold to fly, so he couldn’t speak for performance.

Alaska in the winter. So not on my bucket list. 

Now, intuitively, we pilots understand that lower density altitude gives us better performance. But can we quantify it?

Just as we interpolate between the lines of our performance charts, can we interpolate off the bottom of the chart — which terminates at sea level on a standard day — to determine the improvements in performance that negative density altitude gives us?

I reached out to airplane designers to find out. Steven McArthur, an aerospace engineer with Design, Analysis and Research Corporation of Lawrence, Kansas, says that, “Yes performance charts can be extrapolated for negative density altitudes.”

He notes that, “If the existing performance charts for the plane have temperature or density corrections, these could be extrapolated for extreme cases, but I would caution that often the edges of the charts you are extrapolating may not have been verified by flight testing, and were likely extrapolated to begin with.”

So we’d be doing guesswork based on guesswork. Great.

Jonathan Wing, a junior aeronautical engineer for Sling Aircraft, also says yes, but adds a few provisos.

He writes, “The inverse relationship between light aircraft performance and density altitude can be seen as a linear one, to a point, and the same rules apply when the density altitude goes negative/below current elevation.”

But he hastens to point out that the engine and prop performance increase from negative density increases at a different rate than the increased friction drag from the thicker air, “so the relationship won’t be perfectly linear.”

“One could predict aircraft performance until a point, where that specific performance metric is no longer linear,” he says, adding, “I am not aware of this being proactively done in general aviation.”

Meanwhile, Al Lawless, an engineer with Aurora Flight Sciences — and one of the authors of the Society of Flight Test Engineers Reference Handbook —cautions us about another factor to consider, and that’s that engines are typically designed to deliver 100% power at sea level on a standard day.

“Feeding it higher density air would get you more than 100% power, but less than five minutes running time before incurring damage,” as that’s the standard exceedance time in the industry, he says. Worse, notes Lawless, “If you boost it high enough, then you could blow it up right away.”

So there you have it. The wings and prop are happier the lower the density altitude goes. But our engines can suffer from too much of a good thing, at least at full throttle. I guess the answer to your question is that at maximum negative density altitude we’ll fly better than ever. 

Right up to the second the engine blows up.

William E. Dubois is a “double” Master Ground Instructor accredited by both NAFI and Master Instructors, a commercial pilot, a two-time National Champion Air Racer, and is based in New Mexico where negative density altitude never happens.