Questions from the Cockpit: Dense about density altitude

Franklyn, a sport pilot from Arizona, writes: Call me dense, but I’m having a hard time sorting out one element of density altitude. I get that when it’s hot, the air around us (and our airplanes) expands and acts as if it’s higher. I understand how that negatively affects the lift our wings generate, the power our engines produce, and the thrust our props can bite out of the air. So it makes sense to me that our takeoff roll is longer, and our climb performance sucks.

But what I don’t get is why the landing roll is longer. With the plane performing so poorly, shouldn’t landing distance be shorter? Shouldn’t it just plop on the runway and stop flying? Help me get a handle on this!

True confession: When first I read your question, I too, found myself scratching my head for a few minutes. We all know that airplanes fly worse the higher they go, although I personally experienced the phenomena in reverse.

The first time I took Race 53 from our mile-high base down to an airport at sea level she flew like she never flew before, at least in my experience. At the flare, she floated down the runway. And floated. And floated. And floated. About the time I was contemplating a go around, she finally kissed the pavement. If we’d been competing in a spot landing contest, we’d’a been screwed. 

In that thick air, she just didn’t want to stop flying — very different from what I was used to, where convincing her to fly at all was the trick. My other great surprise was how crazy-short all the runways at lower elevations were. I was used to a mile of pavement at even the smallest of airports in my high elevation part of the country, where you need a lot of runway to get off the ground into the thin air.

But, of course, you also need a lot of runway to get back down again at higher elevations.

We need look no farther than the performance charts of any general aviation aircraft to see that landing rolls increase with altitude: Be that altitude in feet above the oceans or in feet of density altitude.

What’s up with that? Why do planes that really don’t want to fly any more, or are flying poorly — either due to heat or height — need more landscape to land on? I grant you, it does seem counter-intuitive.

And the explanation for this is that, even though your plane is not happy, it’s flying a heck of a lot faster than it’s telling you. Yep. Your airspeed indicator is a lying sack of s–t.

I’m sure that you recall that the speed indicated on your airspeed indicator is pure fiction 99.9% of the time. It only shows you how fast you’re really going if you’re flying a float plane an inch above the surface of the sea on a standard day, which is defined as a barometric pressure of 29.92 at 59° Fahrenheit, a temperature rarely seen on the beaches I like to visit at sea level.

As you go higher in the atmosphere, or as the day gets warmer, the airspeed indicator shows a slower speed than the speed you are truly plying the air.

As a rule of thumb, the airspeed indictor is “off” by about 2% per 1,000′. In a 100-knot airplane flying at 10,000′, we can expect to see an airspeed indication of 80 knots.

Put another way, you are flying 20% faster than you think you are. Hold on to that thought. I’ll come back to it in a moment.

Now, thanks to an irony of aerodynamics, the airspeed indicator error doesn’t matter. That’s because the V-speeds we use to set up landings, deploy flaps, and keep us safe from stalls, also change with altitude.

As the air gets thinner — either from its height above the world’s oceans or from expansion due to heat — all of our operational speeds increase to compensate for the decreased density.

The miracle is that the amount of this increase is roughly equivalent to the degree of false-low reading on the airspeed indicator, thus approach and landing speeds appear to be the “same” no matter if we are landing at Furnace Creek in Death Valley (elevation -210′) or Lake County in Leadville, Colorado (elevation 9,993′).

Yes, we are being fooled, but in a good way. But when it comes to the landing roll, our planes are not as easily fooled as we are.

Remember that example above, where we’re zipping along at 100 knots, even though the airspeed indictor says we’re going 80? Let’s just pretend for a moment that this same plane also has a final approach speed of 80 knots. Of course, in our fictional 10K density altitude example, when your tires kiss the ground at 80 indicated, you are actually rocketing down the runway at 100 knots.

Is it any wonder we need more runway to land in thin air? It really has nothing to do with how doggy the plane is flying. It’s simply flying faster than it’s telling you it is.

And it’s this increase of speed — well, more correctly, the decrease in indicated airspeed — that stretches our landing distance by about 3.5% per 1,000′ in altitude, either MSL or density altitude.

So there you have it: When you are landing hot, even though the plane isn’t performing at its best, you really are landing hot! And that takes a lot of runway.

William E. Dubois is a commercial pilot, ground instructor, and air racer who loves flying to lower elevations so that his airspeed indictor gives more impressive readings.