Density

Hal Stoen

©Hal Stoen, Stoenworks

May, 2002

The runway lights roll by faster than your airspeed is increasing. A look over at the engine gauges shows that the manifold pressure is a little lower than you're used to, and the RPM is down a bit too. Still, the pre-flight inspection showed nothing unusual, and your run-up prior to departure was normal. And yet, here you are screaming down the runway, in your mind's eye going faster than you have ever gone before and you're still on the ground. The airspeed indicator slowly twitches up to your lift-off speed and you pull back on the wheel. The aircraft struggles into the air, a small turbulence bump makes the stall warning horn go off briefly.

You know your aircraft well, and have a mental picture of how much runway it will use, and how the deck angle is at climb. But here you are after eating up an unusual amount of asphalt on departure, and your climb angle is so shallow that you are barely climbing. What the heck is going on?

Let's back up a bit here. It's a lovely summer day as you and 3 friends pile into your Speedmerchant 500, a normally aspirated (not turbocharged) twin-engine general aviation airplane. You're at a Denver, Colorado General Aviation airport, heading off for a weekend of mountain activities in Colorado's "Hill Country". You are a conscientious pilot, and are familiar with your aircraft. This is your first trip out West, to date all of your operations have been from airports in middle and eastern America.

What went wrong?

Density Altitude

The air that we fly in is constantly changing. Winter and Summer, cold and dry, hot and humid. As these variables occur, the air density varies. And, most importantly, as the air density varies so does the performance of your aircraft. "Density Altitude" is the term used when these variables are taken into account. In our above example, as far as the Speedmerchant 500 was concerned you were departing at an airport that was thousands of feet higher than it actually was.

Performance Figures

But, what about the performance figures for the aircraft? They say that it will takeoff at so-and-so airspeed in so many feet. What about that? Well, "that" is a pretty much a fictitious day. It's called a "Standard Day" in airplane talk, and is used so that we have a common basis for discussing and comparing airplane performance.

Standard Day

A Standard Day is at sea level, with an outside temperature of 59 degrees (F), and an altimeter setting of 29.92 inches. As a pilot, you should have these numbers blazed in your mind, for if any of the three of them change your aircraft's performance will change too. And, looking back, how many times have you ever taken off from an airport at sea level?

A Brief Word About Humidity

I remember a sports announcer saying one time "It's a hot and humid night out here at the old ballpark. Fly balls won't carry very far tonight, the air is so thick that you could cut it with a knife."

He was wrong.

Moist air is less dense than dry air. That seems to fly in the face of "logic", but it's true. Relative humidity is not taken into account in most Density Altitude calculations, but when it is at an extreme, it should be. At the end of this tutorial is a link that has a Density Altitude calculator that will factor in humidity.

The "Three H's"

Hot. High. Humid. When any one of these atmospheric conditions is present it will have a significant effect on your aircraft's performance. If all three are present, take a very serious look at your performance tables.

For example, let's look at the Colorado airport that our Speedmerchant 500 was trying to get out of at the beginning of this tutorial and plug in some numbers.

Air temperature: 96 degrees, F.
Altimeter setting: 29.40 inches
Dew Point: 70 degrees, F.
Field elevation: 5,100 feet

Given these numbers, the old Speedmerchant 500 was performing as if it was taking off from a field elevation of 9,400 feet.

Wow.

And don't get the impression that this only applies to high-altitude operations. Given the same weather conditions, but at an airport with a much lower elevation of 500 feet, the Density Altitude would be 3,857 feet.

The Airspeed Indicator And Density Altitude

The beauty of the airspeed indicator is that it "compensates" for Density Altitude. All the pitot tube cares about is the number of air molecules that hit it. If the air is less dense because of Density Altitude, the pitot tube will just require that more air be rammed into it for a given airspeed- ie, a faster relative airspeed. Automatic compensation.

What Can You As A Pilot Do?

First off, just be aware of Density Altitude and the severe effect that it can have on your aircraft's performance.

Have access to a Density Altitude chart or computer and use it.

Be aware of the fact that even after you get off of the ground that climb performance will be affected, and that if you lose an engine you may not be able to climb at all.

Consider either taking off early in the day, or later in the evening when the temperature is lower. Even at the infamous La Paz Airport commercial operations don't take place at Summer's mid-day.

Lower your gross weight by taking on less baggage, passengers, or fuel.

Pay attention to your engine gauges so that you do not exceed maximum temperatures. Density Altitude operations are stressful on powerplants.

Ignore the external visual cues that you are used to, and pay stricter attention to the airspeed indicator. Do not lift off prematurely. Be aware that your climb angle will be more shallow and that your rate of climb will be considerably less than you are used to.

During the approach and landing phase in high Density Altitude operations be aware that your groundspeed will be considerably higher than you are used to because of your higher actual airspeed. Constantly refer to the airspeed indicator during approach and landing. Just as you use up more runway during takeoff, you will do the same during landing. Avoid landing "long" as your ground speed at touchdown will be higher than normal and you will use considerable real estate during the roll out and braking phases. The "ground effect" cushion will be decreased- landings tend to be a little more "firm" during high Density Altitude operations.

If you are operating a normally-aspirated piston-powered aircraft (non-turbocharged), do a full-power run-up before departure. Select an area that is clean so that the props. won't pull up surface debris causing damage to them. Set the parking brake. Stand hard on the brakes. Bring full power up on the engine(s). Slowly lean the mixture(s). As you do so, the RPM(s) will increase until they reach a maximum before starting to fall off again. Go back to the maximum RPM(s) and enrichen 100 degrees on the EGT(s). This will eliminate the extra-rich condition brought about by the thin air of high Density Altitude, and just might be the edge in performance that you will need for a successful departure.

And Lastly

Be an informed pilot. Fly safely.

More Research

For an excellent Density Altitude calculator, see: http://wahiduddin.net/calc/calc_da.htm

For a discussion of a Standard Day, see: http://www.usatoday.com/weather/wdenalt.htm

This tutorial is available on a CD

This tutorial, along with additional content, is available on a CD. Click here for more information.

Written after suggestions by Bård H. Einset and Winston Domingo. Thank you Bård and Winston.

© Hal Stoen

May, 2002

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