I did my initial training in a high altitude area, where it got pretty hot in the summer, so I'm not at all surprised when I hear an ASOS announce:

Caution: Density Altitude (a few thousand above field elev.)

But recently, I was flying in SE. Kansas when it was extremely cold outside. The ASOS announced:

Caution: Density Altitude minus 1800 ft

I understand that cold winter air can make the air extremely dense, leading to a negative density altitude. But I don't understand the caution part of it.

As far as I'm aware, low density altitudes mean your engine generates more power, and your wings generate more lift. Everything is better with low density altitudes, right?

What is the caution for? What should I be looking out for?

up vote 20 down vote accepted

Low density altitudes mean your engine generates more power, and your wings generate more lift. Everything is better with low density altitudes, right?

The potential danger in low density altitudes is that your engine generates more power than it can handle. Even without exceeding the maximum rated RPM and manifold pressure, you can exceed the rated power of an engine under low density altitude conditions.

At low temperatures the air becomes denser. At standard sea level conditions air has a temperature of 15 degrees Celcius and a density of 1.225 kg per cubic meter. When the temperature is -30 degrees Celcius, the density increases to 1.452 kg per cubic meter. The equivalent altitude where this density is found under standard atmosphere conditions would be about -5900 ft. This is called the density altitude.

When air with these properties enters the combustion chamber of your engine and is combusted to the normal exhaust temperature it expands (increases in pressure) far more that air normally would, resulting in excessive power.

This FAA paper on Winter Flying Tips says the following about the risks associated with low density altitudes:

Do not overboost supercharged engines. This is easy to do because at very low density altitude, the engine "thinks" it is operating as much as 8,000 feet below sea level in certain situations. Care should be exercised in operating normally aspirated engines. Power output increases at about 1% for each ten degrees of temperature below that of standard air. At -40 degree F, an engine will develop 10 percent more than rated power even though RPM and MP limits are not exceeded.

  • Actually, manifold pressure probably would get exceeded, right? I mean, DA is essentially pressure, which means your engine would reach max MAP earlier than it normally would. – egid Dec 18 '13 at 19:05
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    Since the temperature is very low, the density altitude is much lower than the pressure altitude. DA is not pressure under these circumstances. You can have sea level pressure with temperatures below 30 degrees Celcius, resulting in a density altitude of -6000 ft. – DeltaLima Dec 18 '13 at 20:21
  • Right, brain fart. :) – egid Dec 18 '13 at 20:23
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    @DeltaLima great answer, but it'd be even better if you included links to further reading (the Lycoming SB or anything else which helps explain your point) in the answer so that sort of thing doesn't get buried in the comments trail... simply a suggestion. – Bret Copeland Dec 19 '13 at 0:16
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    @BretCopeland. Thanks for the suggestion. It's way past midnight here so I will call it a day and expand on my answer tomorrow. – DeltaLima Dec 19 '13 at 0:26

The caution is probably the default prefix for a nonstandard DA notice, not anything particular to the fact that it's negative. You're correct that lower DA will improve performance, and an FAAsafety.gov document on density altitude (PDF) doesn't discuss low DA at all.

In addition to what DeltaLima answered, also remember that denser air means the isobars are closer together and that in turn means that if your altimeter reads correctly on the ground, the same baro-altitude gives you less clearance from terrain. Some instrument procedures have minimal temperature for this reason.

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