Caution: Low Density Altitude?

abelenky
  • Caution: Low Density Altitude? abelenky

    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 annouce "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?

  • 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.

  • 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.

  • Rather than over-performance being cause for the warning, I would suggest that the more likely reason for the "Caution" would be that you don't really want to try to land at -1800 ft if you just flew from somewhere with a DA of 0. Reset your altimeter. ;-)

    EDIT: Oops. Ignore me here. This is what happens when my mouth runs before my brain fully engages. Abelenky is right. The DA wouldn't affect your indicated altitude if you set your altimeter on the reported pressure. It would just affect the performance of the aircraft.

    On an opposite note, I grew up flying in Louisiana where we were essentially at sea level, but we'd have days in the summer where the density altitude would be well over 5000 ft. It sucked pretty bad to be sit in a non-airconditioned plane in that kind of humidity. And it was just as bad to watch so much runway scoot under you while your Cessna wasn't lifting off.

Related questions and answers
  • 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?

  • How do turbochargers improve engine performance at high altitudes / density altitudes? Could you also address how this benefit could become a liability at low density altitudes, for example when the density altitude is negative?

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