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3 Answers

Density altitude

Asked by: 4951 views Student Pilot

This past week during some inclimate weather the DA went from 2700 to 2300 to 1400, how does this apply to airplanes and how would this effect flight if flight was possible?This weather was not for student pilots and limited IFR as I take it.

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3 Answers

  1. pittss2a on Sep 27, 2010

    Remember from your basic knowledge that every aspect of flight depends on the density of air through which an airplane is moving. Without going through the mathematics the lift sustaining the airplane in flight varies directly with the air density. 
    So if nothing else changes an airplane needs to fly faster to maintain a given altitude if the air density decreases. On the runway a lower air density means reduction in thrust and the need for higher take off and landing speeds, meaning a longer take off run. If you were at a short field then a reduction in payload may be in order to reduce take off weight to so the airplane can meet TO performance criteria. Obviously with a propeller driven airplane the blades are also less efficient and produce less thrust with a reduced air density.
    So beware of the hot and high conditions at unfamiliar airfields and obviously low time pilots need to be taught this before they learn through experience!

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  2. Brian on Sep 27, 2010

    Hi James,
    Density altitudes applies to an aircrafts performance information for one. It defines the performance a pilot can expect out of the aircraft, particularly to climb rate. A Cessna POH time, fuel, and distance to climb chart is an example of a time we need to use density altitude.
    This chart gives us the climb performance for standard temperature conditions. If we calculate our density altitude to be 3,000 feet we would want to use this altitude for our climb performance calculations. We would not want to use the field elevation, whatever it may be, because that elevation does not account for non-standard temperature. Density altitude corrects for this non-standard temperature to give us accurate performance information when the charts or graphs do not provide it.
    This information also sheds light on some non aircraft issues. In the case of rapidly falling density altitudes, inclement weather might be expected. I am not an instrument instructor and have not studied this topic in detail recently. That said I will stop here and let someone who is more weather savvy pick up the ball.
    Fly Safe,

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  3. Pete Kemble on Oct 01, 2010

    One important thing to note for new students – the difference between density altitude and air density. someone just starting out can easily mix the two terms up. the terms get thrown around a lot, but have inverse characteristics while talking about the same thing; the amount of “air molecules” in a given unit of atmosphere:

    Performance suffers as density altitude *increases*
    … which also means the air density *decreases*
    … which means performance suffers.

    without launching into a novel about the relationship between temperature, volume and pressure – the reason the DA dropped (and air density increased) is most likely due to the temperature dropping as the inclement weather came in – N.B. inclement weather does NOT mean the temp will drop, this is just most likely what happened in this particular case. As the temperature dropped, the air molecules got closer together, and the air density increased (the air “thickened”) Did a cold front pass through by chance?

    Read up on temperature, volume and pressure as it relates to the weather and the example above will make sense. chapter 3 of the old standby “Aviation Weather” (AC00-6A) will explain it all – and it’s free online. Hope this helps.

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