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3 Answers

Altimeter (non-pilot question)

Asked by: 4376 views General Aviation

Isn't it true that altimeters measure height above sea level?  Is this because there is no accuarate way, without some type of radar, to measure height above ground?

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3 Answers

  1. Koehn on Dec 29, 2010

    It’s true that pilots don’t have a good way of determining altitude Above Ground Level (AGL) in most aircraft; but knowing your AGL isn’t always helpful.
    Imagine you’re flying along contentedly (thanks to your radar altimeter) knowing that you’re 3000′ AGL. Ahead of you are some mountains: are you high enough to clear them? Well, the chart won’t tell you their altitude relative to your altitude; the only meaningful way to chart their altitude is by measuring it relative to Mean Sea Level (MSL). So you as a pilot need to know your own altitude relative to Mean Sea Level. Hence the pressure altimeter.
    Another scenario: you’re landing at an airport, but you can’t see it because of low clouds. You know that you’re currently 500′ AGL, but at what altitude is the runway? Again, you need a common reference for measuring altitude, and that reference is MSL.

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  2. Keith on Dec 30, 2010

    Koehn is right, it’s essential to have a common reference. It can also be good though to know your height above the ground. Flying the <a href=http://www.howitflies.com/Douglas-A-4-Skyhawk”>A-4 Skyhawk</a> during Navy training we learned to make good use of a Radar Altimeter (radalt). Besides being good for low level navigation you can use it in instrument work by setting an alarm for decision heights, etc.

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  3. Earl Kessler on Jan 17, 2011

    We also use MSL with current altimeter setting so that we can maintain accurate separation from other traffic.  If ATC assigns an altitude to IFR pilots, and you are flying VFR, you may only be separated by 500 ft from closing IFR traffic.  It is essential that you be on your assigned altitude to ensure safe separation.  Keeping your altimeter set to local AWOS, ATIS, ASOS or ATC-given altitudes keeps this safety factor.  Don’t be shy to ask ATC for a current altimeter setting if it’s been a few minutes since your last adjustment.

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