Pressure alitude ?

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Im in process of learning for pvt pilot and to take
the written test. Dont understand the differences
of Pressure altitude and altitude and true altitude and any
other alttiudes I forgot ?
If I set the altimeter inside airplane to the current
atis or ATC local pressure they report I usually get the
Hanscom field (here in Bedford MA) 133 feet above sea level.
So I think I understand that - depending on that particular hour
or so of local weather and pressure.

If I was to put in 29.92 into the Altimeter inside plane - what
does that give me for feet above sea level ?
Does it depends I suppose whether a Low or HIgh is going by ?
Will my planes feet show higher or lower than 133 feet (when
on the ground )?

I understand density altitude pretty good (how high temps and
humidity ) can raise possibly the altitude and performance of the

But what do they mean by Pressure vs True Altitude ?
Hope someone can explain in simple terms - I've read all the
Jeppesen Prvt Pilot book stuff but still confused

-- jim kenn, April 29, 2010


Imagine cooling the Earth to a temperature of 1 degree Kelvin. In this extreme cold, the atmosphere would shrink to a height of perhaps one inch. The altimeter does not measure altitude. It measures the percentage of the atmosphere above which you have climbed. So once you'd climbed to 1/2 inch above sea level, the altimeter would read "18,000 feet". If you relied on that reading to attempt to clear a 3,000' hill you would be very disappointed when your airplane smacked into the side of the hill at a true altitude of 1/2 inch.

-- Philip Greenspun, April 29, 2010

An altimeter measures the static pressure around the plane and compares it to the pressure you dial into the window (called the Kollsman window) of the altimeter and displays the difference as an altitude. Roughly, the rule is that pressure decreases by 1 inch per 1000 ft.

The pressure you dial into the Kollman window from the ATIS is the sea-level pressure at your location. So, if you dial in 30.00 (inches of mercury) and the static pressure outside your plane is measured as 28.00 inches, your altimeter will read 2,000 feet and you are at 2,000 feet indicated altitude.

True altitude is your actual altitude above sea level. Why doesn't it equal indicated altitude when you have dialed in the correct sea-level pressure setting from the ATIS? Because the atmospheric pressure does not drop exactly at 1 inch per 1,000 feet and the rate varies based on temperature. On hot days, the whole atmosphere expands and gets taller, so pressure levels are higher. In other words, on a hot day, your true altitude is higher than indicated. The reverse is true on a cold day.

Pressure altitude is simply the altitude you obtain when you dial 29.92 into the Kollsman window. It is used for flight at or above 18,000 feet. This is Class A airspace. Here, all pilots use a setting of 29.92 in their altimeters and the altitude is referred to as a flight level by dividing the altitude by 100. For example, if you dial in 29.92 and read 18,000 feet on your altimeter, you are flying at FL180.

The practical reason for using pressure altitude is that there is no need for pilots to keep adjusting their Kollsman window at high altitudes because it is important only that all pilots use the same setting in the same area for separation, so they just have everybody use the same setting everywhere.

Also, because 29.92 is the arbitrary official definition of sea-level pressure for a standard atmosphere, pressure altitude can be thought of as how high you would be in a standard atmosphere to read the static pressure you are reading. This definition is useful only for understanding that density altitude can be calculated by adjusting pressure altitude for non-standard temperature.


-- Todd Ramming, April 29, 2010