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Author Topic: Altimeter  (Read 5280 times)
mielsonwheals
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« on: October 22, 2011, 04:20:16 PM »

Hello,
I am a student pilot and want to make sure I understand how the altimeter works.

Lets say the location is Hanscom Field in Bedford, MA, airport elevation of 133 MSL. Local altimeter setting is 30.10 for current day.

The plane is on the tarmac before taxi. Altimeter is set to the correct setting of 30.10 and displays 133. So, in this local geographic area and in the current local airmass, at the theoretical 0 ft. MSL the outside air pressure is 30.10 inches Hg. The altimeter works by comparing the outside pressure at Hanscom field, through the static tube, to the 30.10 inches mercury pressure. Therefore, displaying the correct true, or MSL altitude, of 133 ft.

Is this explanation correct? Thanks.

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StuSEL
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« Reply #1 on: October 23, 2011, 12:12:00 AM »

It's a static port, not tube, though there is a pitot tube. But yes, correct.
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CFI ASEL
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Jason
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« Reply #2 on: October 23, 2011, 10:11:19 AM »

Here's a good image which helps explain the internal workings of an analog altimeter.  The aneroid wafers expand and contract with atmospheric pressure which is mechanically translated through inner gears to the indicator pointers.



The plane is on the tarmac before taxi. Altimeter is set to the correct setting of 30.10 and displays 133. So, in this local geographic area and in the current local airmass, at the theoretical 0 ft. MSL the outside air pressure is 30.10 inches Hg. The altimeter works by comparing the outside pressure at Hanscom field, through the static tube, to the 30.10 inches mercury pressure. Therefore, displaying the correct true, or MSL altitude, of 133 ft.

If the altimeter is set to 30.10" Hg (current altimeter setting in your example) and reads 133 ft. MSL, that represents indicated altitude. True altitude is different because it corrects for non-standard temperature.
« Last Edit: October 23, 2011, 10:17:35 AM by Jason » Logged
mielsonwheals
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« Reply #3 on: October 23, 2011, 12:30:30 PM »


If the altimeter is set to 30.10" Hg (current altimeter setting in your example) and reads 133 ft. MSL, that represents indicated altitude. True altitude is different because it corrects for non-standard temperature.

Isn't it density altitude that corrects for non-standard temperature?
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Jason
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« Reply #4 on: October 23, 2011, 04:24:12 PM »

Isn't it density altitude that corrects for non-standard temperature?

Density altitude also corrects for non-standard temperature and pressure but unlike true altitude, density altitude represents the height in the International Standard Atmosphere (ISA) at which air density would be equal to the actual air density where the observation is taking place.

True altitude describes the actual height above mean sea level, corrected for non-standard pressure and temperature.  Density altitude on the other hand describes the height in terms of air density.

For example, given a field elevation of 133 feet (OP's example) on a hot and humid day on the ground at KBED, density altitude may equal 2,000 feet but true altitude is still 133 feet.  The 2,000 foot density altitude provides a quantitative value on which one can evaluate aircraft performance. The aircraft will react as if it is being flown in ISA conditions at 2,000 feet while actually flying at 133 feet above mean sea level.
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mielsonwheals
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« Reply #5 on: October 23, 2011, 07:52:30 PM »

Okay, that makes sense, thanks. That is why the SLP listed in the METAR does not usually equal the altimeter setting in the same METAR. The SLP reading is corrected for non-standard temp. Right?
« Last Edit: October 23, 2011, 08:13:27 PM by mielsonwheals » Logged
Jason
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« Reply #6 on: October 23, 2011, 09:18:40 PM »

Okay, that makes sense, thanks. That is why the SLP listed in the METAR does not usually equal the altimeter setting in the same METAR. The SLP reading is corrected for non-standard temp. Right?

Yes, SLP is corrected for non-standard temperature when station pressure is reduced to sea level.
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mielsonwheals
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« Reply #7 on: October 23, 2011, 09:39:05 PM »

Okay, last question. So why do we not use SLP for the altimeter setting? Would it give a less accurate reading on the altimeter for MSL altitude, and why?
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Jason
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« Reply #8 on: October 23, 2011, 10:51:34 PM »

Okay, last question. So why do we not use SLP for the altimeter setting? Would it give a less accurate reading on the altimeter for MSL altitude, and why?

My understanding is that an altimeter setting is a simpler calculation (less accurate) which allows the mechanism inside the altimeter to read altitude instead of pressure.  Since all aircraft have to be on the same source (QNH vs. SLP) to ensure vertical separation of aircraft, the international standard is altimeter setting (or QNH).
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