why always high Qnh in mountain airports?
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Depends on the age and sophistication of the altimeteter.
As already pointed out, the instruments are designed to reduce errors to a minimum. The actual extent of such an error is something you'd have to ask a specialist engineer.
As already pointed out, the instruments are designed to reduce errors to a minimum. The actual extent of such an error is something you'd have to ask a specialist engineer.
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Bluskis
Altimeters and ASI’s are both designed to react to pressure changes only.
i.e. if the static pressure is A and the Pitot pressure is B then the Altitude will be C and the IAS will be D etc.
Neither instruments ‘cares’ what the temperature is. If as in Huggy’s example a local temperature drop has resulted in an increase in pressure then the altimeter is designed to react to the pressure increase by indicating a lower altitude. That is basically the definition of pressure altitude.
One parameter where we do require temperature to be incorporated in our calculations is True Airspeed (TAS) . To calculate TAS density must be known and this can be calculated by modifying IAS (or CAS to be precise) with temperature. This is the reason that larger aircraft have a Total Air Temp (TAT) probe.
Having said all of that all of these instruments will have some internal temperature compensations that are purely for MECHANICAL REASONS e.g. a lever may be a bit longer when hotter, a capsule may not have the same expansion characteristics at one temperature than another.
It is easy to get the two types of temperature compensation muddled.
In the TAS example all TAS indicators (or more usually an Air Data Computer) will calculate TAS from CAS using the same calculations derived from the Standard Atmosphere model.
In the case of compensation for mechanical reasons individual instruments will differ in the amount of compensation required depending on construction methods material used etc.
Hope This Helps
Max
Altimeters and ASI’s are both designed to react to pressure changes only.
i.e. if the static pressure is A and the Pitot pressure is B then the Altitude will be C and the IAS will be D etc.
Neither instruments ‘cares’ what the temperature is. If as in Huggy’s example a local temperature drop has resulted in an increase in pressure then the altimeter is designed to react to the pressure increase by indicating a lower altitude. That is basically the definition of pressure altitude.
One parameter where we do require temperature to be incorporated in our calculations is True Airspeed (TAS) . To calculate TAS density must be known and this can be calculated by modifying IAS (or CAS to be precise) with temperature. This is the reason that larger aircraft have a Total Air Temp (TAT) probe.
Having said all of that all of these instruments will have some internal temperature compensations that are purely for MECHANICAL REASONS e.g. a lever may be a bit longer when hotter, a capsule may not have the same expansion characteristics at one temperature than another.
It is easy to get the two types of temperature compensation muddled.
In the TAS example all TAS indicators (or more usually an Air Data Computer) will calculate TAS from CAS using the same calculations derived from the Standard Atmosphere model.
In the case of compensation for mechanical reasons individual instruments will differ in the amount of compensation required depending on construction methods material used etc.
Hope This Helps
Max
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Most ATC these days read the QNH off the dial on the console and have no inkling of what is involved in determining it.
Many moons ago I was instructed in the gentle art of measuring the pressure using a mercury barometer, then applying the corrections. I don't remember what they all were now, but one was historical, a correction for noted differences between ISA and reality for that particular location. All done by mirrors now...
Many moons ago I was instructed in the gentle art of measuring the pressure using a mercury barometer, then applying the corrections. I don't remember what they all were now, but one was historical, a correction for noted differences between ISA and reality for that particular location. All done by mirrors now...
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One possible correction here: although an altimeter does only respond to pressure (and measures pressure very well) pressure does not correspond to altitude, except at ISA lapse rates.
If you fly from a hotter region to a colder region your altitude will decrease, although air pressure (and altimeter) may very well not change.
For example, take a procedure commencement altitude of 3000 feet indicated, QNH 1013hPa or 29.92inHg (how convenient yes?). .If the temperature is 30°C (North Africa) true altitude will be around 3210 feet. If the temperature is -20°C (Arctic), true altitude will be 2690-ish feet.
My apologies if I have misunderstood you Max Motor. I'm sure you know all the above, but artflyer may not. In any case, anyone can check my working with a CRP1 or Jeppeson slide rule.
To get back to the original question - has anyone answered it conclusively yet? I couldn't actually tell! But if the air is hotter than ISA for a particular pressure altitude, you will have to wind up the subscale over QNH to get True Altitude (or aerodrome elevation if you are on the ground at your mountain strip.) This would make mountain QNH higher than "real" sea level pressure at the bottom of the cliff.
In other words, as per my example, your altimeter says 3000 feet at your local North African 30°+ mountain airfield, but the airfield is actually at 3210 feet AMSL. So wind up the QNH subscale to get the altimeter to read 3210 feet...
cheers,. .O8 <img src="smile.gif" border="0">
[ 15 February 2002: Message edited by: Oktas8 ]</p>
If you fly from a hotter region to a colder region your altitude will decrease, although air pressure (and altimeter) may very well not change.
For example, take a procedure commencement altitude of 3000 feet indicated, QNH 1013hPa or 29.92inHg (how convenient yes?). .If the temperature is 30°C (North Africa) true altitude will be around 3210 feet. If the temperature is -20°C (Arctic), true altitude will be 2690-ish feet.
My apologies if I have misunderstood you Max Motor. I'm sure you know all the above, but artflyer may not. In any case, anyone can check my working with a CRP1 or Jeppeson slide rule.
To get back to the original question - has anyone answered it conclusively yet? I couldn't actually tell! But if the air is hotter than ISA for a particular pressure altitude, you will have to wind up the subscale over QNH to get True Altitude (or aerodrome elevation if you are on the ground at your mountain strip.) This would make mountain QNH higher than "real" sea level pressure at the bottom of the cliff.
In other words, as per my example, your altimeter says 3000 feet at your local North African 30°+ mountain airfield, but the airfield is actually at 3210 feet AMSL. So wind up the QNH subscale to get the altimeter to read 3210 feet...
cheers,. .O8 <img src="smile.gif" border="0">
[ 15 February 2002: Message edited by: Oktas8 ]</p>
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One ought not to forget plain old altimeter failures .... the Cause and Circumstance article in BCA, April 2000, pp90 is, I suggest, very good reading for altimeter awareness and crosschecking ....not to mention a few other discipline-related things ....
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Ok, found this in the dictionary. Should clarify some things...
Altimeter, device for measuring altitude. The most common type is an aneroid barometer calibrated to show the drop in atmospheric pressure in terms of linear elevation as an airplane, balloon, or mountain climber rises. It shows height above sea level, but not above such land features as hills, mountains, and valleys. The radio altimeter, or terrain-clearance indicator, is an absolute altimeter; it indicates the actual altitude over water or over terrain, however uneven. It operates by first sending either continuous or pulse radio signals from a transmitter in an aircraft to the earth's surface. A receiver in the aircraft then picks up the reflection of the signals from the surface. The time it takes for the signals to travel to the earth and back is converted automatically into absolute altitude that can then be read from a calibrated indicator. The radio altimeter is used in the automatic landing systems of aerospace vehicles; systems developed from radio altimeters can automatically control military aircraft flying at high speeds and low altitudes.
Sounds right to me <img src="smile.gif" border="0">
Altimeter, device for measuring altitude. The most common type is an aneroid barometer calibrated to show the drop in atmospheric pressure in terms of linear elevation as an airplane, balloon, or mountain climber rises. It shows height above sea level, but not above such land features as hills, mountains, and valleys. The radio altimeter, or terrain-clearance indicator, is an absolute altimeter; it indicates the actual altitude over water or over terrain, however uneven. It operates by first sending either continuous or pulse radio signals from a transmitter in an aircraft to the earth's surface. A receiver in the aircraft then picks up the reflection of the signals from the surface. The time it takes for the signals to travel to the earth and back is converted automatically into absolute altitude that can then be read from a calibrated indicator. The radio altimeter is used in the automatic landing systems of aerospace vehicles; systems developed from radio altimeters can automatically control military aircraft flying at high speeds and low altitudes.
Sounds right to me <img src="smile.gif" border="0">
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O8
I completely agree with you conclusions the altimeter will rarely display true altitude. Indeed this is the point I was trying to make. There seemed to be some suggestion in this thread and previous threads that the altimeter would take temperature into consideration and correct this error. As you point out that is not the case.
Just to reinforce this. To use your example, in each case the pressure acting on the altimeter capsule will be 26.82in Hg and the altimeter will indicate 3000 ft. As you point out this will only be the true altitude on the standard day. But 26.82in Hg = 3000 ft period. The altimeter will not attempt to correct for true altitude by taking temperature into account, that is up to you the pilot (or anyone else who maybe interested.)
What the altimeter will try and do is ensure that 26.82 ALWAYS equals 3000ft regardless of the temperature. For example in the Artic the lower temperature makes the capsule become more elastic and it will expand slightly more than it would do normally, even though it has the same pressure acting on it. If uncorrected this would cause the altimeter to overead. But at the same time a bi-metallic spring acting on the capsule slightly increases its force to compensate, so the net movement is the same as it was in standard conditions.
As Long Range Cruise has pointed out an altimeter is nothing more than an aneroid barometer (except that is calibrated to display altitude instead of pressure)
Max
[ 15 February 2002: Message edited by: max motor ]</p>
I completely agree with you conclusions the altimeter will rarely display true altitude. Indeed this is the point I was trying to make. There seemed to be some suggestion in this thread and previous threads that the altimeter would take temperature into consideration and correct this error. As you point out that is not the case.
Just to reinforce this. To use your example, in each case the pressure acting on the altimeter capsule will be 26.82in Hg and the altimeter will indicate 3000 ft. As you point out this will only be the true altitude on the standard day. But 26.82in Hg = 3000 ft period. The altimeter will not attempt to correct for true altitude by taking temperature into account, that is up to you the pilot (or anyone else who maybe interested.)
What the altimeter will try and do is ensure that 26.82 ALWAYS equals 3000ft regardless of the temperature. For example in the Artic the lower temperature makes the capsule become more elastic and it will expand slightly more than it would do normally, even though it has the same pressure acting on it. If uncorrected this would cause the altimeter to overead. But at the same time a bi-metallic spring acting on the capsule slightly increases its force to compensate, so the net movement is the same as it was in standard conditions.
As Long Range Cruise has pointed out an altimeter is nothing more than an aneroid barometer (except that is calibrated to display altitude instead of pressure)
Max
[ 15 February 2002: Message edited by: max motor ]</p>
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The rule of thumb I have for correcting the indicated altitude for non-standard temperatures is: 4' per thousand for each degree C the temperature varies from ISA. Example: At 5,000' indicated OAT is 0. Thats 5 degrees less than ISA. So 5(thousand)X 4 = 20'. 20(-5) = -100'. Actual altitude 5,000 - 100 = 4,900'.
Since the difference is more significant at higher altitudes I read that some approach charts at higher altitude airports make adjustments in the MDA for very cold temperatures.
No, I never did find out why the QNH settings in the mountains around here always seem to be higher. Maybe it is just a local trend.
Since the difference is more significant at higher altitudes I read that some approach charts at higher altitude airports make adjustments in the MDA for very cold temperatures.
No, I never did find out why the QNH settings in the mountains around here always seem to be higher. Maybe it is just a local trend.
