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#21
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Pressure & temperature
"Doug" wrote in message ups.com... The local altimeter measures the weight of the air above it. When temperature is higher, the air expands (less density), but it's the same amount of air in a taller column. The atmosphere expands you see and is not bound tightly by a blocade above (it is bound by gravity actually). Anyway another item of interest is what you are setting the altimeter to is "sealevel" barometric pressure. Imagine a hole in the ground and the altimeter is lowered to the bottom of the hole (at your airport). Of course they dont actually have a hole, they have another way of adjusting it. Standard barometric pressure at sealevel is about 29.92. If you fly an airplane with a manifold pressure guage, it gives the local "absolute" pressure (non-sealevel adjusted). Anyway consider these principals and your knowledge will expand. Standard pressure at sea level is 29.92 at 15 degrees C. The weight of the air contrary to what you wrote above does vary hence high and low pressure weather systems. With low pressure air rises up the column diverges at the top and then descends giving a high pressure somewhere else. with a high pressure, the air converges high up, descends and then diverges when it gets to the ground. The principle of the altimeter is that it works on pressure, and pressure is the force exerted by the air and one common force is weight. When the air is less dense there are fewer molecules, the mass of air is less and therefore the column of air weighs less, (mass x gravity) hence the lower pressure. when you understand these principles then you will understand a bit more about altimetry, weather systems, high pressure, low pressure and winds all of which are about the forces of air. |
#22
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Pressure & temperature
"Peter" wrote in message ... "akjcbkJA" wrote In Europe its the easiest way to fail an IR checkride by failing to compensate for the ISA deviation in the winter and being deemed to have busted the limits Is this your personal experience? I've never heard of anybody here (UK) using anything other than the published minima, directly. One would be landing with the airport QNH set, and altimeter error caused by non-ISA temperature is negligible at more or less any airport in Europe that has an instrument approach. And at high altitute airports one would not be using the airport QNH. They have some other system. An altimeter is a pretty inaccurate device once well away from the ground. Its in the JAR instrument test standards and features within the learning objectives for the ATPL Met exam. Work it out. The difference is small on approach and greater at high altitude but the examiner would be looking to see whether you have taken account of the fact that temperature does make a difference. In JAR land some say they are anal, others would take the view that they do it properly. |
#23
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Pressure & temperature
"Peter" wrote in message ... "S Green" wrote Its in the JAR instrument test standards and features within the learning objectives for the ATPL Met exam. Work it out. The difference is small on approach and greater at high altitude but the examiner would be looking to see whether you have taken account of the fact that temperature does make a difference. In JAR land some say they are anal, others would take the view that they do it properly. Is the pilot supposed to get the ATIS to get the surface temp and work it out there and then? What is the typical correction, for an airport at say 1000ft AMSL which is ISA+10? +40 feet. Being the height in thousands * 4 * ISA deviation In this case 1 * 4 * +10 Temperatures - well you could get the temperature from the ATIS but you also having sitting in front of you on the temperature gauge too. I am sure the examiner will be looking at that and doing the mental calculations. With a tolerance of +50 - 0 for at the decision height, it does not take too much working out that you could blow the tolerances when its either much warmer or much colder than ISA. Say you are planning to fly the check ride a bit too close to the DH. On a cold day, you would be below the DH. On a very warm day, you could well above it and outside the tolerance especially if you decided that you would aim to be about say 40 ft above the DH. A 30 degree C day would have you actually higher than indicated at say 252 ft and busted. When the temp is less than ISA the aircraft will be lower than indicated height by 4ft /1000 for each degree c different. So at ISA dev -15 C at 200 ft above touch down, the actual height will be 188. ISA -25 C at 200 ft above touch down, the actual height will be 180. Its not hard to find places in Europe in winter where the outside temperature is ISA -25 C after all, thats only -10 C. Hence why tables are produced to help pilots especially in cold climates to make allowance. Here is a helpful presentation - I have flow into all three of the field used as examples http://williams.best.vwh.net/smxgigpdf/smxgig2000.pdf Another one from Transport Canada and I suggest you scroll down to fig 9 where there is a correction table laid out. The numbers differ from mine because they round up so my 12 ft, they round up to 20 ft. |
#24
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Pressure & temperature
The correction at 10,000 feet MSL is zilch when you include
the altimeter of the reporting station, it isn't the altitude above sea level, it is the amount of altitude between the aircraft and the field elevation, which will be only 2,000, the errors below that altitude are corrected and cancelled when they take the altimeter setting. "S Green" wrote in message ... | | "Peter" wrote in message | ... | | "S Green" wrote | | Its in the JAR instrument test standards and features within the learning | objectives for the ATPL Met exam. | | Work it out. The difference is small on approach and greater at high | altitude but the examiner would be looking to see whether you have taken | account of the fact that temperature does make a difference. | | In JAR land some say they are anal, others would take the view that they | do | it properly. | | Is the pilot supposed to get the ATIS to get the surface temp and work | it out there and then? | | What is the typical correction, for an airport at say 1000ft AMSL | which is ISA+10? | | +40 feet. Being the height in thousands * 4 * ISA deviation | | In this case 1 * 4 * +10 | | Temperatures - well you could get the temperature from the ATIS but you | also having sitting in front of you on the temperature gauge too. I am sure | the examiner will be looking at that and doing the mental calculations. | | With a tolerance of +50 - 0 for at the decision height, it does not take | too much working out that you could blow the tolerances when its either much | warmer or much colder than ISA. | | Say you are planning to fly the check ride a bit too close to the DH. On a | cold day, you would be below the DH. | | On a very warm day, you could well above it and outside the tolerance | especially if you decided that you would aim to be about say 40 ft above the | DH. A 30 degree C day would have you actually higher than indicated at say | 252 ft and busted. | | When the temp is less than ISA the aircraft will be lower than indicated | height by 4ft /1000 for each degree c different. | | So at ISA dev -15 C at 200 ft above touch down, the actual height will be | 188. | ISA -25 C at 200 ft above touch down, the actual height will be 180. | | Its not hard to find places in Europe in winter where the outside | temperature is ISA -25 C after all, thats only -10 C. | | Hence why tables are produced to help pilots especially in cold climates to | make allowance. | | Here is a helpful presentation - I have flow into all three of the field | used as examples http://williams.best.vwh.net/smxgigpdf/smxgig2000.pdf | | Another one from Transport Canada and I suggest you scroll down to fig 9 | where there is a correction table laid out. The numbers differ from mine | because they round up so my 12 ft, they round up to 20 ft. | | |
#25
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Pressure & temperature
The 40 foot error is from a altitude of 10,000 feet if the
pressure was based on sea level readings. An E6b manual true altitude procedure will show the method that should be used. The true altitude calculation is based on the difference between altitudes, not MSL altitude. "Peter" wrote in message ... | | "S Green" wrote | | +40 feet. Being the height in thousands * 4 * ISA deviation | | In this case 1 * 4 * +10 | | I don't understand this. You are landing with the *airport* QNH set on | the altimeter. How can the altimeter reading be 40ft out when it's | reading 200ft, purely due to non-ISA temp? |
#26
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Pressure & temperature
On Oct 26, 5:07 pm, "S Green" wrote:
"Peter" wrote in message ... "S Green" wrote Its in the JAR instrument test standards and features within the learning objectives for the ATPL Met exam. Work it out. The difference is small on approach and greater at high altitude but the examiner would be looking to see whether you have taken account of the fact that temperature does make a difference. In JAR land some say they are anal, others would take the view that they do it properly. Is the pilot supposed to get the ATIS to get the surface temp and work it out there and then? What is the typical correction, for an airport at say 1000ft AMSL which is ISA+10? +40 feet. Being the height in thousands * 4 * ISA deviation In this case 1 * 4 * +10 You are applying the calcualtion incorrectly. At a DH of 200ft, you would be only (200/1000)*4*(+10) = 8ft in error, not 40ft. The error of +40 ft would be correct only if you are at 1000 ABOVE the airport, at which point the 40ft is insignificant. At very large temperature shifts, (ISA-50) for example, the error could become significant, which is why special procedures are published for cold areas. For all other areas, you are really splitting hairs. If this is the only thing the examiner is worried about during a checkride, I'd say candidate is doing a fine job. |
#27
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Pressure & temperature
On Oct 27, 1:12 pm, Andrew Sarangan wrote:
On Oct 26, 5:07 pm, "S Green" wrote: "Peter" wrote in message .. . "S Green" wrote Its in the JAR instrument test standards and features within the learning objectives for the ATPL Met exam. Work it out. The difference is small on approach and greater at high altitude but the examiner would be looking to see whether you have taken account of the fact that temperature does make a difference. In JAR land some say they are anal, others would take the view that they do it properly. Is the pilot supposed to get the ATIS to get the surface temp and work it out there and then? What is the typical correction, for an airport at say 1000ft AMSL which is ISA+10? +40 feet. Being the height in thousands * 4 * ISA deviation In this case 1 * 4 * +10 You are applying the calcualtion incorrectly. At a DH of 200ft, you would be only (200/1000)*4*(+10) = 8ft in error, not 40ft. The error of +40 ft would be correct only if you are at 1000 ABOVE the airport, at which point the 40ft is insignificant. At very large temperature shifts, (ISA-50) for example, the error could become significant, which is why special procedures are published for cold areas. For all other areas, you are really splitting hairs. If this is the only thing the examiner is worried about during a checkride, I'd say candidate is doing a fine job.- Hide quoted text - - Show quoted text - Here is a table that lists altimeter errors due to nonstandard temperatures: http://www.faa.gov/airports_airtraff...7/aim0702.html |
#28
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Pressure & temperature
Maybe the FAA isn't so dumb. They are anal, I'd give you that.
It just could be that the worst case errors are already factored into the minimums to begin with; no adjustment required. At 500' AGL, the errors aren't zero but they would be small. Isn't that how you would do it? We like simple. Bill H. On Oct 26, 10:04 am, "S Green" wrote: "Peter" wrote in message ... "akjcbkJA" wrote In Europe its the easiest way to fail an IR checkride by failing to compensate for the ISA deviation in the winter and being deemed to have busted the limits Is this your personal experience? I've never heard of anybody here (UK) using anything other than the published minima, directly. One would be landing with the airport QNH set, and altimeter error caused by non-ISA temperature is negligible at more or less any airport in Europe that has an instrument approach. And at high altitute airports one would not be using the airport QNH. They have some other system. An altimeter is a pretty inaccurate device once well away from the ground. Its in the JAR instrument test standards and features within the learning objectives for the ATPL Met exam. Work it out. The difference is small on approach and greater at high altitude but the examiner would be looking to see whether you have taken account of the fact that temperature does make a difference. In JAR land some say they are anal, others would take the view that they do it properly.- Hide quoted text - - Show quoted text - |
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