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#1
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low altimeter and FL180
Was flying in the Denver area today and approach told a plane that FL 180
was unavailable die to low altimeter, which was 28.87 at the time. What is the reason for this? Confusion with 29.92 readings? Thanks, Michael |
#2
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"Michael 182" wrote in message news:V3Bnb.36695$mZ5.183695@attbi_s54... Was flying in the Denver area today and approach told a plane that FL 180 was unavailable die to low altimeter, which was 28.87 at the time. What is the reason for this? Confusion with 29.92 readings? Because there would be less than 1000 feet separation between aircraft at 17,000 MSL using 28.87 as an altimeter setting and aircraft at FL180 using 29.92. In this case, FL190 is also unavailable for the same reason. |
#3
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"Michael 182" writes: Was flying in the Denver area today and approach told a plane that FL 180 was unavailable die to low altimeter, which was 28.87 at the time. What is the reason for this? Confusion with 29.92 readings? FL180 is 18000 indicated feet with the altimeter set to 29.92. That same airspace region is at 16950 ft indicated with the actual altimeter setting. In order to keep traffic using the two altimeter settings apart, FL180 is not used, nor is FL190 (17950) in this case. - FChE |
#4
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"Frank Ch. Eigler" wrote in message news "Michael 182" writes: Was flying in the Denver area today and approach told a plane that FL 180 was unavailable die to low altimeter, which was 28.87 at the time. What is the reason for this? Confusion with 29.92 readings? FL180 is 18000 indicated feet with the altimeter set to 29.92. That same airspace region is at 16950 ft indicated with the actual altimeter setting. In order to keep traffic using the two altimeter settings apart, FL180 is not used, nor is FL190 (17950) in this case. To put it another way, flight levels don't exist below the transition altitude (18,000 Feet MSL In the US). If FL180 would be at 16950, it's not available. |
#5
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Thanks for the replies. Makes sense now.
Michael "Ron Natalie" wrote in message m... "Frank Ch. Eigler" wrote in message news "Michael 182" writes: Was flying in the Denver area today and approach told a plane that FL 180 was unavailable die to low altimeter, which was 28.87 at the time. What is the reason for this? Confusion with 29.92 readings? FL180 is 18000 indicated feet with the altimeter set to 29.92. That same airspace region is at 16950 ft indicated with the actual altimeter setting. In order to keep traffic using the two altimeter settings apart, FL180 is not used, nor is FL190 (17950) in this case. To put it another way, flight levels don't exist below the transition altitude (18,000 Feet MSL In the US). If FL180 would be at 16950, it's not available. |
#6
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AIM Section 7 Chapter 2
"Michael 182" wrote in message news:V3Bnb.36695$mZ5.183695@attbi_s54... Was flying in the Denver area today and approach told a plane that FL 180 was unavailable die to low altimeter, which was 28.87 at the time. What is the reason for this? Confusion with 29.92 readings? Thanks, Michael |
#7
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Ron Natalie wrote:
"Frank Ch. Eigler" wrote in message news "Michael 182" writes: Was flying in the Denver area today and approach told a plane that FL 180 was unavailable die to low altimeter, which was 28.87 at the time. What is the reason for this? Confusion with 29.92 readings? FL180 is 18000 indicated feet with the altimeter set to 29.92. That same airspace region is at 16950 ft indicated with the actual altimeter setting. In order to keep traffic using the two altimeter settings apart, FL180 is not used, nor is FL190 (17950) in this case. To put it another way, flight levels don't exist below the transition altitude (18,000 Feet MSL In the US). If FL180 would be at 16950, it's not available. I hope you'll forgive the intrusion from a lurker and non-pilot (well, a wannabe-pilot). Would someone please explain a few points about this to me? I understand that altimeters are re-calibrated to correct for the current barometric pressure (extrapolated to MSL). My questions: - Why would people be using two different altimeter settings anyway? - Why would this problem affect just FL180 and FL200? Why not FL160, 220, and every other altitude? - Portions of other posts make it sound like there's something special about calibrating ones' altimeter to 29.92, as if that's commonly done in certain situations. Is there something different about that specific number? or am I misinterpreting other posts? I don't have any reference books mentioned in another post, so cannot look up these answers myself. Thanks for any light you can shed. - Bo |
#8
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Robert Lyons wrote: - Why would people be using two different altimeter settings anyway? Because the regulations require it. In the States, all aircraft flying at or above 18,000' ASL are required to set their altimeters to 29.92. Aircraft flying below 18,000' are expected to set their altimeters to the local barometric pressure. - Why would this problem affect just FL180 and FL200? Why not FL160, 220, and every other altitude? It only affects flight levels at which the actual altitude of a plane using the 29.92 setting would be low enough to conflict with aircraft using the actual pressure setting. In this case, a plane flying at 18,000' using 29.92 would be at 16950 ft indicated with the actual altimeter setting. If another aircraft using the local barometric pressure is flying at 17,000', there is a possibility of collision. One pilot "knows" he's at 18,000' and the other pilot "knows" he's at 17,000', but they're actually at nearly the same altitude. Since altimeters can be slightly inaccurate, they could easily be at exactly the same altitude. Since ATC is tasked with ensuring a certain amount of separation between IFR aircraft, ATC will simply not assign 18,000' as a flight altitude. FL 200 and higher don't present problems here. FL 210 would be an altitude of 19,950', and there will be no aircraft at that altitude using the local setting. George Patterson You can dress a hog in a tuxedo, but he still wants to roll in the mud. |
#9
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"Robert Lyons" wrote in message
... - Why would people be using two different altimeter settings anyway? Well, the situation at hand is that one airplane is flying at or above "flight level 180" while the other is flying below "flight level 180". By definition, altitudes "in the flight levels" are referenced to standard atmospheric pressure (29.92) even if non-standard conditions prevail. So anyone flying at or above FL180 has their altimeter set to 29.92. However, for all flying below the flight levels, the altimeter will generally be set to the prevailing conditions (sea-level corrected, of course). This is, in fact, the whole point with respect to the question at hand. The reason for using a single altimeter setting for all airplanes at or above FL180 is that most aircraft flying at that altitude travel so quickly that it would be impractical for them to have to keep changing their altimeter setting as they fly through different weather systems. They are at a high enough altitude that the error between indicated and true altitude won't cause any problem with terrain (at least in the US), which is the main reason for using a local altimeter setting in the first place. - Why would this problem affect just FL180 and FL200? Why not FL160, 220, and every other altitude? It affects all flight above FL180. There is no "FL160" in the US, and I'm not sure why you got the impression that flights at FL220 are unaffected. - Portions of other posts make it sound like there's something special about calibrating ones' altimeter to 29.92, as if that's commonly done in certain situations. Is there something different about that specific number? or am I misinterpreting other posts? 29.92" is the pressure at sea level in standard atmospheric conditions. It just happens to be a nice "middle-ground" number. Nothing special other than that. By the way, we typically reserve the word "calibrate" for an infrequent operation in which the accuracy of the altimeter is ensured. Setting the altimeter setting to a specific number in flight is generally referred to as "setting the altimeter setting". Pete |
#10
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Thanks VERY much, George and Peter, for your cogent answers to my
questions. I understand now. - Bo |
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