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#11
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On Mon, 03 May 2004 20:34:22 GMT, "John R. Copeland"
wrote: Along that line, perhaps you've noticed that the new VNAV approaches typically carry a note to the effect that Baro-VNAV is not authorized at temperatures below some limiting number. ---JRC--- Yes, I have. The RNAV (GPS) Z RWY 1 at KPQI states Baro-VNAV not authorized below -16°C (4°F). Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA) |
#12
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Ron Rosenfeld wrote:
It is my understanding that in Canada there is a requirement to alter minimums for certain approaches in very cold weather. No such requirement exists (Part 91, at least) in the US. I don't believe there have been any accidents in the US due to this phenomenon. You're correct about the Canadian regs, though I don't know how many Canadian pilots remember to apply them. In any case, many parts of the northern U.S. can see temperatures down to -35 degC or -40 degC just as much as Canada can; when you combine that with the allowed +/-50 ft altimeter error, you could be down below 100 ft when you think you're at 200 ft DH. A more likely problem, though, would be early in an IAP, when you're still a few thousand feet above the station elevation and have to clear a mountain or a tower on a hill. All the best, David |
#13
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Bob Gardner wrote:
AIM 1-1-20 (a)(8) tells us not to rely upon GPS to determine aircraft altitude. Right -- ATC counts on the altimeter's temperature errors for traffic separation (since every plane in the same area will share the same errors). Our altimeters are pretty inaccurate at cruising altitudes, but as long as we're all inaccurate in the same way, the system works. In any case, I don't think that the poster was using his GPS for altitude -- he was just concerned that his altimeter might be wrong. I expect that this will be a FAQ now that a lot of people have WAAS-enabled GPS's (they used to think that the altimeter was right and the GPS altitude wildly inaccurate). All the best, David |
#14
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SOURCE DOD FLIGHT INFORMATION HANDBOOK
(https://164.214.2.62/dafif/dafif_0404_ed6/plan/fih.pdf) 3. TEMPERATURE ERRORS (AFFSA/AFFSA LTR) a. Pressure altimeteres are calibrated to indicate true altitudes under International Standard Atmospheric (ISA) conditions. Any deviation from these standard conditions will result in an erroneous reading on the altimeter. This error becomes important when considering obstacle clearances in temperatures lower than standard since the aircraft’s altitude is below the figure indicated by the altimeter. b. The error is proportional to the difference between actual and ISA temperature and the height of the aircraft above the altimeter setting source. Height above altimeter source is considered to be published HAT or HAA for the approach. The amount of error is approximately 4 feet per thousand feet for each degree Celsius of difference. c. Corrections will only be made for Decision Heights (DHs), Minimum Descent Altitudes (MDAs), and other altitudes inside, but not including, the Final Approach Fix (FAF). The same correction made to DHs and MDAs can be applied to the other altitudes inside the FAF. TEMPERATURE CORRECTION CHART (FEET) AIRPORT TEMP °C 0 0 20 20 20 40 40 40 40 60 80 90 110 120 140 180 240 300 -5 10 20 30 30 50 50 60 60 80 110 120 150 160 180 240 320 400 -10 20 20 40 40 60 60 80 80 100 130 150 180 200 230 300 400 500 -15 20 30 50 50 70 80 90 100 120 160 180 220 240 280 360 480 600 -20 20 40 60 60 80 100 100 120 140 180 210 250 280 320 420 560 700 -25 30 50 60 70 90 110 120 140 160 210 240 290 320 370 480 640 800 -30 40 60 60 80 100 120 140 160 180 240 270 330 360 410 540 720 900 -35 40 60 70 90 110 130 150 180 200 260 300 360 400 460 600 800 1000 -40 40 60 80 100 120 140 160 200 220 290 330 400 440 510 660 880 1100 -45 50 70 90 110 140 160 180 210 240 310 360 430 480 550 720 960 1200 -50 60 80 100 120 160 180 200 220 260 340 390 470 520 600 780 1040 1300 200 300 400 500 600 700 800 900 1000 1300 1500 1800 2000 2300 3000 4000 5000 HAT/HAA NOTE: Must round interpoldated values to nearest 10 feet. On Sun, 2 May 2004 13:50:02 -0500, smf wrote: My Altimeter is accurate when on the ground or at lower alt. When I climb up to 7k and above and check it against GPS and approach controls read out it is about 300+ feet off. Is it the altimeter I'm having problems with? Steve -- Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/ |
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