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#21
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Setting altimeters with no radio
agreed
"Neil Gould" wrote in message m... | Recently, Mxsmanic posted: | | "Jim Macklin" writes: | | GPS, even a $100 hikers model will solve the problem. But I | just say, look at the ground, you can judge 1,000 feet | pretty well and you only need to apply the hemisphere rule | above 3,000 AGL. | | GPS is far less accurate than an altimeter, and I don't think the | regulations say "if you have no radio, use GPS." | | I'm not sure why you think that "GPS is far less accurate than an | altimiter...", as an altimeter only need be accurate to 75' to be legal. | GPS can do much better than that, and are unaffected by barometric | pressure; the result is a potential source of problems that require pilots | to fly by the altimeter, not the GPS. None of this has anything to do | with regulations, of course. | | To answer your original question, the prudent pilot will take the | barometric pressure of their destination into consideration during | preflight planning, and adjust the altimeter accordingly. Most of the | time, the pressure won't change all that drastically at the destination in | the time it takes to fly 100 miles, and non-radio VFR pilots aren't likely | to fly in weather where the pressure is changing too rapidly. | | Neil | | | |
#22
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Setting altimeters with no radio
Jim Macklin schrieb:
they give altitude, accurate to within a few feet. just set the altimeter to read the same. NO, dont' do this. The whole aviation system (airspace, separation...) is based on pressure altitude, not geometric altitude. Stefan |
#23
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Setting altimeters with no radio
Well, kind of. Most, if not all altimeters now, compensate for pressure
variations to show geometric (if that's the right term for it) altitude. With SA disables in the GPS system, altitude is much more accurate than it was. How accurate, I don't know. When I compare my altimeter with my GPS's, they're usually about 100' apart. I'm not sure which is more accurate. mike "Stefan" wrote in message . .. Jim Macklin schrieb: they give altitude, accurate to within a few feet. just set the altimeter to read the same. NO, dont' do this. The whole aviation system (airspace, separation...) is based on pressure altitude, not geometric altitude. Stefan |
#24
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Setting altimeters with no radio
The question was IF no radio available to get a corrected
altimeter setting... if a GPS is available the GPS altitude is better than no correction at all. If I take-off in a airplane with no radio and fly through a cold front in VFR, the altimeter setting is likely to change enough to have a 500 foot or greater error with a 100 mile flight. A GPS can be used to get closer than that. Probably, most NORDO flights are at or below 3,000 AGL under VFR. Terrain clearance is more critical to such a flight based on the altimeter than is the mid-air collision problem since the "rule" only applies above 3,000 AGL. Someday, the rules may change, but within today's rules, using GPS "as the best available" or only source is better than no source at all. "mike regish" wrote in message . .. | Well, kind of. Most, if not all altimeters now, compensate for pressure | variations to show geometric (if that's the right term for it) altitude. | | With SA disables in the GPS system, altitude is much more accurate than it | was. How accurate, I don't know. When I compare my altimeter with my GPS's, | they're usually about 100' apart. I'm not sure which is more accurate. | | mike | | "Stefan" wrote in message | . .. | Jim Macklin schrieb: | they give altitude, accurate to within a few feet. just set the | altimeter to read the same. | | NO, dont' do this. The whole aviation system (airspace, separation...) is | based on pressure altitude, not geometric altitude. | | Stefan | | |
#25
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Setting altimeters with no radio
Mxsmanic wrote:
You can find the answer in FAR 91.121(a)(iii). Found it, thanks. I'm surprised that just setting it at the airport would suffice--one could conceivably fly for hundreds of miles VFR, and the altimeter could change significantly along the way. That Reg does not apply to flight within 3,000 above the surface. You have to look at two other Regs to conclude that. Therefore, under 3,000, an adjustable altimeter is not required per 91.205. FARs don't apply to you; stay away. F-- |
#26
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Setting altimeters with no radio
mike regish schrieb:
When I compare my altimeter with my GPS's, they're usually about 100' apart. I'm not sure which is more accurate. The point is not which one is more accurate. The point is that everybody uses the same frame of reference. Stefan |
#27
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Setting altimeters with no radio
Jim Macklin schrieb:
The question was IF no radio available to get a corrected altimeter setting... if a GPS is available the GPS altitude is better than no correction at all. Agreed. Probably, most NORDO flights are at or below 3,000 AGL under VFR. Terrain clearance is more critical to such a flight In such a flight you usually just look out of the window for terrain clearance. Stefan |
#28
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Setting altimeters with no radio
Agreed also, but if at night, the ground can be hard to see
if there is no moon or ground lights. The question is some what academic, even aircraft without radios often have a handheld and altimeter settings can be had from ATIS, etc. The point about GPS, it is accurate in relation to terrain. It is not good for IFR separation or ATC boundaries. "Stefan" wrote in message ... | Jim Macklin schrieb: | The question was IF no radio available to get a corrected | altimeter setting... if a GPS is available the GPS altitude | is better than no correction at all. | | Agreed. | | Probably, most NORDO flights are at or below 3,000 AGL under | VFR. Terrain clearance is more critical to such a flight | | In such a flight you usually just look out of the window for terrain | clearance. | | Stefan |
#29
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Setting altimeters with no radio
My point is that they both are, basically, the same frame of
reference-height above sea level. mike "Stefan" wrote in message ... mike regish schrieb: When I compare my altimeter with my GPS's, they're usually about 100' apart. I'm not sure which is more accurate. The point is not which one is more accurate. The point is that everybody uses the same frame of reference. Stefan |
#30
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Setting altimeters with no radio
mike regish schrieb:
My point is that they both are, basically, the same frame of reference-height above sea level. Which is wrong. Stefan |
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