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
|
|
|

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

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

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
|
|

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--

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

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

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

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

mike regish schrieb:
My point is that they both are, basically, the same frame of
reference-height above sea level.

Which is wrong.

Stefan