Setting altimeters with no radio

mike regish schrieb:
Well, what exactly are their reference datum(s?).

GPS altitude and pressure altitude are completely different and, most
important, *incompatible* systems.

GPS altitude gives you a geometrically derived value above some
reference surface. Mostly it's the WGS84 geoid. Of course this can be
converted to MSL altitude if you wish. But this isn't the point.

The point is that the pressure altimeter measures, well, a pressure, not
an altitude. It displays a value in feet, but actually, this is wrong.
It may be true in very specific conditions, but the real atmosphere very
seldom matches the theoretical ISA atmosphere model on which the
altimeter is based. And, most important, there is no way to calculate
the real (geometric) altitude from the displayed pressure altitude. (Of
course it could, if you knew the complete atmosphere profile, but you
usually don't.)

So the point is: GPS gives you the true altitude in reference to the
earth, but this doesn't help you, because the whole aviatic system
(airspace boundaries, ATC clearances, traffic separation) is based on
pressure altitude, and there is no practical way to convert one into the
other. If you are given an ATC clearance for a certain pressure altitude
but fly GPS altitude instead, then you act exactly like that bozo who
drives on the wrong side of the road.

The only use for GPS altitude in aviation is to calculate the final
glide of a glider. (Or an IFR approach, but I don't know enough about
IFR flight to comment this.)

But I'm sure you knew all this before, because after all, you've passed
a written.

Stefan

Above 18K feet, everybody is on pressure altitude. Down in the sewers, where
I fly, we're all on local pressure.

mike

"Mxsmanic" wrote in message
...

Not true. Altimeters show substantial differences with geometric
altitude as altitude increases. Air traffic control is based on
pressure altitudes, not geometric altitudes.

The altimeter is much more accurate.

That's what I would assume with my particular GPS.

Do those amounts vary with location? How large a difference is there between
the 2? I remember reading about the 2 standards, but forget how the mean
geoid is determined.

But you're right. If that's true, and I don't doubt it is, GPS would be
better suited to terrain avoidance and less so to aircraft separation.

mike

"Mxsmanic" wrote in message
...
mike regish writes:

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

Nope. GPS is height above the mean surface of the geoid, altimeter is
height above mean sea level. They can be hundreds of feet apart.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Gee, that was actually a well thought out, well explained, and in my case
anyway, very informative post.

It's been 9 years since my written, but I don't remember a whole lot about
geoid reference in it. I do remember the WSG84 reference in my handheld
(which I never use for altitude, BTW) though, so it must have been there.

mike

P.S. I got a 96


"Stefan" wrote in message
...
mike regish schrieb:
Well, what exactly are their reference datum(s?).

GPS altitude and pressure altitude are completely different and, most
important, *incompatible* systems.

very knowledgeable post snipped

But I'm sure you knew all this before, because after all, you've passed a
written.

Stefan

"TxSrv" wrote in message
. ..
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.

The requirement stated in 91.121a1iii does not include any exception for
flights under 3000' AGL. If you have an adjustable altimeter, you have to
set it according to 91.121, regardless of your altitude.

Therefore, under 3,000, an adjustable altimeter is not required per
91.205.

According to 91.205, an adjustable altimeter is only required for IFR
(regardless of altitude).

--Gary

My filter failed and this message popped up... Not an issue of any
importance in itself and fixed again with two mouse clicks...
However, seeing the list of you tripping over yourselves to answer his
trolling have me howling with laughter...

denny

Mxsmanic wrote:
Since it is possible to fly without voice radio equipment, and given
that (if I understand correctly) pilots are supposed to have their
altimeters set correctly to a reference located not more than 100
miles from their position, how does an aircraft without a radio keep
its altimeter properly set as it travels?

--
Transpose mxsmanic and gmail to reach me by e-mail.

"TxSrv" wrote in message
. ..

For VFR, an adjustable altimeter is not required. Therefore, such a 100
mile rule doesn't exist.


What aircraft altimeter is not adjustable? IFR flight requires a sensitive
altimeter adjustable for barometric pressure, VFR flight simply requires an
altimeter. But even altimeters that cannot be adjusted for barometric
pressure are adjustable. My Champ' came with a single-needle altimeter,
once around the dial was 10,000 feet. I hated it, the needle never seemed
to move. But there was an adjustment knob that rotated the dial so the
needle would point to field elevation.

The regulation requires aircraft without radios to set the altimeter to the
elevation of the departure airport or an appropriate altimeter setting
available before departure.

Mxsmanic wrote:
Since it is possible to fly without voice radio equipment, and given
that (if I understand correctly) pilots are supposed to have their
altimeters set correctly to a reference located not more than 100
miles from their position, how does an aircraft without a radio keep
its altimeter properly set as it travels?

--
Transpose mxsmanic and gmail to reach me by e-mail.

So MXsmanic, you strike again.

For those who wonder why people give you the answers you get, there's a
good reason.
Now it's my turn to tell people about you. I also did some searching on
your past postings too. You show up here pretending to be a pilot, and
a lot of people who haven't fallen for the act try and give you helpful
advice, to which you aren't thankful. What the heck is this game you
are playing with everyone? I'm VERY grateful that I was tipped off to
your game by some honest posters last week.

If you were more grateful, I might feel differently. Or if you told
people in your posts that you aren't a pilot, and don't ever intend to
be one, that would be fine too. But all of your questions, and there
are a lot of them, are worded so that people are misled. Could you
please start including in your posts that you are looking for info for
your MS flight simulator? Don't get me wrong, I like flight simulators,
and there's nothing wrong with non-pilots asking questions- they
deserve the same respect. But the way you decieve people here is
disrespectful. Don't take advantage of the fact that pilots are
inclined to help eachother.

"Cirrus" wrote in message
oups.com...
If you were more grateful, I might feel differently. Or if you told
people in your posts that you aren't a pilot, and don't ever intend to
be one, that would be fine too.

Sorry, but I think you're overreacting a bit. It's true that in a few of his
posts, Mx misleadingly describes his simulator exploits as though they were
real. But the vast majority of his posts don't do that, and many of them
make it explicit that he's not a pilot. In any case, everyone participating
in this thread so far is aware that Mx isn't a pilot.

I think your impression of Mx's posts is probably skewed by the fact that
your introduction to Mx happened to occur with one of the few posts of his
that was phrased misleadingly with regard to his flight status.

--Gary

Dang. I actually learned something from this post.

An' ah already gots one o them pilot licencse thingys.

mike

"Cirrus" wrote in message
oups.com...

So MXsmanic, you strike again.

For those who wonder why people give you the answers you get, there's a
good reason.