Setting altimeters with no radio

"Stefan" wrote in message
. ..
Travis Marlatte schrieb:
VFR, it doesn't really matter, does it?

I've always thought that airspace boundaries are to be respected by VFR
traffic, too.

Stefan

I agree that there are many reasons why I need to know my pressure
compensated altitude to avoid busting airspace or illegally flying to close
to person or property.

The discussion was about the life-saving necessity of an accurate altitude
instrument. MX was claiming that GPS is inaccurate enough to kill you. I
presume that that can only happen in one of two ways 1) I fly into the
ground believing my inaccurate intrument or 2) I fly at an altitude
different than the regulated VFR cruise altitudes and run into another
plane.

I think both are highly unlikely in VFR conditions no matter how inaccurate
the instrument.


-------------------------------
Travis
Lake N3094P
PWK

Travis Marlatte schrieb:

The discussion was about the life-saving necessity of an accurate altitude
instrument.

Sorry to nitpick, but the discussion was about how to set the altimeter
if you can't get the lastest pressure data with the radio. Some guy
proposed to use the GPS altitude as reference, and then some (other?)
guy stated that you better always fly on GPS altitudes, because it's
more accurate anyway. This was the point when I jumped in and said no,
don't do this.

Only then:

MX was claiming that GPS is inaccurate enough to kill you.

Actually, it can. It's accurate within a few meters (depending on how
"enhanced" it is), but only in some 95% or so of the time and in some
90% or so places (the correct numbers don't matter). Which means that if
you happen to be in the wrong place at the wrong time, believing in the
GPS can actually kill you, at least theoretically. Of course I'm rather
looking out of the window when I fly in VMC, so this isn't an issue in
real life for me.

Stefan

Stefan wrote:

Actually, it can. It's accurate within a few meters (depending on how
"enhanced" it is), but only in some 95% or so of the time and in some
90% or so places (the correct numbers don't matter). Which means that if
you happen to be in the wrong place at the wrong time, believing in the
GPS can actually kill you, at least theoretically.

Stefan

Actually the 95% number often associated with an accuracy is a
confidence. It is not an availability value. Thus under the
identical conditions 95 times out of a hundred you would be at that
accuracy or better. No value of the other 5% is given and may not be
very much worse. At least in this case not enough to be
life-threatening. Of course if VFR use your eyes and look outside.

Ron Lee

No one has mentioned using the manifold pressure guage as an altimeter.

Doug wrote:

No one has mentioned using the manifold pressure guage as an altimeter.

Or how many times your ears popped (:

Notice how IFR training doesn't include procedures if your altimeter
fails. Reason?

Jim Stewart wrote:
Doug wrote:

No one has mentioned using the manifold pressure guage as an altimeter.

Or how many times your ears popped (:

Doug wrote:
No one has mentioned using the manifold pressure guage as an altimeter.


Because you can't do it in the air, unless you stop the engine first.

No one has mentioned using the manifold pressure guage as an altimeter.
Because you can't do it in the air, unless you stop the engine first.

Well, you can, sort of, but it probably won't help much. At full
throttle, the manifold pressure will max out at a value that has the
same relationship to altitude as an altimeter. I vaguely recall it's
something like an inch loss per thousand feet high.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.

On a normally aspirated engine, the MAP will be about 1 inch
per thousand feet from 29.92 plus about 1.5 inches for
induction losses on a running engine. So if the maximum
observed MAP is 23 inches, you are at about 5,500 feet
pressure altitude [give or take a thousand feet.
Turbocharged engines make such a check impractical.



"Jose" wrote in message
news | No one has mentioned using the manifold pressure guage
as an altimeter.
| Because you can't do it in the air, unless you stop the
engine first.
|
| Well, you can, sort of, but it probably won't help much.
At full
| throttle, the manifold pressure will max out at a value
that has the
| same relationship to altitude as an altimeter. I vaguely
recall it's
| something like an inch loss per thousand feet high.
|
| Jose
| --
| "Never trust anything that can think for itself, if you
can't see where
| it keeps its brain." (chapter 10 of book 3 - Harry
Potter).
| for Email, make the obvious change in the address.