Manifold Pressure at Rest

Kage,

let's try again, shall we?

Assume close to a standard atmosphere. Engine off.

Case 1: We're at a sea level airfield.

1. The MP will read the ambient pressure at field elevation, which is
29.92.

2. The altimeter, when set to the altimeter setting of the day of
29.92, will read the field elevation of 0 feet.

3. When the altimeter is set to indicate 0 feet at the field elevation,
the altimeter's Kollsmann window will read the ambient pressure at
field elevation, which is 29.92.


Case 2: We're at an airfield at 2000 feet elevation.

1. The MP will read the ambient pressure at field elevation, which is
27.92.

2. The altimeter, when set to the altimeter setting of the day of
29.92, will read the field elevation of 2000 feet.

3. When the altimeter is set to indicate 0 feet at the field elevation,
the altimeter's Kollsmann window will read the ambient pressure at
field elevation, which is 27.92.

Thus, when you set the altimeter as described in 3. (i.e. to indicate
the field elevation) it will show the same as the MP gauge. FWIW, the
altimeter setting that produces an indication of 0 feet at the field
elevation is called QFE in ICAO speak (which the US isn't good at ;-)).
The "altimeter setting" you guys in the US know is called QNH elsewhere
in the world.

This setting is what I was trying to describe in my posts. Where,
pray, tell, is that "not even close" and "completely erroneous"? On the
contrary, your "It [the MP gauge] should be close to the setting [on
the altimeter] seen when field elevation is dialed in." is definitely
wrong.

--
Thomas Borchert (EDDH)

Robert,

I thought
you were referring to zero MSL but it would be zero AGL.


Yep. I tried to explain in detail in another post to kage.

--
Thomas Borchert (EDDH)

Thomas,

(i.e. to indicate
the field elevation)


Oops, messed up there. That should read "i.e. to indicate 0 feet at the
field elevation".

--
Thomas Borchert (EDDH)

"kage" wrote in message ...
Thomas,

Well it might be wrong, but it will be close, like I said----not exact. And
it is a good preflight check to see if there are gross errors in the
instruments.

True, but atmospheric pressure isn't involved in this test at all. You
are comparing altimeter setting with altimeter reading. You are just
verifying the altimeter setting. Pressure really has nothing to do
with it. They should never have labeled the altimeter with numbers
like 29.92. They should have just called them 123, etc.

If the actual pressure did change, it would change both your altimeter
reading and altimeter setting, so pressure isn't really relevant to
testing your altimeter against the altimeter setting.

-Robert