Manifold Pressure at Rest

[Andrew Gideon]

Did I misunderstand my reading on the subject, or should the Manifold
Pressure on an engine that's off be the local atmospheric pressure?

I've just noticed that this is not the case in my little R182.

- Andrew

[Dave S]

Should be the same value if the engine is not operating and the gauge is
correctly calibrated. I would suspect the gauge is out of calibration.

Dave

Andrew Gideon wrote:
> Did I misunderstand my reading on the subject, or should the Manifold
> Pressure on an engine that's off be the local atmospheric pressure?
>
> I've just noticed that this is not the case in my little R182.
>
> - Andrew
>

[Dave S]

Let me amend my answer to be "same value if the engine is not operating,
the gague is correctly calibrated and you are AT SEA LEVEL".

Remember that local altimeter settings are corrected for sea level when
you are at a locale that is at elevation. 29.92" setting in Denver is a
"corrected reading" for what the pressure would be if you were at sea
level. So, ballpark.. the value is 1" per thousand (without digging out
reference material) pressure change/altitude. Denver on a standard day
(setting 29.92") should have a manifold pressure gauge reading (5000
ft/5" less) just under 25" when the engine is off.

Sorry for my initial error.
I'm sure if there are others, they will be quickly pointed out.
Dave

Dave S wrote:

> Should be the same value if the engine is not operating and the gauge is
> correctly calibrated. I would suspect the gauge is out of calibration.
>
> Dave
>
> Andrew Gideon wrote:
>
>> Did I misunderstand my reading on the subject, or should the Manifold
>> Pressure on an engine that's off be the local atmospheric pressure?
>>
>> I've just noticed that this is not the case in my little R182.
>>
>> - Andrew
>>
>

[John Gaquin]

Where are you?

"Andrew Gideon" > wrote in message
online.com...
> Did I misunderstand my reading on the subject, or should the Manifold
> Pressure on an engine that's off be the local atmospheric pressure?
>
> I've just noticed that this is not the case in my little R182.
>
> - Andrew
>

[Andrew Gideon]

John Gaquin wrote:

> Where are you?

I assume you're answered by: Less than 200' above sea level.

- Andrew

[BTIZ]

at less than 200ft MSL, then the MP indicator with the engine at rest (and
sitting on the ground) should read the approximate value of the barometric
pressure or about 30in

Out here (2200MSL) the MP indicator would read about 2 inches or more lower
or about 28in (or closer to 27.5in) MP before engine start

BT

"Andrew Gideon" > wrote in message
online.com...
> John Gaquin wrote:
>
> > Where are you?
>
> I assume you're answered by: Less than 200' above sea level.
>
> - Andrew
>

[Thomas Borchert]

Andrew,

> I've just noticed that this is not the case in my little R182.
>

Should be. Otherwise, your MP gauge might need repair. Ours is
indicating about 0.5 inches too much, so we correct our settings for
that.

--
Thomas Borchert (EDDH)

[Robert M. Gary]

Andrew Gideon > wrote in message e.com>...
> Did I misunderstand my reading on the subject, or should the Manifold
> Pressure on an engine that's off be the local atmospheric pressure?
>
> I've just noticed that this is not the case in my little R182.

It should be the same as ambient pressure, but not necessarily the
same a altimeter setting. Altimeter setting is not pressue but some
made up number that makes the altimeter read correctly (that's
actually the definition!).

-Robert

[Thomas Borchert]

Robert,

it should read the same pressure as the altimeter in the Kollsmann
window when the altimeter is set to zero feet, right?

--
Thomas Borchert (EDDH)

[g n p]

On Wed, 18 Aug 2004 17:57:19 +0200, Thomas Borchert
> wrote:

>Robert,
>
>it should read the same pressure as the altimeter in the Kollsmann
>window when the altimeter is set to zero feet, right?

......altimeter set to ***airfield elevation***, correct??
The old QFE vs QNH settings thing...
GNP, Socata TB-9 SX-ADE, LGTG, Greece, (grounded due to Olympics in
progress, next flight 29 September)

[kage]

No.

It should be close to the setting seen when field elevation is dialed in.

Karl


"Thomas Borchert" > wrote in message
...
> Robert,
>
> it should read the same pressure as the altimeter in the Kollsmann
> window when the altimeter is set to zero feet, right?
>
> --
> Thomas Borchert (EDDH)
>

[BTIZ]

I cannot adjust my altimeter to "zero feet", the adjustment does not go that
far..

BT

"Thomas Borchert" > wrote in message
...
> Robert,
>
> it should read the same pressure as the altimeter in the Kollsmann
> window when the altimeter is set to zero feet, right?
>
> --
> Thomas Borchert (EDDH)
>

[BTIZ]

well.. if he is at EDDH.. he is close to zero feet..

BT

"kage" > wrote in message
...
> No.
>
> It should be close to the setting seen when field elevation is dialed in.
>
> Karl
>
>
> "Thomas Borchert" > wrote in message
> ...
> > Robert,
> >
> > it should read the same pressure as the altimeter in the Kollsmann
> > window when the altimeter is set to zero feet, right?
> >
> > --
> > Thomas Borchert (EDDH)
> >
>
>

[Jack]

BTIZ wrote:

> I cannot adjust my altimeter to "zero feet", the adjustment does not go that
> far..

BTIZ, you're high again.


Jack

[Robert M. Gary]

Thomas Borchert > wrote in message >...
> Robert,
>
> it should read the same pressure as the altimeter in the Kollsmann
> window when the altimeter is set to zero feet, right?

Altimeter setting is defined to be the number required to make your
altimeter indicate field elevation on the field. It was a mistake many
years ago that they used the same scale as pressure does. It will be
somewhat close, but not the same, especially on a hot day.

-Robert

[Thomas Borchert]

Robert,

> Altimeter setting is defined to be the number required to make your
> altimeter indicate field elevation on the field.
>

I know that.

--
Thomas Borchert (EDDH)

[Thomas Borchert]

G,

> ......altimeter set to ***airfield elevation***, correct??
>

That will make the Kollsmann show the QNH. And, as has been mentioned
in this thread, that is not ambient pressure at the field's elevation.
Thus, it is NOT what MP will show.

--
Thomas Borchert (EDDH)

[Thomas Borchert]

Kage,

> It should be close to the setting seen when field elevation is dialed in.
>

I think you're wrong. When you dial in field elevation, the Kollsmann will
show the altimeter setting for the field on that day. And that, as has been
mentioned in this thread, is not ambient pressure at the field's elevation,
it is rather the pressure as corrected for MSL. Thus, it is NOT what MP
will show.

--
Thomas Borchert (EDDH)

[kage]

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.

Your:

"it should read the same pressure as the altimeter in the Kollsmann
window when the altimeter is set to zero feet, right?"

won't even be close, unless you're at sea level, and is completely
erroneous.

Karl

"Thomas Borchert" > wrote in message
...
> Kage,
>
> > It should be close to the setting seen when field elevation is dialed
in.
> >
>
> I think you're wrong. When you dial in field elevation, the Kollsmann will
> show the altimeter setting for the field on that day. And that, as has
been
> mentioned in this thread, is not ambient pressure at the field's
elevation,
> it is rather the pressure as corrected for MSL. Thus, it is NOT what MP
> will show.
>
> --
> Thomas Borchert (EDDH)
>

[Robert M. Gary]

Thomas Borchert > wrote in message >...
> Robert,
>
> > Altimeter setting is defined to be the number required to make your
> > altimeter indicate field elevation on the field.
> >
>
> I know that.

I may have misunderstood what you meant by "zero feet" then. I thought
you were referring to zero MSL but it would be zero AGL.

-Robert

[Thomas Borchert]

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)

[Thomas Borchert]

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 Borchert]

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)

[Robert M. Gary]

"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