flyrotary@lancaironline.net Mailing List Archive

Bill;

Yes; at or near sea level the correction factor is 1.000. If your airport is at 5000 ft, the correction factor is 0.832. So if the ATIS (or the altimeter setting to give you field elevation) is 29.92, then your engine off MAP should read 24.9”. And, yeah; you’re right – who needs a chart, the correction is about 1” per 1K ft. My field elevation is about 1400’, and the correction is 0.95, so my engine off MAP is .95 x altimeter setting; if 29.92 then my MAP should read 28.4”.

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Bill Bradburry
Sent: Tuesday, June 23, 2009 10:00 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: progress report #347c

Al,

Ok, Part of my misunderstanding lies in the fact that my field elevation is 55 feet. So both will read close to the same here. You are saying that his readings would have to be corrected for his 5000 foot altitude which is about 1 inch per 1K feet?

Bill

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Al Gietzen
Sent: Tuesday, June 23, 2009 2:12 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: progress report #347c

Al,

Hmmmm, Further explanation would be helpful here. I thought that the

altimeter was actually reading the air column weight or pressure at its

location. If you set the altimeter to the field elevation, it seems it

should read the barometric pressure in the window

Gees; I was hoping you wouldn’t ask – then I wouldn’t have to think about itJ.

Well; it goes something like this; your altimeter setting is the barometric pressure of the station as if the station were at sea level. So the altimeter setting is barometric pressure with an altitude correction applied. Your MAP is absolute ambient pressure.

Absolute pressure is measurement relative to a perfect vacuum, as in outer space. A zero reading occurs when the pressure is reduced to near perfect vacuum conditions. The ambient atmospheric pressure at sea level is approximately 29.92 inches of mercury, or 14.7 pounds per square inch, or 760 mm of mercury. As you go up the ambient atmospheric pressure decreases and your Absolute pressure gauge (MAP) reading also decreases.

I have a chart of the barometric vs ambient pressure, but I don’t know where I got it. Anybody know? I’ll check the internet and see if I can find something; or I could scan it and send it.

Al G