----- Original Message -----

From: Al Gietzen

To: Rotary motors in aircraft

Sent: Monday, June 04, 2007 12:23 PM

Subject: [FlyRotary] Re: EC-2/EM-2 manifold pressure

Well Bill,

if the Kolsman window is where you set your altimeter setting (mb or inches mercury) you will never see field elevation (pressure) unless you are at sealevel.

 

E.g. if you have 29.92" at sealevel (and in the window) you still have 29.92 in the window (you set it there!) when  you get to 6500'. Your actual ambient pressure is around 23.42".... (which you should be reading on your manifold pressure gauge - engine stopped...

 

Unless, I got it wrong the last 15 years and 9500 hours - ohhh my...!!

 TJ

I think you’ve got it right.  It seems to me that if you want to find what your MAP out to be when parked, engine off; you set 29.92 in the altimeter window to get pressure altitude, then either use a formula or chart to get the expected air pressure at that altitude.  For example, if you set 29.92 and it shows your pressure altitude to be 2000 ft, then your chart will tell you that the air pressure where you are is 27.8 in. HG for a standard day, then correct for temp.

 

So unless I’m mistaken, that is the same as calling the tower (or weather observation station, or get out your handy-dandy flight computer) and get the density altitude, then vary the altimeter setting until the altitude reads density altitude, and get the pressure from the Kolsman window.

 

Al