Johannes, et al,

 

I have listed this under a separate topic because not all LML'ers are following the CHT problem.  Johannes had not considered the difference in cockpit pressure and static pressure as measured at the aircraft static port.

 

Every non-pressurized closed cockpit aircraft I have ever had the pleasure of flying or examining has had lower pressure in the cockpit - that is whether it is experimental or conventional and whether it is engined in the front, rear, both or out on the wing. 

 

Why does this matter?

 

1. If the firewall is on the fuselage and it is not sealed, noxious toxic fumes can enter the cockpit.  This can be troublesome if CO (carbon monoxide) is entering.  In my airplane at cruise, the pressure difference is about 7" H2O or .25 psi.

 

2. If an IFR capable airplane utilizes the cockpit air as a backup static source and, in a static line blockage, such air is used then the airspeed, AOA (if present) and altitude are affected - even if you have one of those glass cockpits and a fancy Angle Of Attack device.

 

Conventional certified airplanes, such as Cessna, have an entry in the POH that describes the effect of using the alternate static air - airspeed is falsely increased by X amount and altitude is displayed as Y higher than actual.  Did you make such an entry in your Experimental Lancair POH?  Did you placard the panel (usual in conventional aircraft)?  Do you know the effect in your airplane?

 

Suppose I am cruising in the clouds in my 320 while over the Smokey Mountains at 7000 MSL at 180 KIAS when the static line fails.  Switching to alternate cockpit static air, the altitude display will read about 300 feet higher than actual and the KIAS will indicate about 195 (15 Kts higher) because the pressure is about .3" Hg less in the cockpit.  I hope I don't run into any granite or cell phone towers........

 

The good news is that at low altitude approach/landing speeds (say 110 Knots), the error is much smaller because the pressure differential is much less - say .1" Hg.  Thus, the true altitude is only off by 100 feet and the actual airspeed is about 8-9 Knots less than indicated.  Make sure the ILS glide path is maintained.

 

BTW, there is a small change in cockpit pressure when the cockpit air vents are opened.  It is left as an exercise for the student to determine the change.

 

Let's be safe out there.