Johannes, et al,

I have listed this under a separate topic because not all LML'ers are=20 following the CHT problem. Johannes had not considered the differ= ence=20 in cockpit pressure and static pressure as measured at the aircraft static=20 port.

Every non-pressurized closed cockpit aircraft I have ever had the=20 pleasure of flying or examining has had lower pressure in the cockpit - that= is=20 whether it is experimental or conventional and whether it is=20 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, noxiou= s=20 toxic fumes can enter the cockpit. This can be troublesome if=20 CO (carbon monoxide) is entering. In my airplane at cruise, the=20 pressure difference is about 7" H2O or .25 psi.

2. If an IFR capable airplane utilizes the cockpit air as a backup= =20 static source and, in a static line blockage, such air is used then the= =20 airspeed, AOA (if present) and altitude are affected - even if you have= one=20 of those glass cockpits and a fancy Angle Of Attack device.

Conventional certified airplanes, such as Cessna, have an entry in the=20= POH=20 that describes the effect of using the alternate static air - airspeed is=20 falsely increased by X amount and altitude is displayed as Y higher tha= n=20 actual. Did you make such an entry in your Experimental Lancair=20 POH? Did you placard the panel (usual in conventional aircraft)? = Do=20 you know the effect in your airplane?

Suppose I am cruising in the clouds in my 320 while over the=20 Smokey Mountains at 7000 MSL at 180 KIAS when the static line=20 fails. Switching to alternate cockpit static air, the altitude display= =20 will read about 300 feet higher than actual and the KIAS will indicate=20 about 195 (15 Kts higher) because the pressure is about .3" Hg less in the=20 cockpit. I hope I don't run into any granite or cell phone=20 towers........

The good news is that at low altitude approach/landing speeds (say 110=20 Knots), the error is much smaller because the pressure differential is much=20= less=20 - say .1" Hg. Thus, the true altitude is only off by 100 feet and the=20 actual airspeed is about 8-9 Knots less than indicated. Make sure= the=20 ILS glide path is maintained.

BTW, there is a small change in cockpit pressure when the cockpit air v= ents=20 are opened. It is left as an exercise for the student to determine the= =20 change.

Let's be safe out there. =20

Scott Krueger=20 AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL=20 (KARR)

PS Johannes,=20 I will reply to the CHT problem later as I must go flying=20 now........