Normally Aspirated Skymaster Cabin Heat Shutoff?
 

Has anyone experienced problems getting the cabin heat in the normally aspirated models to completely shut off? Has anyone rebuilt the heater/ventilation control airboxes?

The changed the design of the control airboxes around 1970, and I wonder if the earlier design was prone to leak and if the later design solved the problem. Either way these things look tough to access.

Yes they leak. I have found if you safety wire the control in the full closed position from underneath (and placard the controls "inop" in the summer months) it improves the situation. I think it was a bad design and air flow opens the flap slightly in flight, leaking warm air into the cabin. Also would be real bad of course if your mufflers ever failed...ie no way to really shut off the CO from entering the cabin.

Bill

Easier than that & more effective, simply remove the muffler shroud to heat valve box orange SCAT air hose/duct during the summer months.

Good suggestion. I asked my mechanic about that idea and he was not comfortable with that solution because of what to do with the lower end of the hose relative to it not getting caught up in the cowl flap. Or were you suggesting you remove it at the top end?

I took one of the mixing boxes off and found that the original design had no seals and the valve cannot completely shut off. They allow heated air in the cabin regardless of the control position. Go figure.

I bought some of the 1970's or later mixing plenums to install. They are cylindrical construction and have seals, so they should provide a much cooler ride. I have air conditioning in the plane now, but I sit up front and roast with the engine heat leaking in while the women in back are freezing.

Yes, totally remove the SCAT hose section at both ends so it does not hang loose. Save the hose and clamps to reinstall for cold weather.