From here
With the B36TC (TSIO-520 UB), the alternate door can freeze up, but it can be forced open manually.
The only reason I can think of for the loss of MP above 13K is due the warming of the induction air when the alternate air door opens.
However, WPR14FA094 caught my attention. The most likely reason may have been as simple as not pulling the T-Handle to force the alternate air door open.
The GA8 has the same, but there’s no altitude limit. But the GA8 has no turbocharger so engine performance at 10.000’+ is marginal under the best of circumstances anyway.
I can imagine that the alternate air door has a relatively small aperture, which means that above a certain altitude the airflow is restricted to something less than the engine needs. Haven’t seen the arrangement in the B36 but in the GA8 the air filter is about 15×20 cm, while the alternate air flap/door is about 8×4 cm. Also, the alternate air door draws air from inside the engine compartment, after it has been heated by the cylinders, so that might reduce the density.
The assertion in the original thread (SR22T engine stoppage at FL230 – cause never determined as the aircraft was immediately sold) was that the fuel inlet somehow froze up and the alternate air door was already frozen in the shut position.
It could happen in the TB20 too – some data here. And that’s a manual door. I think using prop TKS prevents it happening in the TB20 because the inlet duct fills up with TKS fluid droplets, and TKS also protects from fuel servo icing in the same way.
