alternate air in visible moisture

[RMarkSampson]

Believe everyone is tracking “as identified in your POH” as the correct answer. There are multitude of variables in chemistry and engineering which we all trust has been tested in our certified aircraft. The best and right answer should be reflected in our POH.

Per the “why” behind the "visible moisture" POH entries - believe you would need to jump back into your high school chemistry class and Stoichiometry where we balanced those equations of reactants and products. The ideal air-fuel ratio for gasoline engines is around 14.7:1 – a little less air for a diesel engine at 14.5:1. I would say essentially the same between a 100LL and diesel engine for our discussion of the use of alternate air. However, combustion is possible even is the mixture is different than stoichiometric, between 6:1 and 20:1 – but for us 16.5:1 is lean and 13.7:1 is rich. We work this equation every time we lean or enrichen our engines. Note that the ideal combustion process makes two main products, water and carbon dioxide. Both of these products cycle back into larger chemical engine we call Mother Earth... (please let us not fall into that GW conversation)

The fuel mixture level connects to our fuel injection system – not the air induction system. So when we adjust our fuel mixture we are changing the amount of fuel we put into the system – we do not change the amount of air. With a nice clean filter or the use of alternate air we really don’t change the availability of that air. However, if your filter is clogged then you would affect the amount of air available – certainly flying into icing conditions is a prime example of how to clog your filter. Thus the use of alternate air improves air availability and resultantly combustion and power availability.

Per the rain discussion, the cross-section of our filters would be passing both air and water. Thus the volume of air coming through the filter can diminish with the increase in rain - and tend to enrichen the air-fuel mixture. Thus it may be beneficial to not force the air and rain through the filter while flying in rain by the use of alternate air. This is why your POH may recommend that procedure. Of course, this means other contaminates can be ingested into your induction system - like that occasional 6K MSL-flying bug that ascends during some thunderstorm updraft. I’ve seen a few of those things show up on my windscreen. But I digress…

Ultimately the numbers of factors are complex. The engine makes water during the combustion process – but more water will not change the chemical reaction. But more water can change the temperature of that reaction – and modify the flame front that dances within our engine during each and every stroke. The only way to know is to test and test which should have been done during the certification process. So that is why the only right answer is to “follow the POH.”

[Rick]

Nicely done, Mark! Another great example of why this forum is so useful. Thanks for the explanation!