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Hi,

If the standard cessna 172 from FSX comes in severe icing conditions the engine power drops evenly severe.
Since its Lycoming engine is fuel injected there is no carburator heat.
For me that almost ended in a disaster one flight.

I started Googling for the specific engine type and studied the specifications.
Found out that the solution for such a scenario is - Alternate air control - wich is basicaly a bypass around the air intake filter where the ice is buiding up. It may only be used in clean air.

I did not know about this because I never checked out the purpose of the Alternate air knob wich is sitting on the panel.

Unfortunately the thing doesn't have any effect.
So digging into the FSX SDK I found out that there is a settable variable called - RECIP ENG ALTERNATE AIR POSITION.
I assume that this is related to the knob on the panel but don't find it in the related aircraft.cfg.

Does anybody know if this is usable or not.
Or is it only accesable through gauge development or our Accusim gurus?

Forgive me please asking about a stock FSX aircraft(!) but I use it with the free FDE from Realair Simulations wich I find remarkebly well done.

Hope someone will take the time for me.
Thanks.

Hi,

AFAIK it is not modelled and doesn't work.

Second - you shouldn't fly ANY aircraft into severe icing.

Best regards,
Robin.

_________________
A2A/AccuSim. 'nuff said!

Right!

Thanks.

Hi All,

In the Cessna 172 the alternate air knob is an alternate static pressure source in case of static vent blockage, it sources static pressure from inside the cabin which is slightly higher than the ambient, outside air pressure. This leads to a slight under-reading on the Airspeed Indicator and Slight under-reading on the altimeter.

What you are thinking of, the Alternate Air system for the air intake manifold, is a spring-loaded trap door in the air intake system after the intake filter if the filter becomes blocked the reduction in pressure in the intake causes the door to open and supply the engine with air from inside the cowling, resulting in a slight loss of power from the warmer, less dense air.

Basically in the stock 172 the system is supposed to be automatic and therefore you shouldn't run into a loss of power caused by airframe icing blocking the intake. From what you've said it happens anyway and I guess that supports what Vulcan said, probably not even modelled by Microsoft! Anyway, I fully agree that you shouldn't be flying into severe icing, even if it is one of those aircraft types rated for flight into known icing conditions, severe icing can easily overload de-icing and anti-icing equipment..

Hope this makes sense!

Thanks for correcting me man!
Fine info.

No worries, glad I can help! Lol I've just spent too much time in an R model (CPL training and flight test) to forget this stuff!

hey, I was just playing around with this; I know this is an old thread, but if someone else is searching...

The 1999 Skyhawk Cessna 172 I'm flying in the real world has fuel injection (so no carb heat). It does, however, have "Intake Heat" (labeled "alternate air").

So I cranked up the icing in FSX on the weather page, flying the default Cessna 172, and pulled the "Alternate Air" knob; just like the OP, no good, it's the alternate static air knob, not the intake heat! I started sinking as the RPMs started dropping!

I searched all over for the knob/lever/anything related to carb heat/alternate air; but found nothing.

PRESS H!!!! Press the letter "H", and even though you don't have a visible control, it still seems to work! I tried it, and when I press H, the RPMs slowly come back and control is regained. If I press H again (turning it back off), I again lose RPMs until the plane is coming down out of the sky.

So you do have that feature, it's just invisible!

Funny, I never thought about trying the carb heat via 'H' because there is none in this fuel injected engine.
But that way we can simulate the alternate air system.
I'll give it a try next flight.
Thanks very much.

It sort of makes sense. With either carb heat (carbureted engine) or alternate air (fuel injected engine), you are opening a vent (the alternate air source) to permit airflow around the air filter where impact icing usually accumulates, thereby permiting air to flow into the intake manifold. It's just that with carb heat, you are also directing air from the shroud around the engine exhaust to the carburetor, as well.