Cherokee engine gradually dies

[Fizzelle]

Twice now, my engine has died on me. It's flying along purring away and then just slowly but surely fades away like an old soldier.

A complete overhaul fixes it but I really wonder why it might be happening. There's a fascinating discussion on this website ( https://backcountrypilot.org/ ) where the problem has been narrowed down to failing mags, faulty ignition switches etc. I'm not sure if A2A model things like this internally.

I have pulled info from all over the web regarding this fault but cannot pinpoint the exact cause. In my case, both times it has been raining, or a thick sea mist (both near sea level). Temperature has been around 55-70F, icing doesn't appear to be a problem here. Nevertheless I have tried all mixture settings, flown it by the book and experimented with carb heat too, all to no avail. I can't pull the wiring out to check for sparks etc, and I can't run a diagnostic on individual engine components - although the compression check was A-OK, and all engine items were in the green.

So in the context of A2A's programming - what part of the engine did you model that would cause this behaviour and, if it happens again, is it possible to fix that item rather than run a complete overhaul and reset my hours?

So not really tech support (kinda), but a fascinating behaviour I'd like to get to the bottom of.

And for those of you still reading, yes I managed to land in a field both times safely, with full flaps and no damage! Although it has been expensive to get the aeroplane towed to the nearest repair hangar

Thank you.

[Oliver Branaschky]

I'd still wager carb icing. Keep in mind that the Venturi effect can lower the temperature by 30-40° F. Add in moisture and there you go (tumbling down towards a field). Pulling the carb heating should have prevented these situations.


Viele Grüße | Best regards,
Oliver

[AKar]

To add a bit to the above, the biggest cooling effect in the carburetor occurs due to fuel evaporation. The temperature drop due to venturi effect alone is somewhat small, and also just transient (any moisture condensing or depositing off the air actually warms it). The fuel injection system used in most Lycomings is structurally somewhat similar to carburetor, but is relatively free of icing issues in most conditions, as the fuel evaporation takes place at the intake port area, not in the throat / throttle valve area, which is vulnerable to ice formation.

-Esa

[Nick - A2A]

In my case, both times it has been raining, or a thick sea mist (both near sea level). Temperature has been around 55-70F, icing doesn't appear to be a problem here. Nevertheless I have tried all mixture settings, flown it by the book and experimented with carb heat too, all to no avail.

I'd put my money on carb icing too. Rainy/misty weather in the sim is a real 'smoking gun' here and the temperature you describe puts you right in the danger area.

For more info, I'd suggest a look at this rather good ATSB article and accompanying graph: Melting Moments: Understanding Carburettor Icing.

Also, bear in mind that when using carb heat in the Cherokee, partial application isn't recommended. Carb heat should be applied fully and preferably before symptoms develop. In the sim, if you've already suffered a more-or-less complete power loss, it may take a few minutes of full application to get rid of the ice. In real life, such a situation may be even less forgiving of late carb heat application, as the engine may no longer be able to provide enough heat via the exhaust system to melt the accumulated ice!

Cheers,
Nick

[AKar]

For more info, I'd suggest a look at this rather good ATSB article and accompanying graph: Melting Moments: Understanding Carburettor Icing.

An excellent writeup. It is good to read one which correctly identifies the "fuel chill" as the primary issue instead of all-too-inaccurate venturi explanation. Also, it notes (again, correctly) that rich mixture settings can be more prone to carburetor icing issues. This is certainly something that is not understood nor studied too much, perhaps because much inaccurate description of the phenomenon in most material.

-Esa

[Lewis - A2A]

Yes I would concur with the others, the effects you describe is 1:1 match of carb icing.

thanks,
Lewis

[Fizzelle]

Well thank you all for your efforts to help me track down this phenomenon. I did pull the carb heat but obviously left it somewhat late!

I'll re-fly the same route and see if I can replicate the problem and fix it "on the fly".

It's nice to meet so many pleasant people; thank you again.