Carb Heat Issue

[Garrett Gee]

Hey everyone,

    Starting to cool down here in Mississippi, and right now the conditions are ever so slightly conducive to carb ice.  Now from what I have seen and read, the Rotax isn't bad about developing carb ice, but I have read a couple of incidents where it may have been a contributing factor.  Anyway, I digress.  I was flying the other day, and just for grins and giggles, pulled the carb heat knob (I don't really use it unless conditions warrant), and noticed no change (or maybe only a slight 10-20 RPM change) in RPM or otherwise (CTLS).  I have read different things, with some places saying this is normal (specifically a CTsw thread), and some saying that you should experience an RPM decrease (specifically said a CTLS).  I know on the Lycomings I fly, the decrease is usually in the 50-100 RPM range.  Anyway, out of curiosity and concern, I pulled the pilot side instrument panel, and lo and behold, the cable sleeve where it mates with the knob mechanism is broken.  The cable is in tact, and I can work the cable in and out if I hold the cable sleeve.  What I did find is that if I push the knob in and hold the sleeve, the cable moves like it should,  but if I don't hold the sleeve, the cable will pull out normally, but if I try to push the knob back in, the whole cable sleeve moves.  The cable moves with ease, which leads me to believe it is working, but the lack of any indications makes me worry.

   So, my concerns are:

     1)  Is the carb heat actually working if I pull the knob?  This is obviously for safety reasons in the event of carb ice, and my main concern.

     2)  If the carb heat door/valve is stuck, is there an easy way (given that the box is in such a pain in the neck place) to see if it is stuck open or closed, or in between?  If closed, I can work with that, but if stuck open, I am not going to fly much until I can get it fixed.  Drawing in unfiltered air scares the mess out of me.

 

Any insight would be greatly appreciated!

Safe flying!

Garrett Gee

[Roger Lee]

It's probably working just fine. You can hear the butterfly in the air box move open and close if you put your ear up there. The most you MIGHT see is 25 rpm change and you usually won't feel anything change. If there was real carb ice and you applied the heat it may cause a tad roughness until it cleared.

[Garrett Gee]

Just now, Roger Lee said:

It's probably working just fine. You can hear the butterfly in the air box move open and close if you put your ear up there. The most you MIGHT see is 25 rpm change and you usually won't feel anything change. If there was real carb ice and you applied the heat it may cause a tad roughness until it cleared.

Thanks Roger,

     I did not think about listening for the butterfly valve.  I will check that next time I am at the airport to see if I can hear it.  It seems to work if I need to pull it on, I will just have to pull the panel to turn it off once on the ground until I can get the sleeve fixed.

 

Best regards

Garrett Gee

[Tom Baker]

I have seen what you describe before with the cable. If you pull the knob your carb heat will turn on, no problem. When you push the knob back in an the cable housing pushes back the carb heat stays on.

The reason you didn't see a drop is likely because the carb heat was already on, from the last time the knob was pulled.

Until you fix the cable housing the only way to turn off the carb heat will be to remove the panel and hold the cable housing and push the knob back in.

[CT2K]

Please check page 5-2 in the following file (looks like a link):  Maintenance and Inspection Procedures Manual.pdf