Mx tip that could save your life..

[John McClanahan]

Hey Group - Since it sounds like many of you are interested in professional maintenance training, I thought I'd share this.

As some of you know, I’ve been helping Haze Skipper with the rebuild of his Cessna 180 at Falcon.

Today he was asking me how to properly hook up a mixture cable.  Here’s what I told him - 

My observation is that around our airport, this simple thing isn’t taken seriously enough.  And lack of attention to detail here can certainly kill you so here goes.

First, the cable clamp..  If the cable clamp slips, your mixture can go to cut off so it needs to be secured properly.

Ok, go to Spruce and get an AN742 plain clamp and go with that because that’s what Spruce offers.  Nope.  Big mistake.  Here’s why -

Here’s an AN742 -

So what’s wrong with this?  Compare it to this - 6317 series from McFarlane.  Notice the indentation?

Here’s another pic.  If you look closely, you'll see the indentation is actually formed at an angle to match the spiral of the Bowden cable.  These hold securely and won’t slip..

 

And absolutely do not use standard Adel clamps here.  The rubber gets oily, deteriorates over time, and can allow the cable to slip.

Next, let's consider the clamp bolt.  There are many designs for this critical item and most of them suck.  Here are two that Spruce cells on page 156 of the on-line catalog, items 05-16100 and 05-16210 - 

I find two problems with the bolt type.  First, you don’t want threads in the area where the bolt goes through the lever arm.  The threads are sharp and wear the arm - the hole wallows out.  Second, if the bolt/hole isn’t exactly the correct length, tightening down on the nut can cause the assembly to bind in the arm.  This is critical because if the bolt doesn’t swivel, the cable will be forced to bend when you move the cockpit control..bend-bend-bend, break, crash..

So I prefer the second type since it can’t bind in the arm.  Note the cable is held by a set screw.  Put blue loctite on the set screw and take action if the unit starts to get sloppy in the hole in the lever arm.

Finally, here’s a trick I learned a long time ago from the EAA - Get some of these model airplane wheel holder-oners.  Du-Bro and GreatPlains makes them.  eBay is a good source - 

And what do you do with these?  Put them on the cable on either side of the clamp bolt (and use loctite again).  That way if the primary set screw on the clamp slips, the cable still can’t slide.  Check tightness of all these connections, starting at the sleeve clamp on every condition inspection.

By the way, a friend of mine was seriously injured in a crash of a Cessna Cardinal because of oversight of this stuff.  When he pushed up the throttle on a go around, the carb heat cable slipped leaving carb heat on.  At the weight and density altitude he was at, he didn’t have enough power and went into the trees off the end of the runway..bad ju ju..

So the lesson learned from that is give equal consideration to your carb heat cable.

And, also, by the way, I’ve found that Lycomings don’t need carb heat unless in extreme conditions.  I’ve seen this on at least three different airplanes with carb temp gauges (if you have a carb, you do have a carb temp gauge, right? Right?) - With Lycomings, because the carb is mounted on the bottom of the oil pan, the carb temp actually rises significantly when power is reduced which is the opposite of Continentals used in so many of the training airplanes (C-150s for example) we learned on.  And that’s because the airflow through the carb actually cools the carb in a Lycoming so less airflow=warmer carb.  So if you’re using carb heat during landing on your Lycoming, you might want to re-think that since it’s unnecessary and that air is unfiltered.  

I hope this helps some of you out there.

As always, happy to hear your comments. 

And I’m curious if anyone has a better way to secure Bowden cables because the machinist in me isn’t very impressed with any of the current offerings from Spruce.  Airplanes aren’t lawn equipment..

John McClanahan

[Ralph "Trapper" Trapaga]

Wow, excellent and well-articulated advice as you always supply, John. Thanks for taking the time to put this together!

Ralph "Trapper" Trapaga

165 Turnberry Cir

Fayetteville, GA  30215

H: 770-631-8272

C: 678-571-9595

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[vic syracuse]

Good practice. I always wrap a couple of turns of safety wire around the clamp and then cross it over to the other side of the clamp and then wrap a couple of turns on that side and secure it. 

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[Randy Weston]

Excellent tips!

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[Scott Calkins]

Excellent tips and advice. I’ll take a close look at that next time I pull the cowling off. 

Thanks John!