Bowden cable break

prkaye · #1 10-11-2011, 06:37 AM

On final approach after my last flight of the weekend, I pulled the carb heat on and the cable freely came right out of its socket! I didn't experience any carb icing and landed without incident.

After pulling the airplane back in the hangar, i took the cowls off to inspect. The carb-heat bowden cable had broken near the end where it attaches to the arm that actuates the carb-heat door. I had the cable attached near the top of the actuator arm, which meant that as I opened and closed the carb heat door the end of the cable moved in an arc, causing it to bend slightly. My theory is that this repeated bending led to fatigue and failure of the cable. I have ordered a new cable from Spruce, and will attach it to a lower point on the actuator arm so that it moves in a smaller arc (moves straighter).

Anybody have similar stories, or alternative fixes?

Sam Buchanan · #2 10-11-2011, 07:16 AM

Quote:

Originally Posted by prkaye

On final approach after my last flight of the weekend, I pulled the carb heat on and the cable freely came right out of its socket! I didn't experience any carb icing and landed without incident.

After pulling the airplane back in the hangar, i took the cowls off to inspect. The carb-heat bowden cable had broken near the end where it attaches to the arm that actuates the carb-heat door. I had the cable attached near the top of the actuator arm, which meant that as I opened and closed the carb heat door the end of the cable moved in an arc, causing it to bend slightly. My theory is that this repeated bending led to fatigue and failure of the cable. I have ordered a new cable from Spruce, and will attach it to a lower point on the actuator arm so that it moves in a smaller arc (moves straighter).

Anybody have similar stories, or alternative fixes?

Phil, the cable attachment must be installed so it can rotate in the arm as the arm swings through its arc. If this is the case you should not have problems with the cable breaking.

As a general rule, carb heat is not considered necessary during approach with the Lycoming engine unless icing conditions are severe. Carb heat anytime rpm is reduced is more of a Continental thing. The only time I've experienced carb ice in 1050 hrs in my RV-6 is a couple of times during extended taxi under prime icing conditions.

prkaye · #3 10-11-2011, 07:24 AM

Quote:

the cable attachment must be installed so it can rotate in the arm as the arm swings through its arc.

Ok, good to know - I probably had that castle nut holding the bowden cable-end thingie too tight, not allowing it to rotate.

Quote:

As a general rule, carb heat is not considered necessary during approach with the Lycoming engine unless icing conditions are severe. Carb heat anytime rpm is reduced is more of a Continental thing.

Also good to know! There seem to be a few things you learn in flight training that get adjusted when you stop training and operate in the real world. For example, in flight training you do a run-up before every flight. The guy who owns the flight school I trained at (an old ex mil pilot with experience on all kinds of stuff) told me once that "that's just flight training" and that he would only do a runup before first flight of the day in his own airplane.
Good to know about the Lycoming and carb heat - in that case, being a VFR pilot, I would probably never use my carb heat!

cfiidon · #4 10-11-2011, 07:27 AM

(After posting, I saw two others had jumped in...)

Do you have a good swivel on it Phil?

http://www.aircraftspruce.com/catalo...ireswivels.php

Don

prkaye · #5 10-11-2011, 07:36 AM

I have a b-nut, which would swivel freely I think if I just keep the castle nut loose (using a cotter pin for security).

Sam Buchanan · #6 10-11-2011, 07:51 AM

Quote:

Originally Posted by prkaye

There seem to be a few things you learn in flight training that get adjusted when you stop training and operate in the real world. For example, in flight training you do a run-up before every flight. The guy who owns the flight school I trained at (an old ex mil pilot with experience on all kinds of stuff) told me once that "that's just flight training" and that he would only do a runup before first flight of the day in his own airplane.
Good to know about the Lycoming and carb heat - in that case, being a VFR pilot, I would probably never use my carb heat!

I consider a runup (which includes checking carb heat) essential for every flight .

Use carb heat anytime you see rpm dropping with no change in the throttle. But this will happen very rarely and only under prime icing conditions for the VFR pilot.

RV7ator · #7 10-11-2011, 08:28 AM

Besides allowing B-nut rotation, chamfer and polish the edges of the wire hole. Before learning to do this, I had a wire crack precisely at the sharp edge of the hole.

Falling MP without moving the throttle is the carb ice indicator for a constant speed prop equipped plane. Fixed pitch sees RPM drop.

John Siebold

XOverZero · #8 10-11-2011, 08:32 AM

I had exactly the same failure in exactly the same way some years ago on a Champ. The bug nut had gotten loaded with enough grime to prevent it from swiveling freely. It didn't take much. It's been a checkpoint ever since.

Bob Axsom · #9 10-11-2011, 08:48 AM

Mine failed even though I never use the carb heat. It is a common problem and it happens fast. I installed a rod end bearing on the end of the cable with a small aluminum angle part (home made) and screws. There is one AN3 bolt mounted in the control arm that is fixed (does not move or rotate) that is the post for the rod end bearing. I hit the rod end bearing with triflow spray for lubrication at the annual condition inspection and no more problem. This failure used to happen on my Archer II carb heat cable every 1500 hours or so but in the RV setup the whole arm will wear out in short order if the rotation takes place at that location.

Some searching in this forum will provide photos of various failures.

Bob Axsom

Snowflake · #10 10-11-2011, 10:48 PM

My installation has a B-nut as well, that seems to rotate freely. I've never been happy with the geometry of the B-nut installation though, it's always looked like something that's "designed to fail" at some point. I inspect it at every cowling removal, as here in Canada carb ice is more common than down in the warmer latitudes... I occasionally see carb ice while flying in winter.

This may be a stupid question, but is there any reason people don't use a proper clevis in this application?