Mixture Cable Attachment

How to keep the Cessna 170 flying and airworthy.

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Mixture Cable Attachment

Postby lowNslow » Thu May 13, 2010 6:24 pm

My mixture cable broke at the carb end so I replaced it with the Spruce unit suggested by George. I noticed that the attachment to the mixture arm on the carb was a bolt with hole in it for the cable and then bolted tight against the arm. It seems that this fitting should be able to rotate to allow for the arc of the mixture arm movement. Is there a special fitting for this? I couldn't find anything in the parts manual.
Karl
'53 170B N3158B SN:25400
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Re: Mixture Cable Attachment

Postby minton » Thu May 13, 2010 7:12 pm

The bolt you speak of is drilled at a point where there should be enough room for the washer to be installed after the cable being threaded through the hole. Then the castle nut is tightened down and alined with the cotter pin hole. At this point you should check to see the the bolt rotates freely in the mixture arm. If it does, check the rigging for full travel, stop to stop on the arm with 1/8-1/4" coushion on the cable @ full rich. If all is good, install cotter pin, put a 90 degree kink in the cable just after the bolt for safeties sake.

Potential problems would be:
The bolt won't rotate as the hole is to close to the arm. If so, drill a new bolt. (round the edges of the hole)
The washer is to thick, not allowing the cotter pin enough room. (use thinner washer)
The cable housing is anchored to close to the mixture arm causing the cable to bend excessively as it's moved in and out. This alone can cause the cable to break. (anchor housing farther away)

Hope this is of some help.

There are other potential issues but due to my CRS I can't think of them right now. :lol:
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Re: Mixture Cable Attachment

Postby n2582d » Thu May 13, 2010 8:44 pm

Aircraft Spruce has what you are looking for here. They are a little cheaper directly from McFarlane Aviation.
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Dynamic Propeller has an overpriced nylon sleeve that helps prevent wear on the control cable wire. It's STC'ed for several aircraft but not the C-170. I'd call it a minor alteration myself.
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Last edited by n2582d on Thu May 13, 2010 9:36 pm, edited 1 time in total.
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Re: Mixture Cable Attachment

Postby minton » Thu May 13, 2010 8:53 pm

Was noticing that the A/C Spruce bolt was'nt drilled for a cotter pin??? 8O
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Re: Mixture Cable Attachment

Postby n2582d » Thu May 13, 2010 9:17 pm

minton wrote:Was noticing that the A/C Spruce bolt was'nt drilled for a cotter pin??? 8O

The IPC shows it with a locknut rather than a castle nut.
Mixture Contol Attachment.jpg
Of course one could drill a hole for a cotter pin and use a regular castle nut. Or if you wanted to go with the belt and suspenders approach, use a self-locking castle nut (MS17825). :D
ms17825selflock.jpg
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Re: Mixture Cable Attachment

Postby minton » Thu May 13, 2010 9:54 pm

You go right ahead and use something that can unwind on one of the most important (life supporting) controls. Not me. 8)

Remember that an engineer contributed to that IPC. I doubt that any A&P or pilot did.

Any bolt or nut that is or can be subject to any amount of torque or twisting (same thing), should have a positive locking device. Give a look at your throttle cable end at the throttle arm as a good example. They even stack a large area washer onto the bolt to protect from a end bearing failure!! The carb base-- lock washer, nut, pal nut. Get my drift?? Steel or elastic nuts or loc-tight won't get it. This is one part of the equation that over kill is OK. End of story in my book.
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Re: Mixture Cable Attachment

Postby n2582d » Thu May 13, 2010 11:23 pm

Milton, where's your sense of adventure? :lol: I agree with you here--a castle nut and cotter pin is the way to go. It would be tempting to add a light spring to the mixture control to ensure that if the cable broke the mixture would go toward full rich rather than idle cutoff. Karl, what did your mixture control do when the cable broke?

Another place the engineer rather than the pilot or mechanic did the drawing was in the trim control attach. I would ensure any plane I flew had a castle nut/cotter pin rather than a regular AN365 locknut as illustrated in the IPC.
Trim tab control attach.jpg
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Re: Mixture Cable Attachment

Postby minton » Fri May 14, 2010 12:25 am

I saw the aftermath of a missing trim tab bolt. The guy had to write a big check for a new elevator and trim tab. :?

AMEN!! The big engines (round motors) using Stromberg carbs incorporated a "cruise" failsafe mechanism "IF" something vital broke loose you got a automatic cruise power setting. All that you had to do then is figure out how to land that puppy with cruise power going on. I would imagine that the elevator and rudder peddles got a good workout. :lol:

I've given the spring consideration myself.
Last edited by minton on Fri May 14, 2010 12:34 am, edited 1 time in total.
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Re: Mixture Cable Attachment

Postby gahorn » Fri May 14, 2010 12:29 am

Last week I was asked by a fellow member and AP/IA (who will remain anonymous and who we will all refrain from criticizing) to come provide an aircraft check-out "for insurance purposes" . His airplane has been a several-year MAJOR restoration with an engine conversion and many, many other mods, all with either STC or field approvals. The new engine had been "ground-run" only, and this would be it's first flight.

I flew to his private field and looked the airplane over and while not-yet ready for "show" it appeared very airworthy with only a few issues after I pre-flighted it. (A few loose fasteners in the upper flap rollers and a couple other minor issues, which were quickly satisfied.) I confirmed the annual inspection was completed and signed off, with "approval for return to service."

We took it airborne and remained directly over the field at high power settings and speed (for cooling) for an hour, watched the CHT drop off about 50 degrees in about 30-40 mins, and then returned for a landing. A second takeoff/landing was made and was uneventful.

A postflight showed no major leaks and all was fine. It was lunchtime and with another local pilot, we all hopped into my airplane and went over to a nearby larger airport with a cafe for lunch. After lunch we returned and prepared for currency-checkout of 3 TO/LNDgs/stalls/etc..

First TO was cacelled during taxi due to rough throttle movement during power application. A check under the hood showed the castellated nut on the carb-lever had no cotter and the bolt had backed out and was rubbing the carb body.
The AP/IA owner blushed :oops: and after that was corrected we made a second effort and the takeoff roll resulted in a reject due to fluctuating prop RPM. A static runup only resulted in the prop going to redline and then surging between redline and 300 RPM lower, repeatedly. Taxi back and take a look at the prop gov lever, and find another castellated nut with no cotter had fallen off. More :oops: :oops:

Corrected that and more thorough look/see at everything under the hood and off we go again. Climb to 5K and do a clean stall. Beautiful. Now for a dirty-stall. Beautiful....except the flaps won't retract but partially. Decided not to change a thing and descend and land with about 30-degrees flaps.

Postflight shows left flap loosey-goosey as usual and right flap jammed with inboard, lower roller-bolt fractured at threads with the roller jammed sideways in the track. (New-appearance AN bolt but all the threads are missing and the nut/threaded portion is down inside the flap.) Since this will require additional investigation/repair and since 3 takeoffs/landings have been accomplished by this very experienced tailwheel-qualified owner/pilot/AP/IA...it's decided to call it quits and he, with considerable embarrassment, admits he's looked at this airplane so much for the last several years he no longer trusts himself and promises to get another to inspect it before further flight.

Moral? Major restorations might need a different set of eyeballs before return-to-service. Or four.
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Re: Mixture Cable Attachment

Postby minton » Fri May 14, 2010 12:39 am

Amen again. Thats definately a short comming of the GA system as it exists. 135, 121 etc. requires two sets of trained eyes.

And the preflight walk around?? :oops: :oops:
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Re: Mixture Cable Attachment

Postby lowNslow » Fri May 14, 2010 1:39 pm

Thanks guys, the bolt I have is similar but looks to be home made. The hole is to high on the bolt so that you have to snug the cable to the arm to get it tight so that the bolt does not rotate - which is why my cable broke.
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Re: Mixture Cable Attachment

Postby blueldr » Fri May 14, 2010 4:51 pm

Karl,

Wrong bolt. Hole in wrong place. Make new bolt. Nut should clamp wire tight but bolt should rotate freely on mixture arm with washer between wire and arm.
Best with washers under bolt head, between wire and arm, and between wire and nut. Shim and adjust with half thickness washers. Use castellated nut and cotter pin.
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Re: Mixture Cable Attachment

Postby Brad Brady » Sat May 15, 2010 1:29 am

A misplaced washer or a washer of the incorrect thickness, in this application can make all the difference in the world.
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Re: Mixture Cable Attachment

Postby jwsowles » Thu Nov 04, 2010 9:22 pm

N2582D mentioned a safety spring on the mixture control at carburetor to ensure carb goes rich if cable breaks? Does anyone have a pattern to follow? Mine let go, fortunately with no power loss.
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Re: Mixture Cable Attachment

Postby lowNslow » Thu Nov 04, 2010 10:42 pm

n2582d wrote:Milton, where's your sense of adventure? :lol: I agree with you here--a castle nut and cotter pin is the way to go. It would be tempting to add a light spring to the mixture control to ensure that if the cable broke the mixture would go toward full rich rather than idle cutoff. Karl, what did your mixture control do when the cable broke?

Gary, sorry I just noticed you had asked me a question in this post. Luckily my cable broke when I went to cutoff after parking, not in flight.
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