What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Throttle Cable Severs in Flight

A

airshawn58

Member
I was turning base to final yesterday and discovered that my throttle control was "disconnected" from the fuel distribution system (RV7a/IO360). The cable had actually completely severed. I was fortunate, made the end of the runway by a few feet. I've consulted with both Vans and my A/P. I'm not shaming or blaming anyone; just reiterating that the power cables and connections should never show any actual bare cable and if it does, more obviously, should not be frayed.

If it's not already part of your Condition Inspection, put it on the must-check list.
 

Attachments

  • ThrotCab1.jpg
    ThrotCab1.jpg
    218.1 KB · Views: 871
  • ThrotCab2.jpg
    ThrotCab2.jpg
    279.5 KB · Views: 827
Last edited:
That’s an interesting failure. I’ve never seen it before. Not sure there is any (reasonable way) to inspect for that. (?)
 
Neither had the Van's guy. He just confirmed that on visual inspection, no bare cable should be showing north of the fire wall.
 
I’ve never taken one apart but pretty sure that stranded cable is swaged to the sleeve which attaches to the throttle arm. That swaged assbly is then encased in the outer sleeve clamped to the retaining bracket. No way to put eyes on the stranded cable that I can think of.
 
Did the throttle cable actually break, or did it just pull out of the swaged fitting?
Are there any remnants of the cable inside the swaged area?
 
Yes, there are "ends" of the severed cable just inside the swaged fitting that are clearly visible.

Yes, the swaged cable is inside the sleeve. Your point about "putting eyes" on any exposed part of the cable is well taken. I'll continue to try and think/visualize what we should be trying to check for.

I'm not a builder; just a lucky pilot who took the bottom cowling off and am reporting what I saw/experienced.
 
airshawn58 said:
Yes, there are "ends" of the severed cable just inside the swaged fitting that are clearly visible.

Yes, the swaged cable is inside the sleeve. Your point about "putting eyes" on any exposed part of the cable is well taken. I'll continue to try and think/visualize what we should be trying to check for.

I'm not a builder; just a lucky pilot who took the bottom cowling off and am reporting what I saw/experienced.
Click to expand...

Could we see fotos of the other end?
 
Cable too short?

Shawn

Looking at the photos it looks like the cable has no free play between the bracket and the firewall. These engines shake a lot on startup. Tugging on the cable may well cause it to break.
Also the bracket has been modified to make it fit. There must be a reason why the builder did this. Again possibly because the cable was too short.

I suggest getting someone experienced in rv’s to have a good look at your engine setup.

Regards Peter
 
First of all, Holy Cow, congratulations on an emergency landing with an unbent airplane.

Secondly, as far as inspecting during condition inspection, since these cables are sealed units, about the only thing you can look for is routing, external chafing, and does it feel like its binding up or "gritty" during operation.

Was there any advance warning to thins at all? Did it feel funny when you move it?

It would also be nice if you shared about how many hours were on it.

In 30+ years of knocking around airplanes, including a M.S. degree in Aviation Safety and a career at brand C, I've never heard of this happening without some warning signs, so if it just up and broke, I think the guys who suggest there isn't enough slack in the way it was routed might be on to something.
 
It's not super clear in the photos, but if you enlarge them a little the sheered strands are visible.
 

Attachments

  • IMG_3777.jpg
    IMG_3777.jpg
    176.4 KB · Views: 427
  • IMG_3776.jpg
    IMG_3776.jpg
    195.5 KB · Views: 359
  • IMG_3775.jpg
    IMG_3775.jpg
    590.8 KB · Views: 374
Thank you for the kind words. I've owned the N736S since March, 2020. It has a TT just over 500 hrs (about 125 are mine including a cross country trip from Cleve, OH to/from Enterprise Oregon in June). All Condition Inspections/ADs are current.

The throttle never felt funky, grabby, strange in any way. Although about 30 mins before my forced landing, I struggled for a few seconds trying to slow the plane down. I leveled out just after take off and my airspeed was nearing 200TAS. I pulled back on the throttle but was still accelerating so tried a couple other things, attitude, trim, etc. and then everything returned to normal. I was busy/distracted and once everything seemed normal, it never occurred to me that I had a throttle issue; again, it felt/moved in/out as usual.
 
Last edited:
kiwipete said:
Shawn

Looking at the photos it looks like the cable has no free play between the bracket and the firewall. These engines shake a lot on startup. Tugging on the cable may well cause it to break.
Also the bracket has been modified to make it fit. There must be a reason why the builder did this. Again possibly because the cable was too short.

I suggest getting someone experienced in rv’s to have a good look at your engine setup.

Regards Peter
Click to expand...

I agree with Peter.

In many years working with these airplanes, I don’t recall ever hearing of an engine control cable failing in this manner.
That makes me suspect something specific about this particular airplane having an influence.
The straight line (no slack) path of the cable is undesirable and could very well have been the cause.

I recommend that you don’t replace with the exact same length (I.E., don’t buy the same part #).

And just to reiterate, it is not possible to inspect the control cable to see evidence of a pending failure at this location but considering how it failed based on the photos, it is likely a progressive fatigue failure of the wire strands at the point where they exit the swedged on rod.
 
Last edited:
To the group: is the end of that smaller diameter sleeve supposed to be “beaded” to help retain it inside of the larger diameter sleeve? I’ll have to take a really good look at one of these cable assblys the next time I get a chance.
 
Photos

airshawn58 said:
It's not super clear in the photos, but if you enlarge them a little the sheered strands are visible.
Click to expand...

Could you retake these so the end is a little more clear? The end we need to see is too blurry to make out much.....:confused:

Looking at the original photos, it looks like the cable end is square without any "stranding" indicating it either pulled out of the end fitting or just snapped off, which would be a little strange for a stranded cable. With the lack of free play from the firewall, pulling out would make more sense as there is a LOT of movement between the firewall and your carburetor. The few strands in the fitting might be what was left hanging on to the fitting rather than it breaking. I agree: something isn't adding up.....:confused:
 
airshawn58 said:
About the best I can get with my 1Phone X.
Click to expand...

If you touch the screen on your phone at the place on the failed part you are wanting to emphasize, it will focus on that spot. You may already know this so my apologies in advance if this is the case.
 
Broken cable

airshawn58 said:
Improved picture.
Click to expand...

OK: I can see the broken ends of the cable still in the fitting a bit more clearly. Could you also send a picture of how the linkage was attached to the swing arm/throttle on your carburetor? It looks like the problem might be how the cable was flexing against the fitting itself; ie, flexing too much between the support for the housing (the white metal in the first pictures) and the throttle control arm of the carburetor. Was it in the hole closest to the throttle shaft or in the middle hole or one furthest away? If it was in the last hole, that would be way too much flexing between the housing and the cable itself. Very odd, I must say. I had to go out and look at my set up. :eek: The only way it could break like that would be from constant flexing where the cable went into the fitting. :eek::confused:
 
Control arm attached to fuel system. Everything looked aok.
 

Attachments

  • IMG_3768.jpg
    IMG_3768.jpg
    380.3 KB · Views: 414
  • IMG_3756.jpg
    IMG_3756.jpg
    328.9 KB · Views: 351
It looks as though the cable is too short for the application, and adjustments were made to the bracket. The cable would be at full extension at full throttle with the least amount of sleeve on the main housing to support the extended rod. And then vibration took it from there.

The one time the throttle was stuck open, it was telling you that the rod extended out of it's housing.... Remember, if the throttle is stuck wide open, your mixture becomes your power control.
 
I’m still impressed by the statement “I struggled for a few seconds trying to slow the plane down. I was climbing out and my airspeed was nearing 200TAS”
 
Adding data point

At about 25 hrs I had a throttle cable bind/lock near the swage at the fuel servo side. It jammed on the idle retard for a landing flare so it wasn't consequential, but it was effectively useless from that moment on.

Straight run from firewall to servo, more than 4" from anything hot and also heat shielded. Nothing interesting in the installation.

I sent it back to the manufacturer for an engineering investigation and nothing ever came from that.

Eyes wide open: Mysterious cable problems can and do happen. Spreading the word is important.
 
Taltruda said:
I’m still impressed by the statement “I struggled for a few seconds trying to slow the plane down. I was climbing out and my airspeed was nearing 200TAS”
Click to expand...

Agreed. If you have a 7A that does 200 in a sustained climb, you're going to be a very rich man - once you do a little reverse engineering. :D
 
gasman said:
Boot and housing guide missing....??
View attachment 49491
Click to expand...

Yes: As I was out walking my Cat (yes, really), it dawned on me that the cable might not have been supported from the housing nut to the throttle arm. If it was just pushing against the brass fitting without support, there would have been too much force on the cable itself at the fitting. When you look at the fitting attached to the throttle arm, could you see the cable itself? Was it enclosed in any supportive housing? It should have been...:eek:; and if it was, the cable would not have been visible. It would have been enclosed in its housing.
 
Whaaaaaaaaat???

airshawn58 said:
It's not super clear in the photos, but if you enlarge them a little the sheered strands are visible.
Click to expand...

OK: I'm confused now. What was the brass fitting in these photographs attached to besides the cable? There is a nut at the end attaching it to ?? Then the rod attached to the carburetor arm is silver. What was it attached to?
Could you back out and take a picture of the housing support fixture (white), the control rod (silver) and where the brass fitting went that broke off of the cable....:confused::confused::confused:
 
Whaaaaaaaaat???

Roadjunkie1 said:
Yes: As I was out walking my Cat (yes, really), it dawned on me that the cable might not have been supported from the housing nut to the throttle arm. If it was just pushing against the brass fitting without support, there would have been too much force on the cable itself at the fitting. When you look at the fitting attached to the throttle arm, could you see the cable itself? Was it enclosed in any supportive housing? It should have been...:eek:; and if it was, the cable would not have been visible. It would have been enclosed in its housing.
Click to expand...

What it should kind of look like.......
 

Attachments

  • IMG_5506.jpg
    IMG_5506.jpg
    469.6 KB · Views: 415
Accelerator pump fitting

airshawn58 said:
Control arm attached to fuel system. Everything looked aok.
Click to expand...

......except the silver jam nut on your (yellowish) control arm for (I assume) your accelerator pump linkage is not jammed onto anything......:eek: And the linkage bolt to the throttle arm should probably be drilled and have a castellated nut and be keyed rather than the friction nut. It is on a moving part and might move itself..... IMHO.....
 
Last edited:
Sam Buchanan said:
Something’s not adding up here…….
Click to expand...

I have heard of clutch cables in cars suddenly failing if the cable is being used as an unexpected ground return in lieu of a good ground path. The heat generated when the electrical power uses the clutch cable for a return path heats the cable to the point the cable fails. This just came to mind when I read about this throttle cable failure which seems so strange. Check your ground returns and be sure that this is not the case here.
 
Roadjunkie1 said:
......except the silver jam nut on your (yellowish) control arm for (I assume) your accelerator pump linkage is not jammed onto anything......:eek: And the linkage bolt to the throttle arm should probably be drilled and have a castellated nut and be keyed rather than the friction nut. It is on a moving part and might move itself..... IMHO.....
Click to expand...

Wrong assumption...

The photo shows a throttle body for a fuel injection system.
The gold colored linkage is pretty standard for fuel injection servos.
The silver "star" is the adjuster for the length of the link, used for adjusting the idle mixture. It is safetied by the spring that is engaged on one of the indentations of the star in the side away from the camera.

I still don't understand why the original builder thought they needed to modify the control cable mounting bracket. It should have worked fine, even with vertical induction fuel injection.
Perhaps they were un-aware that you can reclock the control arm on the throttle shaft if needed, though it doesn't look that would have been necessary.

I still feel pretty strongly that the failure was caused by the straight line path of the control cable from the firewall to the bracket with no slack in the cable to account for the movement of the engine, which down low at the carb. or injection servo, is way more than most people would think.
 
Roadjunkie1 said:
......except the silver jam nut on your (yellowish) control arm for (I assume) your accelerator pump linkage is not jammed onto anything......:eek:
Click to expand...

I believe the silver jam nut between the yellowish control arms you're referring to is the idle mixture adjustment. The spring on the back side keeps that knurled knob in place. At least that's what it is if we're looking at the same pic...there have been several posts on this thread, so maybe I misunderstood.

Sam brings up an interesting theory on it possibly being an inadvertent ground path. It seems like that should be one thing that's definitely run to ground :rolleyes: as well as the possibility that the control arm or cable was binding. If you're not sure what to look at, please, please grab somebody knowledgeable to help you with this. This time, it was merely an inconvenience, but the potential is there for an off-airport landing.
 
rsultzbach said:
I have heard of clutch cables in cars suddenly failing if the cable is being used as an unexpected ground return in lieu of a good ground path. The heat generated when the electrical power uses the clutch cable for a return path heats the cable to the point the cable fails. This just came to mind when I read about this throttle cable failure which seems so strange. Check your ground returns and be sure that this is not the case here.
Click to expand...

That's a good thought, I've seen the same thing a few times with no or loose eng/bat ground cable, usually the item that carried the current starts to glow!
Back in my muscle car days clutch cables were common and under a lot of stress, especially when you installed a high-pressure clutch pressure plate.
Maybe borrow one of the high magnification microscopes from a builder with LCP .:)
 
Last edited:
Cable Support Tube

Roadjunkie1 said:
What it should kind of look like.......
Click to expand...

If the original poster's plane had the outer tube removed from the cable end that is shown in your picture, then the swaged part of the cable would see a lot of bending load from engine vibration right where the cable enters the swage fitting. Bending load here would effectively cause the swaged cable end to cut through the cable over time. The missing outer tube ensures that the cable and the swaged end remain in-line with each other so that there is no bending at this point.

I am guessing that the original builder couldn't get enough travel towards the idle position on the throttle and may have removed the support tube instead of making adjustments elsewhere such by reclocking the servo control arm or installing a shorter arm (Avstar does sell different length and shape throttle and mixture arms). If this was the case, bad idea...

Skylor
 
Roadjunkie1 said:
......And the linkage bolt to the throttle arm should probably be drilled and have a castellated nut and be keyed rather than the friction nut. It is on a moving part and might move itself..... IMHO.....
Click to expand...

There is no relative motion here. The original plans call for a metal lock nut. Nothing wrong with this, however, many feel the way you do.

For the OP - I would clean up the corrosion on that bracket although it has nothing to do with the failure of the cable.
 
cable

The throttle connection is on the inboard side of the throttle arm. Should be on the outboard side. Probably the reason for the modified bracket which in turn caused less or no slack between the firewall and the bracket.
The more significant issue is whether the cable had the proper amount of swivel at the forward end or possibly no swivel capability at all.
Push pull cables come in many different varieties. Some of them I would not use on a lawnmower much less an airplane.
 
In the second picture of the OP, the threaded end.... That rod is supposed to slide within an outer guide tube that is swaged to the fitting on the end of the flex cable. This outer tube has some angular articulation at the fitting, but is otherwise fixed in place. Was this outer tube present when the cable broke?
 
Last edited:
Custom bracket

I’m wondering if the vans supplied bracket was for a carb and the builder has modified it for fuel injection setup.
In doing so they have tried to use the original cable which would now would be too short.
So really both the bracket and cable need to be addressed to avoid a repeat of this in the future.

Regards Peter
 
Would someone please splain.....

kiwipete said:
I’m wondering if the vans supplied bracket was for a carb and the builder has modified it for fuel injection setup.
In doing so they have tried to use the original cable which would now would be too short. So really both the bracket and cable need to be addressed to avoid a repeat of this in the future. Regards Peter
Click to expand...

OK: it looks like the bracket was modified to drop the cable attachment about an inch lower. That would not make that much change in the length of the cable needed.

BUT: here's what I don't understand. Will someone please explain (say, the OP....): in the OP post #1, it shows the modified bracket with the cable housing attached and the broken cable protruding from that. The steel rod that attaches to the throttle body is also blurry but visible. Post #13 shows a brass rod that the cable was once attached to that has now broken off. That brass rod or tube has a threaded end with a nut on the end not hooked to the cable. Post #24 shows the steel push rod attached to the throttle body arm. Where does the brass fitting that was attached to the cable come in? What was it pushing on? What was it sliding through, if anything? Was that in the cable housing? There should be a sleeve that might have departed company with the cable when it broke. Was it ever there or was the brass rod coming out of the cable housing before the cable broke? But then what is the purpose of the brass nut on the brass fitting? :confused::confused::confused:
 
I'm not entirely sure what you are asking, as the pictures I provided didn't show the set up before the incident. I asked the Vans guy if they ever considered adding a spring to the arm to default wide open in case the cable broke. He said that that would have had to have come Lycoming.

I'm skeptical to believe the builder made any serious mods to the cable path. Everyone who has inspected my plane have said it was a completely professional build; no red flags or concerns at all.
 
Shawn

Although it looks minor just lowering the cable one inch, that might be just be enough to cause a problem. (You must have slack between the firewall and bracket)
Also the bracket might just be the wrong distance from the servo control arm. Again it might not look wrong but these two things together may cause a problem.
I know in my case, I had to make a custom mount for my-7, because I used a superior engine.

Regards Peter
 
Last edited:
I’m with the OP on this one and again, commend him for his good practice of staying above the glideslope. THAT is what made the difference here and everything else seems to be noise and speculation.
 
Roadjunkie1 said:
OK: it looks like the bracket was modified to drop the cable attachment about an inch lower. That would not make that much change in the length of the cable needed.

BUT: here's what I don't understand. Will someone please explain (say, the OP....): in the OP post #1, it shows the modified bracket with the cable housing attached and the broken cable protruding from that. The steel rod that attaches to the throttle body is also blurry but visible. Post #13 shows a brass rod that the cable was once attached to that has now broken off. That brass rod or tube has a threaded end with a nut on the end not hooked to the cable. Post #24 shows the steel push rod attached to the throttle body arm. Where does the brass fitting that was attached to the cable come in? What was it pushing on? What was it sliding through, if anything? Was that in the cable housing? There should be a sleeve that might have departed company with the cable when it broke. Was it ever there or was the brass rod coming out of the cable housing before the cable broke? But then what is the purpose of the brass nut on the brass fitting? :confused::confused::confused:
Click to expand...

I don't see a brass rod in post 13. I see a stainless rod that is either reflecting a yellow "hue" from the background or a photo that is off in white balance.

Skylor
 
You must log in or register to reply here.
Back
Top