Cable damage

[pudestcon]

A couple of weeks ago during a daily inspection I found some damage to the port side empennage bracing wire, top side between horizontal stabiliser and the fin, on my T500 Thruster. I have no idea how the damage occurred but this cable was not replaced during the recent rebuild of this aircraft, so I have no idea how old it is. Anyway, the purpose of this thread is to show you the damage once I had unravelled each single strand of wire that formed part of the cable. The cable used is 7x19 3/16" stainless steel, or at least I thought it was stainless steel until the magnetised pointy nosed pliers stuck to a strand of wire!!

 

 

This shows the general construction of the cable

 

 

This shows what I found once unravelling the cut out damaged section of cable. There are 11 strands (shown top left and centre of photo of wire either broken (9) or damaged (2), so 11 in total. These came from 5 bundles making up the 3/16" cable. The 2 bundles shown left bottom of photo, being the inner most cable and 1 of the cables that twist around the outside, are the undamaged bundles. The 2 damaged strands came from 1 bundle, and the other broken strands came from the remaining 4 bundles. I had no way of inspecting each strand for stretching or distortion - other then running each strand between my finger nails:smile:. Bear in mind that each bundle has 19!! Yes 19 very small filament strands - you can see the pile I unravelled on the right side of the photo. There are 84 strands in that pile; go on, count them!!!

 

So, by my calculations, and not allowing for any strands that were stretched or otherwise sustained damage I could not detect, 8.27% of this cable was rendered useless. Does this mean the strength of this cable was reduced by 8.27%, or that the cable was still, largely, doing the job?

 

By the way.... The cable was replaced forthwith.

 

Unfortunately I did not take a photo of the cable insitu on the aircraft, but I can tell you the damage was not easily visible, but certainly easily detectable by feel. Every accessible flying wire now receives the 'finger' treatment at the daily inspection.

 

Do you inspect your wire cables regularly?

 

Pud

 

 

[Riley]

Pud

 

Dunno about your calculations but I agree that only once does one ever have to find a broken strand (OUCH!) whilst running their fingers down a control/support cable and, from then on - forever, I wager they will very, very carefully inspect (and respect the importance of) cable stranding during pre-flight inspection. A pierced finger (which ultimately could avert a busted rectum), proves to be a very cheap 'heads-up'. cheers

 

 

[Tomo]

Can't remember the calculations required to establish cable strength, but I looked up my book and found this where it said -

 

When broken wires are detected, the number and position, along with the examiner's opinion of the general rope condition, will decide whether or not the rope should be discarded.

 

If local concentrations or groups of broken wires are found, the rope should be discarded when:

 

a)Three or more broken wires are found in the close proximity of the termination,

 

b)Three or more broken wires are found in one strand or

 

c)Five broken wires are found between two adjacent strands within a length of 10 x rope diameter.

 

 

[turboplanner]

I've seen a lot of cables peel, in farm applications and they all seem to progress very quickly once the first strand snaps, so it would be an interesting exercise if someone has tension gauges capable of reading the tension at breaking point of the cable, and of the strand.

 

From my farm experience I suspect the rolling of both steel cables and hemp rope provides a friction which reduces stretching of the individual strands, but have no definitive evidence of that.

 

What I would hope is that aviation cable is being used on home builts.

 

Your 8.27% calculation, whether right or wrong is a good method of picking up factors which may escape eyesight. If for example you know that an elevator surface has 27% more area than a previous design, it gives you at least a pointer to the fact that it might be a lot more sensitive and may require different leverages to avoid over control for example. In other words it's a handy tool for comparisons. For example 7/16" through bolts are about 16.7% stronger than 3/8"

 

 

[Guest Maj Millard]

A pro will run down the cable with a rag, it'll pick up a broken strand less painlessly than a bare finger. Cables often will rust on the inner cores first, and so it is unseen, until the out cores start to let go. There are well over 100 different types of stainless steel, and yes some are magnetic !!...and some will also rust quite rapidly if given the chance.

 

CASA some years ago came up with some suggestions on proper methods for cable inspections..............the removal of a cable (especially a control cable ) for the bend check as described by Steve Bell recently in his tech column is good stuff, and often highlights a deffective cable that looked fine initially............................................Maj...

 

 

[Guest Maj Millard]

 

And whilst we are on the subject of cables...and thanks Pud for starting this helpfull thread. I recently was asked by one of my customers with a very early Slepcev Storch to look at an engine problem he was having. His complaint was that the engine RPMs would flucuate at mid and high range with no movement of the throttle.

 

I did my usual Rotax recommended checks of the carb adjustments and made some fine adjustments, but found nothing major. The carb mount rubbers were old and I suggested he order a new set for replacement. I then conducted some ground engine runs and although there was some small variations, all sounded normal and I put those down to the wind blowing through the 3 blade prop.

 

I then conducted a test flight ( never knock back Storch time !) and all seemed normal and strong with the engine.

 

to my surprise a week later the owner Emailed me the following photos of a carb cable he had found during fitting of the new mount rubbers. After buying the aircraft some years earlier he had fitted new steel cables instead of Stainless cables. Things rust quickly in the humid tropics and this cable didn't have far to go before failure ! Could have been exciting had it let go in flight...................................................................Maj...

 

 

 

 

[bones]

ok then onthis subject of throttle cables, if the carb cable broke on this aircraft would the throttles go full open or closed?

 

Just that there was a convo about it a little while baack on the gyro forum, i just want to know which why you guys have the rotax set up.

 

 

[facthunter]

Every four stroke Rotax I have had anything to do with goes to open throttle. The two strokes go closed. It would be hard to change the two strokes. Nev

 

 

[Guest Maj Millard]

Welcome back bones !.....generally on the 912 if you break a cable it'll go full throttle because of the spring attached to the throttle arm. However I have seen several set-ups where this may not occur. The Titan Tornado for example that has a continous cable loop set up, and some other aircraft that have eliminated the springs for some reason.

 

Personally your probabily going to be in better shape if you went to full throttle, to enable you to get sorted and maybe make somewhere for a suitable landing. Especially so if your involved in climbing out after take off heavily loaded. Once your in good shape you could switch the thing off, and follow up with a glide approach.

 

Without that your commited to going back to idle only, which may not get you very far in most machines....................................................Maj...

 

 

[turboplanner]

That's Ok in the air Major, but what about if it snaps during startup. Say you haven't used the aircraft for a while, and there's a bit of pushing and pulling on the throttle cable trying to get it started during which it let's go. You're going to need very fast reflexes to work out what happened and think to get on the brakes and shut it down at the same time.

 

 

[Guest Maj Millard]

I always like it when things break on the ground Turbo, with the exception of my tailwheel of course...........Maj...

 

 

[dazza 38]

Switches in aircraft are back to front for that reason, and other reasons.Back to front as in you flick downward to turn off.Opposite to light switches.They reckon it is alot quicker and probably more controllable to run your hand down the panel in a emergency to turn off switches, in one fast movement.

 

And no i didnt make this up.

 

 

[Gnarly Gnu]

I thought it was just American. They would argue that the switches are not 'back to front' but exactly the right way - and they probably have the numbers (of switches) on their side.

 

 

[dazza 38]

Hi GG, that could be true as well .I was told the story that I have mentioned by a RAAF Pilot.

 

 

[bones]

ok yeah i forgot the Pud's little wagon was a 2 banger, and obviously it would go full closed, was thinking more along the lines of the 4 stroke side.

 

Maj, some guys turn the carb linkages around and also swap the spring so if the cable breaks it goes closed, personally i prefer open for obvious reasons as stated above, and that is why all my machines are set up this way. thanks for the replys.

 

you guys play nice catch ya in another couple or three weeks :) or until the wet starts up there.

 

 

[Guest davidh10]

...what about if it snaps during startup. .... You're going to need very fast reflexes to work out what happened and think to get on the brakes and shut it down at the same time.

Don't you always apply brake(s) before starting? On smooth level ground, idle is just fast enough to move my aircraft, especially if there's some choke applied, so always start with the brake applied. Actually, the brake wouldn't hold mine if the engine is at full power. Holds up to about 90%.

 

...Do you inspect your wire cables regularly?

Visual inspection is part of the daily. More detailed inspection every airframe 100 hourly.

 

 

[turboplanner]

Yes, brakes on is the first pre start check. I was thinking more of the confusion when you are taxying in and out of aircraft on the apron and you suddenly get the push in the back.

 

However I could also see some of the beach umbrellas having a problem for more than a few seconds with those tiny brakes, and the guys with pull starts who start the aircraft out of the cockpit. I imagine some of those units with high thrust line would almost lift the rear wheels enough to slide.

 

 

[facthunter]

D38, think the s/w orientation was the USA way. Later others adopted it for standardisation.. Nev

 

 

[dazza 38]

D38, think the s/w orientation was the USA way. Later others adopted it for standardisation.. Nev

That makes sense Nev.What happened, was I asked a Raaf Pilot why are the switches, back to front. Without even hesitating he gave me the answer I have mentioned. Maybe true I dunno.

 

 

[facthunter]

Flown a few US planes . Everything is down to off. (Going from memory). It has logic to it. Early Vickers Viscount had switches operating one way and later ones the other way. When you have pilots dual endorsed, this is a recipe for disaster. Nev

 

 

[turboplanner]

Someone have a look at an old Moth? That would tell us.

 

 

[robinsm]

Dazza, that has been my understanding for many years now. My instructors have told me that, I remember reading that in a number of publications and some airforce friends of mine reinforced that when asked. The reason the house light switches are the opposite is that in an emergency a smart downward stroke would turn them on. The orientation of the switches seem to be in the best positioon for emergency use, ie aircraft to turn off, lights in your house to turn on.

 

 

[Yenn]

Flexible steel wire cables in my opinion should be discarded if they are stainless steel and have any broken strands. Stainless steel has a tendency to let go suddenly and once one strand is broken all the others will follow, especially as the load is shared by fewer cables each time one breaks. Gal wires are a lot more forgiving. There is a recommendation for the amount of wear permissable on the outer strands, but I can't lay my hands on it at the moment.