What Breaks?...Common faults, not commonly known.

In the interest of Continued Airworthiness of our fleet, I hereby initiate a thread providing information on your MOST COMMONLY OCCURING MECHANICAL WOES found at annual/100hr inspections. Thank You, Mr. Diggler, for asking the question in one of your very entertaing posts. Hopefully we'll get a not too redundant list.

I'll start with one I've been finding on initial inspections of new customers for years...Not really sure that others have seen this much, the guys who maintained these didn't, though certainly you'll be looking for it now.

I've been finding sheared screws and broken attach brackets on the root (inboard-most) rib/spar attach brackets, usually the aft bracket attaching the root rib to the aft side of the rear spar. Usually, the bracket for the front of the aft spar will have the same. If you give a good shake to the inboard flap hanger you'll know if you've got this.

These brackets have cut-outs in them. They also attach to diagonal truss members of the ribs which will often be cut-out or releived of material to allow for flap cable clearance. The root rib cap strips, it's truss structure, and the attach brackets are somewhat of a "load-terminus" for the inboard flap hanger loads induced to the flap-area false wing-trailing-edge (flap cove skin sheet metal) and then the root rib, by the flap hanger.

Maybe the failure occurences I've seen are related to an "over-speed" flap application incident. Maybe it's just a screw that's not torqued well on assembly. Maybe it's related to the cut-out rib diagonal.

A little thought, a nice bracket doubler, several solid (not pop) rivets, and some thread locker on the new screws have repaired this area to far better than original.

Dave Calkins.

I have tons of others: worn-out cables where they exit fabric, or go around a pulley that's subject to getting dirty. Corroded-through gear legs on -12's and -14's. Cracked-out air scoops on lower cowlings. Broken spinner screw tabs. Broken oil cooler brackets. Broken baffling when the oil cooler is mounted on it without adding doublers to the baffling. Drain grommets that come off the fabric because the "expert" coverer didn't place silver dollar patches over them. Carburetor parting gaskets leaking due to gasket shrinking at initial assembly, and not retorquing after allowing overnight "compressing" of the gasket. Un-varnished wingtip bows warping. Cracked flap and aileron nose-ribs.

I could go on and on, and will, in the days to come. But I'll stop now and leave it open to you. I'm hoping to hear the oddball stuff that you've found that may not be the same as everyone else. Let's not be too redundant.

Dave Calkins.

Alternator bolt, 800.0 TTIS

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Good one Dave,

Here are a few things that come to mind imediately that I don't see on your list, like you said there are many Cub specific things to watch for.

The front seat back frame cracked just above where the diagonal support is welded. Probably happens from rear seat pasengers using the seat back to pull themselves up over the years. You can only find this on annual by pulling up the seat cushion, and most probably don't do this.

The front floorboard cracked around the brake petal when the board is not reinforced in this area.

Rear upper longerons cracked by the rear stab support, or a cracked rear stab support. This is from shacking of the tail feathers from the prop wash.

Rear stab tube seized in the support from lack of lube, a grease fitting here helps a lot.

Oil coolers rubbing the bottom of the nose bowl, possibly due to sagging engine mount bushings.

Non bushed front tail wheel spring attachments crushed.

Loose muffler flame tubes that can only be found by pushing on the flame tube through the tail pipe with a long screw driver or stick.

PA12 door frame tubing rotted under the frame sheet metal due to lack of paint applied at rebuild.

Rudder/elevator trailing edges rotted out, you can find these by gently twisting the trailing edges.

Tailwheels - Always check the 2 main springs closely, they never break at the same time, you can usually find one broken before the other lets go. Always check the tailwheel hardware for torque and distortion. My own tailwheel maintenance schedule is something like - Retorque hardware ever 50 or so hours, replace all hardeware every 100-200 hours, depending on use. Replace springs about every 500 hours, sooner if they start to sag. When replacing springs, also disect the tailwheel for cleaning and inspection. This seems to get the abused parts off the plane before they fail. Note that I really don't like monster tailwheels and springs, I concider the tailwheel and it's associated components consumable items. I'd rather knock off the tailwheel than have it stay on and bend/break the fuselage.

Just noticed kases alternator bolt which is another good one. The bolts that hold the alternator bracket on a wide deck also like to shake loose in the case. All the orginal generator/alternator/oil cooler brackets like to break, and the area in hard to inspect with the nose bowl installed, so this area needs special attention.

Inboard false spar to rear spar attachment. This is in the prop wash, and really likes to shake and break. It will show up externally as cracks in the back of the fuel tank cover. Push down on this area, if any movement is detected, the false spar is loose or broken.



These are some Cub specific things just off the top of my head, there are many more, good topic.

This one's pretty common, especially hangar/ramp queens that have sat around unannualed for a couple of years (shudder to think). The problem being corrosion in the rear window tracks due to the tail low attitude at three point and moisture "pooling" in those low spots. Minor I know, but its corrosion nonetheless.

Speaking of out of annual for a few years....A friend's father almost ferried a ramp queen that the mice had eaten the rib lacing off of. I guess they liked the wax.

And corrosion...how about elevator leading edges corroding from thestick tied BACK, but no drain grommets at the front of the elevators. ALWAYS tie the stick FORWARD, it helps keep the tail down in a big blow from the back, and also lets the snow slide off.

Dave Calkins.

Always drill drain holes in the rear window channels before installing the windows, this lets the moisture drain out.

And another one - Always chech that wimpy little bracket the holds the throttle cable to the engine mount for cracks and security (make sure it doesn't slip), and that it holds the cable away from the muffler so it doesn't get fried.

One more - Shock strut lowers full of H2O and rotted out at the bottom. Look for pin holes, and check with a Maule tester or pick if suspisious of internal corrosion. It happens more with non-hydrosorb shocks, when assembling these, always put a lot of grease between uppers and lowers to make a seal. Put some RTV on the hydrosorb when installing it on the lower to seal it. And always flush any shock strut components with tube seal before assembly.

axles

One other thing: Stock axles. Either sleeve them or replace with HD.

Had a stock one break back here today. The axle itself cracked off but the inner sleeve held...though it did bend.

Oh hey Mark...so you ran into the dreaded muffler flame tube blockage scenario huh? It happened to me once low and slow over a lake with tree's to get over coming up I'm surprised I didn't bend the throttle trying to shove it through the firewall. I like Dane's exhaust more and more!

Glad you got your 'puter running.

This is probably something everyone already knows, but ever notice the Cleveland wheel manual's way of installing the wheel bolts?... They show the bolt heads inboard, towards the brake... Where does a wheel bolt normally break?... In the threaded portion... Soooo, doesn't it make more sense to install the bolts from the outboard side and if it breaks it will migrate out instead of into the brake?... Seems like one of those "Duh" things to me...

kase said:

Alternator bolt, 800.0 TTIS

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I saved my generator bolt that looks exactly like that. 500+hours.

Hey guys,

Excellent thread! (not the bolt, I meant the postings).

This is the kind of thing I would like to organize in a document and post in W&I...

sj

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Alternator Belt either to loose or to tight! To tight, (takes the bearings out) to loose can rub on the baffling and or any other place close.

Pulleys full of sand under floor boards, Broken or missing cable guards.

Over torgued Gear Bolts, squeezing the fittings together, also rust in fittings!

Intake pipes connectors dryed out and clamps not sealing intakes.

overtorqued valve cover gaskets (throw away the cork ones and replace with "real gasket" silicone ones and DON'T OVER TORQUE AND TIGHTEN just a little on annual!!

Bent rear carry through tube (often caused by rear seat belt attached to center of this tube (give me a break) but seen many times!!

PA12 flap handle cable attach bracket not properly welded!! (no service inspection point) This cause me this last week end to make use of the "cub tool kit" to make a field repair so that I could get out of the spot we were camped in. This linkage actually has considerable load on it when pulling on full flaps! The original cub part is made from a 'squashed tube (for a reason) the STC for adding flaps to a PA12 shows this part built out of 'flat stock'. (quite ok if it is adequately supported and lined up directly under the pulley?) Any way this is a good place to check on an annual! or in my case on a weekend trip to the backcountry!


bending caused 1/1/2" of slack in flap cable-only about 5 degree total flap available

Flap attach bracket bent back and splinted with visegrip and .032 safetyw

hole in fabric made necessary to make field repair

I will think of more, and try not to demonstrate the lack of attention to some problem areas on my own equipment.

Tim

This has turned out to be a good thread, thanks for the attention guys.

baffle tie-rod holes worn through the baffles. I repair these with a thin sheet-steel doubler to last forever.

cracked cowling brace angle, the horizontal one attached to the side cowl with the latches mounted on it. these get cracked when the bumper screws and/or latches are adjusted too tight.

aileron pulley misalignment. the one hanging off the front lift strut bolt. the bracket needs to be slightly tweaked for proper pulley alignment with the runs of the cable. Interesting to note that in the Aviat Husky, they copied this problem. this wears the cables a bit.

I've got more. You guys do too, so keep 'em coming.

Dave Calkins

Nobody has said but was in another thread. Worn aileron and flap hinge blocks and worn elevator and rudder bushings. Push the surface stright up and down and watch the clevis. I see this a lot. They get neglected over the years.

Tail wheel bearings, locking tabs on the compression strut not set, flap bell crank wear, cowling wear from baffle seals rubbing them, torqued tail sections from turning under power with big loads on rough ground, torque tube wear, door hinge wear, slop in the wing strut attachment bracket holes (both ends) and gear bracket attachments holes, wear on mixture and carburetor heat cables from vibration, jury strut misalignments and bends from antler hauling.

Trim Pulleys

The aluminum double groove trim pulleys, front and back. Clean out the grooves with a tooth brush and solvent then look at the bottom profile of the pulley groove. The groove should go to a sharp V at the bottom. I've seen pulleys that look great from the outside but are really shot with the bottoms of the grooves rounded out. Grooves rounded out at the bottom will allow the cable to slip when cranking trim, especially in cold weather. Always install new aluminum double groove pulleys if the grooves are rounded out at all on rebuild. Crash

Mine's in bits for a 100-hour now and the oil cooler bracket is sheared in two places (three by the time it had been removed by me ).

A month or two ago my primer line snapped too. (Steve Pierce kindly sent me some little gizmos to fix it FOC. Thanks, Steve!)

BCB

Luckily, I've just had my oil cooler brackets welded, but a new set as per Univair would total $341!

I suspect I'll have to buy a set in the end, but omigod.

BCB

Currious: How many have run into this one. I have found four bad cases over the years but havent heard it mentioned much.

The rear door post just above the door latch with lightweight channel that forms the opening is usually open to the upper window channel. Dirt will accumulate over time and is trapped at the bottom as well as any moisture which runs down the channel and continually soaks the dirt that collects. Over time the tube will corrode through between it and the channel within the lower 1/2 to 1 inch. For some reason the corrosion goes to the tube rather then the channel. If the channel isnt opened up to allow drainage you can expect problems. Try slipping the blade of a pocket knife or an ice pick under the channel at the bottom edge.
Suggest drilling a small drain hole in the lower edge of the channel and also filling any opening at the top with silicone to keep water out.

Good luck
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Ed

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Thought this would go well in this thread.

Primer lines broken at the cylinder fittings......these usually break up close where you can't see the break. Look for fuel stain or give a gentle tug on these lines at preflight.

Check your wire runs...

The day after my 'fresh annual' on my PA-12 the charging system stopped. A simple inspection showed two wires crossing near the alternator that were chafing, and they eventually wore through. Because one of the wires was shielded, it 'looked' fine, but was broken.

Flaps

I had my flight surfaces recovered a couple of years ago during the first annual we noticed one flap would not fully retract? I though I had a weak return spring but after a close inspection we noticed the hing was loose on the flaps spar. We cut open the new fabric and opened up the flap to my surprize there was server corossion where the flap hinge riveted to the spar, my flap was hanging on by only two rivets all the other spar material was gone! The other flap was in about the same shape and I replaced the spar in it also. Niether flap would have held for much longer(thank God for annual inspections). I doubt many cubs would have this trouble but if they have been worked around salt water or on floats I would take a close look at the flap spar next time you do a recover.

Cub_Driver

Severe tail wheel shimmy on my last landing. Turned out the tail wheel bolt had broken, the one that holds the tail wheel to the leaf springs. Got lucky and didn't have the tail wheel come off, just really loose. I am going to start replacing the tail wheel bolts as well as gear bolts every 200 hours.

Tail wheel spring bolt always needs tightning

When I wash my cubs I always take a roll of 2 inch masking tape and tape off the lower portion of all window and airframe to keep water out of the window channels, when finished washing I always fly it to dry it.

This is the kind of thread I have been waiting for. Can some one assemble all of the good ideas in one posting for print out and use at my hanger?

wing tip bow

Has any one used a 1/2"or 3/4" pvc pipe for a wing tip bow?

www.cubframes.com

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Ash wing tip bows are cheap, and work quite well if you hit things every now and then. Those old timers kind of knew what works!

I've replaced two backplates to my spinner in the last three annuals... now I verify that the screws that hold on the spinner are tight prior to every couple flights -- with a screwdriver! Funny how two screws always seem to need a quarter turn. I hate to imagine what it looks like to lose a spinner in flight!

CubCouper said:

I've replaced two backplates to my spinner in the last three annuals... now I verify that the screws that hold on the spinner are tight prior to every couple flights -- with a screwdriver! Funny how two screws always seem to need a quarter turn. I hate to imagine what it looks like to lose a spinner in flight!

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I think you have a bigger problem. Bad spinner maybe. There are backing plates with 4000 hours. Where does it crack?Do you use fiber or nylon washers under your spinner screws? Maybe you should change out the nut plates. But if it's new backing plate they should'nt be worn to the point they let screws loose.