Unexplained Rigging Issue - 172
- Thread starter killbilly
- Start date
inigo88
Composite-lover
You’re clearly a hack.
But also I completely agree, if this is a brand new thing squawk it and stop flying the airplane until a qualified A&P checks it out. I’m also imagining a @CoffeeIcePapers scenario where somebody had a gust lock on and then forced the yoke and stretched a cable.
Pony up the $$$ and get that checked out, if nothing else for the peace of mind. Flight control rigging drifts over time as cables stretch, pulleys wear, etc, but that shouldn’t be an overnight thing. If it is you should be concerned.
CFI A&P
Exploring the world one toilet at a time.
The oldest C172 I flew was a ‘67 model and it had a hole in the yoke shaft from the factory.
The seatbelt around the yoke on a PA28 doesn’t need max tension on it, just enough to dampen the flight controls from moving during wind gusts and propeller blasts.
CFI A&P
Exploring the world one toilet at a time.
Then why do those cables get checked for tension at every 100hr and annual?
Kent-Moore Cable Gauge | Aircraft Spruce
Kent-Moore Cable Gauge Easy to use - just squeeze ball handle, put gauge on cable, release handle and read tension on rotation dial. No conversion charts necessary.
www.aircraftspruce.com
inigo88
Composite-lover
For those on the sidelines who may be curious about this topic, here’s a good kitplanes article to get your feet wet.
www.kitplanes.com
It’s from the perspective of an experimental homebuilder who may be figuring this all out on their own design, but has some good gems in there. For example, a 1/8” stainless steel cable has a minimum breaking strength of 1760 lbs. Cable stretch (plastic deformation) is going to start below that, but still well above human capability.
…Until you take into account the fact that cable and pulley flight control systems employ mechanical advantage, and the force you apply at the yoke isn’t necessarily the force felt in the cables or at the flight controls thanks to force multiplying bellcranks, etc.
Maintenance Matters
Most homebuilt aircraft use control cables, so it’s helpful to understand how to select, install and maintain them.
www.kitplanes.com
It’s from the perspective of an experimental homebuilder who may be figuring this all out on their own design, but has some good gems in there. For example, a 1/8” stainless steel cable has a minimum breaking strength of 1760 lbs. Cable stretch (plastic deformation) is going to start below that, but still well above human capability.
…Until you take into account the fact that cable and pulley flight control systems employ mechanical advantage, and the force you apply at the yoke isn’t necessarily the force felt in the cables or at the flight controls thanks to force multiplying bellcranks, etc.
Roger Roger
Paid to sleep, fly for fun
I’m more worried about the compounding effects of corrosion, fatigue, wear from fairleads/stuck pulleys….For those on the sidelines who may be curious about this topic, here’s a good kitplanes article to get your feet wet.
Maintenance Matters
Most homebuilt aircraft use control cables, so it’s helpful to understand how to select, install and maintain them.
It’s from the perspective of an experimental homebuilder who may be figuring this all out on their own design, but has some good gems in there. For example, a 1/8” stainless steel cable has a minimum breaking strength of 1760 lbs. Cable stretch (plastic deformation) is going to start below that, but still well above human capability.
…Until you take into account the fact that cable and pulley flight control systems employ mechanical advantage, and the force you apply at the yoke isn’t necessarily the force felt in the cables or at the flight controls thanks to force multiplying bellcranks, etc.
killbilly
Vocals, Lyrics, Triangle, Washboard, Kittens
So here's the latest:
I flew the airplane yesterday and - per @Roger Roger 's suggestion, trimmed it out in cruise, straight and level, with the yoke deflected, and then let go to see what it would do. It flew perfectly straight and level for about 40 seconds or so, and then very, very gradually started rolling left. Just very slightly. As it rolled further left, it picked up a little more speed in the change. Again, gradual, just not AS gradual. We repeated this test a couple times just to make sure of what we were seeing.
When I got in this morning, I had a thought and pulled the usage logs on the airplane. Sure enough, it had been in for what was described to me as "extensive" maintenance about 20 hours ago. Which, looking at the usage logs, corresponds to the earliest known report of this behavior. So I took some pictures, emailed the DM.
He thinks maybe they did something at the Mx shop, said it could be deferred for a little while, and to bring it down to them and they could fix it in just a couple of hours.
So there ya go.
I flew the airplane yesterday and - per @Roger Roger 's suggestion, trimmed it out in cruise, straight and level, with the yoke deflected, and then let go to see what it would do. It flew perfectly straight and level for about 40 seconds or so, and then very, very gradually started rolling left. Just very slightly. As it rolled further left, it picked up a little more speed in the change. Again, gradual, just not AS gradual. We repeated this test a couple times just to make sure of what we were seeing.
When I got in this morning, I had a thought and pulled the usage logs on the airplane. Sure enough, it had been in for what was described to me as "extensive" maintenance about 20 hours ago. Which, looking at the usage logs, corresponds to the earliest known report of this behavior. So I took some pictures, emailed the DM.
He thinks maybe they did something at the Mx shop, said it could be deferred for a little while, and to bring it down to them and they could fix it in just a couple of hours.
So there ya go.
Roger Roger
Paid to sleep, fly for fun
That all makes sense.
Rigging controls on little GA airplanes isn’t hard, but it’s finicky, and you spend quite a bit of time making an adjustment, checking, making an adjustment somewhere else, and checking again. Also, GA manufacturers are fond of giving you useless information like “ensure that the up stop of the bellcrank is 1.375” from the aft spar with the yoke neutral” meanwhile said bellcrank is located at an odd angle inside a tiny access panel so good f’n luck setting it to 1.375" without spending another hour fabricating some sort of tool or fixture.
They’ll also tell you to do the impossible, like “set both elevator cable tensions to 55 lbs with the elevator neutral and the yoke at such and such position”. Why is that impossible you ask? Think about how the yoke and the elevator are being held in that position, and what that does to the tension in each cable.
Minuteman
“Dongola”
Roger Roger
Paid to sleep, fly for fun
Roger Roger
Paid to sleep, fly for fun
Also to address the “control cables don’t fail” issue….yes, it’s rare, but they absolutely do. That’s why both Piper and Beechcraft have ADs against control cables. Most new build aircraft, even part 23, have at least suggested (chapter 5) replacement intervals for cables, if not mandatory (chapter 4) life limits.
As mentioned, I can’t count how many cables I’ve changed over the last ≈15 years for wear. Usually due to broken strands where the cable made a big wrap around a small pulley (the PA32 had a couple spots that were infamous for this), or where rubbing on a fair lead of some kind had again resulted in broken strands. Side note, don’t look for broken strands with your fingers unless you like bleeding. The best way is to wrap a rag around the suspect area and work the control, any broken strands will catch on the rag and be very noticeable.
I have seen 2 outright failures, one where a trim cable unraveled so much it jammed the trim system, and another where a flap cable just plain broke. I also changed all the rudder and elevator cables in a floatplane due to corrosion, after a rod end in the elevator system broke, also due to corrosion. This airplane had had an annual like 3 months prior too.
When I was a running a maintenance shop it got to the point where I just ordered a full kit of all the flight control cables for the 206, 207, and 208. Then whenever we used a part from a kit we’d replace it. We had all the cable fab tooling and parts but without actually doing pull tests and stuff (not to mention how easy it was to be 1/4” off and have some real rigging headaches) I was never super comfortable with those for primary flight controls, and I figured those were a totally reasonable use of the boss man’s money.
As mentioned, I can’t count how many cables I’ve changed over the last ≈15 years for wear. Usually due to broken strands where the cable made a big wrap around a small pulley (the PA32 had a couple spots that were infamous for this), or where rubbing on a fair lead of some kind had again resulted in broken strands. Side note, don’t look for broken strands with your fingers unless you like bleeding. The best way is to wrap a rag around the suspect area and work the control, any broken strands will catch on the rag and be very noticeable.
I have seen 2 outright failures, one where a trim cable unraveled so much it jammed the trim system, and another where a flap cable just plain broke. I also changed all the rudder and elevator cables in a floatplane due to corrosion, after a rod end in the elevator system broke, also due to corrosion. This airplane had had an annual like 3 months prior too.
When I was a running a maintenance shop it got to the point where I just ordered a full kit of all the flight control cables for the 206, 207, and 208. Then whenever we used a part from a kit we’d replace it. We had all the cable fab tooling and parts but without actually doing pull tests and stuff (not to mention how easy it was to be 1/4” off and have some real rigging headaches) I was never super comfortable with those for primary flight controls, and I figured those were a totally reasonable use of the boss man’s money.
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ppragman
FLIPY FLAPS!
Lolz
ppragman
FLIPY FLAPS!
“The grand aircraft magi consults the scrolls before jotting <rolls dice> ‘OPS check good, aircraft RTS’ in the log book.”
killbilly
Vocals, Lyrics, Triangle, Washboard, Kittens
Director of Maintenance.
But I *did* put on my robe and wizard hat before I called him.
EDIT - If you're a fan of Charles Stross's books, you'll know why I was wistful that the DM's first name was, indeed, Derek.
ppragman
FLIPY FLAPS!
knot4u
Repeat Offender
I have almost zero GA experience so forgive my ignorance. Do airplanes like a 172 use rig pins? I've changed lots of flight control cables, and without going into a diatribe about the process, installing rig pins into bellcranks or sectors was always considered "zero" and I'll build out from there. The final inspection would always include a visual inspection of everything that had been disassembled, being able to remove/install the pins easily after the cables were tensioned and checking the actual travel of the control surface with a protractor (either mechanical or digital). Inspection of the actual cables was always why they were replaced, @Roger Roger is correct about ordering the cables before you start an inspection. If I'm going to do a tailcone below floor inspection on a G-III I know the cables are going to need replacement because of how the airplane is built, on a G-IV I'll wait until they've been removed and inspected.
Roger Roger
Paid to sleep, fly for fun
Unfortunately the little GA airplanes I’ve worked on don’t use rig pins-sometimes the manual will give you a pattern for a jig for one part or another of the system (Cessna gives you dimensions for a jig to hold the yoke in the neutral elevator position, Piper gives you a drawing for a jig to set the aileron bellcranks, for example), but usually they have you measure some part of the bellcrank from some other part of the structure, which as you can imagine is a pain in the ass especially with the size of aircraft and the access holes we’re dealing with.
knot4u
Repeat Offender
Sometimes rigging almost turns into an artistic endeavor. Hydraulically boosted lateral control devices can be problematic. Elevators and rudders normally use one actuator but ailerons require one on each wing and they have to work in harmony as they move in opposite directions, if those girls can't dance together it makes it tiresome for the guy wrestling with the yoke. I've had more than one airplane that came in with a squawk about the ailerons going into "manual reversion" that actually were the actuators not getting along. There's no real base point to start from other than "it's hard to roll right" so the best course of action after verifying the rigging is to make a small change on one of the actuators, if it gets better keep doing it, if it gets worse go back the other way. This is all done in a hangar with at least one hydraulic mule and force gages to measure how much it actually takes to move the yoke. All it takes is time and a bit of an ability to actually "see" the system.
inigo88
Composite-lover
I know the immediate question is resolved, but for the benefit of future forum lurkers I checked, and Google will readily turn up the PDF(s) for the maintenance manual you seek:
It seems the two most informative places to check are if the secondary cables at the ends of the yoke chain are even with each other, and if both aileron bell crank stop bushings are centered in their slots. Depending on the outcome it could be an easy fix (adjusting the push rods between the aileron bellcrank in the wing and the aileron) or a hard fix (re-rigging the cables). It seems like as much of an art as a science, and I’ve only ever watched guys like @Roger Roger and @knot4u do it and have not done it start to finish myself. Note the custom rigging tooling the A&P is expected to fabricate, like the “bar” holding the two yokes together, etc.
Roger Roger
Paid to sleep, fly for fun
Because it’s been forever since I worked on a 172, and never did control rigging on one (lots on 206/207) I had to find a picture of the aileron bellcrank and the slot.I know the immediate question is resolved, but for the benefit of future forum lurkers I checked, and Google will readily turn up the PDF(s) for the maintenance manual you seek:
It seems the two most informative places to check are if the secondary cables at the ends of the yoke chain are even with each other, and if both aileron bell crank stop bushings are centered in their slots. Depending on the outcome it could be an easy fix (adjusting the push rods between the aileron bellcrank in the wing and the aileron) or a hard fix (re-rigging the cables). It seems like as much of an art as a science, and I’ve only ever watched guys like @Roger Roger and @knot4u do it and have not done it start to finish myself. Note the custom rigging tooling the A&P is expected to fabricate, like the “bar” holding the two yokes together, etc.
this is what I’m talking about, you’re trying to tension the cables so that bolt is in the middle of the travel range of the slot, viewed through an access hole and almost certainly from an angle that doesn’t give you a straight on view of the bellcrank slot, and keep in mind the turnbuckles you’re adjusting are in the headliner (well maybe not, it’s been forever since I touched a 172) so…adjust a half turn, check tension, check left bellcrank out at the end of the wing, check right bellcrank on the other side, adjust out the half turn you did and do a half turn on each of the OTHER two turnbuckles….lather rinse repeat.
Oh and by the way, once you have the cables and bellcranks rigged to the yoke, the ailerons are rigged to the flaps per @inigo88 ‘s post above, so better hope your flaps are rigged properly!
edit: seems to me to keep the yokes straight with each other on 206/207/Cherokee we used to use a chunk of 2x4 and some c-clamps. The sedan your only adjustment to sync the yokes really is the chain so you unfortunately you only get 1 tooth increments to sync the yoke. Of course that airplane is more complicated but also sort of easier because it has 3 separate circuits. So once you get the fuselage circuit set you can adjust each side independently.
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