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M20K O2 Cable Snap


smwash02

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Touched down in Monterrey, MX a couple weeks ago and went to turn the O2 off and heard the snap. Fortunately not too many funny looks when I took off the back panel to shut off the O2 (I have a leak I can't find -- open to ideas here, spraying soapy water has come up short).

Found some good threads on here and will likely order some of the spring back temper wire in case I can't get proper blessing on my 'owner created part'. A friend saw my struggle and helped me machine the gizmo below. Sure beats the wire wrap around the post original design, in my opinion.

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There are a few threads covering the broken wire to control the valve...

A couple on the best soapy water solutions that is sold by one of the O2 equipment guys...

Nice owner supplied part... but...

The nice thing about commercial parts... if there is a failure in the design... there is a good chance that somebody in the herd finds out first, and tells the rest of the herd...

Make sure your two set screws can’t move... probably blue lock-tite for that...

Make sure if the wire does start slipping you can see there has been motion... some form of torque-seal paint might work...

 

Nothing sets up a disaster like an O2 failure...

Landing while sleeping at cruise speed... hasn’t been demonstrated yet...

Dan only went to the extent that it is possible to land while sleeping, but only at Vy... :)

Great start with the pics!

 

Want to discuss what you know /don’t know about the leak?

We have found a few around MS... from mismatched or re-used compression fittings, to improper plastic bits not matching between two separate suppliers...

PP thoughts only, not a mechanic...

Best regards,

-a-

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On the leak side -- I sprayed the T, the regulator, the air fill port, gauge, and pilot Scott fitting. Leak only occurs when the tank is 'open', so I think this precludes the high pressure side (gauge). Leak occurs with nothing plugged into the connectors. Can the system be smoked? That would make it pretty effortless to find. In any case, I put in an order for proper gas leak detection fluid and hope that'll yield better results. I presume the bottle should be able to be left open with nothing plugged in an leak be none or minimal. I'm seeing 250lbs a day.

On the part side -- the commercial design is merely an undersized cable looped around the post. As you mentioned -- a few threads on this since it is the factory design. I'm not too concerned about if my part 'fails' as it'll fail in the spot where it was last touched -- as before when it failed. If there was a spring back system as proposed somewhere it would be a bigger consideration. Both setscrews have loctite, the unfortunate part about this mechanism is it is impossible to see once installed even if it had a marking on it; though, I agree it is a good idea and will implement.

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Annual two months ago with new bottle. Based on how the panel came off I don't believe it was removed. I can see how the puncture could happen, but fortunately I didn't see any while working on it.

I didn't own it prior so can't say if it leaked previously or not, so approaching from the angle of it could be anywhere.

Leak fluid arrives Wednesday, will report what I find. Thanks for the ideas!

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Most low pressure leaks are from the regulator not closing all the way when your knob in the cockpit indicates closed. Start by verifying the regulator arm can't be moved further to closed. Since your cable broke, I'd bet you'll be okay once you properly re-rig the cable.
Also 250 lbs seems to large to only be leaking in flight - another reason why I suggest the regulator rigging.


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13 minutes ago, kortopates said:

Most low pressure leaks are from the regulator not closing all the way when your knob in the cockpit indicates closed. Start by verifying the regulator arm can't be moved further to closed. Since your cable broke, I'd bet you'll be okay once you properly re-rig the cable.
Also 250 lbs seems to large to only be leaking in flight - another reason why I suggest the regulator rigging.


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250lbs is 24 hours of o2 open and nothing plugged into the Scott ports. Is 11lbs/hour loss 'normal'? Seems high to me and will add up quickly in the air over time.

In the closed position, I don't experience any loss. I confirmed travel to be good by setting to closed and pushing on the arm to ensure it couldn't move any further.

@larryb Thanks. I remembered soapy water is not safe around high levels of O2 and decided to stop until I get the proper testing fluid, so will check those next when it arrives this week.

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250lbs is 24 hours of o2 open and nothing plugged into the Scott ports. Is 11lbs/hour loss 'normal'? Seems high to me and will add up quickly in the air over time.
In the closed position, I don't experience any loss. I confirmed travel to be good by setting to closed and pushing on the arm to ensure it couldn't move any further.
[mention=8639]larryb[/mention] Thanks. I remembered soapy water is not safe around high levels of O2 and decided to stop until I get the proper testing fluid, so will check those next when it arrives this week.

I never leave mine open/on except in flight when in use. The ports are not leak proof when on. Leaving it on while not using it will drain down the tank over a weekend. How many psi/hr I don't know, but don't find it to be an issue in flight when breathing on it since the pressure is being vented to the connected ports. I am still on the same O2 fill from when I left San Diego flew to Guatemala to Yucatan and now in Puebla with 2 people mostly at 16K and still have ~800 lbs - enough to get home.
To not loose O2, it's imperative to turn off the regulator when not in use. You shouldn't feel any pressure when you inset your cannulas/mask into a port at the start of a flight. If you do, the line is pressurized (any leaking) from not being entirely closed. I even pull out the devices from the ports to avoid loss in case it's not entirely off.


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Hmmm... my cable is also very stiff. We've worked on in a few times with no improvement. So I just leave mine on all the time. I've not noticed any loss of O2 over the course of a month or more.

I guess I am going to have to look into replacing my O2 port seals. I always assumed that is what the off control was for and that the port seals would naturally leak if left on. Good to know!


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3 minutes ago, kortopates said:


I guess I am going to have to look into replacing my O2 port seals. I always assumed that is what the off control was for and that the port seals would naturally leak if left on. Good to know!

LOL... and I was just thinking I need to bring my 252 to you to fix the cable properly so I can turn it off.

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LOL... and I was just thinking I need to bring my 252 to you to fix the cable properly so I can turn it off.

My cable does operates smoothly. I’ve seen where the cables wears on bends and/or the sheath wears thin. But easiest to try a new cable first.


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I would expect it to behave like @gsxrpilot states, things shouldn't leak.

Mine operated smoothly but I could feel flex when moved to the 'off' position where I believe the cable was flexing because the bottle was closed but there was extra wire at the control. I suspect this contributed to the snap if not wholly caused it.

Re-doing it, I have it such that when it's closed there's not enough for it to flex and start to fatigue. I left a little extra room, but it's minimized a lot compared to the original feel.

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It is easy to fix the cable. Disconnect both ends, pull the old one out, push the new one in with a few squirts of tri flow. Most of the time is spent in removing the pilot seat and sidewall. You could probably just lube the old cable but if going to the trouble of doing this I’d just replace the inner core with new.

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1 hour ago, smwash02 said:

I would expect it to behave like @gsxrpilot states, things shouldn't leak.

Mine operated smoothly but I could feel flex when moved to the 'off' position where I believe the cable was flexing because the bottle was closed but there was extra wire at the control. I suspect this contributed to the snap if not wholly caused it.

Re-doing it, I have it such that when it's closed there's not enough for it to flex and start to fatigue. I left a little extra room, but it's minimized a lot compared to the original feel.

To fully turn off, it should go a little past off so the cable is still pushing a bit at fully off. This won't break the cable - mine has never broken. And from memory, the service manual will echo that. 

Yeah, I guess mine has always leaked through the ports - if not off. I'll have to look into new port seals. From memory they're just o-rings.

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Expect things to be leak free...

Mostly because O2 getting loose can cause a flammability challenge...

Less likely under ordinary circumstances... or in flight with so much air movement....

Stationary in the hangar, having an O2 bottle Go empty in the plane would be sub-optimal...
O2 finding an oily rag could be self igniting...

PP thoughts only, not a mechanic...

Best regards,

-a-

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10 hours ago, carusoam said:

SMW,

Did that screw get tightened?

I keep seeing the first pic at the top. It never seems to get tightened... :)

Best regards,

-a-

 

I couldn't find a torque spec so I went with 100ft-lbs, too much? :)

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Edit: I thought you were making a joke and I was playing along. Appreciate you looking out for me!

I figured that's the one you're referring to.

The mechanism is self contained for controlling the bottle and it mounts to the sidewall with the screw you have circled and threads around the control knob. I unfortunately don't have a photo of the assembly without the knob on, but here's a photo of it mounted to the plane from before I took it apart. The photos at the beginning are with it resting while being worked on.

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Here's a photo from the IPC

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As you can see from the IPC mine was installed incorrectly, which made it extremely difficult to remove.

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  • 3 years later...

IMG_0974.jpeg.1ea296687044e9bc3653203d83a8f3e0.jpeg

Sorry to resurrect an old thread, but thought this might be the best place for this question. Ovation O2 knob assembly missing this nut. Parts manual does not list what size this is. Does anyone have any better information? The actuator does not stick out far enough for the set screw in the knob to grab on without the nut. I tried a 15/32 hex nut I had laying around just for sizing, but it was too big.

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