Compressed Air Instead of Nitrogen in Gear Strut?

HighFlyingA380

HighFlyingA380

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Jim F.
So I went for a flight in the BE-76 today, but ran into many issues. The primary one I'm concerned about is that when I got to the frozen plane, the nose strut was completely flat (no shiny metal to be seen). So I called MX to take a look. From what they told me, all the struts were hydraulic/pneumatic using nitrogen. They said there was no leak in the strut, and the cold air caused the nitrogen to loose pressure. I feel like calling BS on that, since the other two struts were fine, as well as the nitrogen un-feathering accumulators, and the strut was perfectly fine a few days ago, when it was 20 degrees colder. To top it off, their high-pressure nitrogen regulator broke yesterday, and would be several weeks before it could get fixed and re-approved. After about 30 minutes of debate amongst themselves, they decided to fill the strut with shop air. But since the shop compressor couldn't provide high enough pressure to overcome the weight of the plane, they had to lift the plane up in order to get enough air in.

There is no information in the POH regarding the pressures and type of compressed gas to be used, so I went with what they said. But it did worry me that if it were an acceptable fix, why did they have to debate for 30 minutes about if it would be O.K.? I also seem to remember hearing somewhere that all high-pressure compressed gasses for aircraft were supposed to be nitrogen. I'm interested to see if it actually held the pressure when I show up later today, as I feel there has to be something wrong with the seal.

So, are my fears/uneasiness in this situation warranted, or is this a perfectly acceptable fix for this? Thanks for you input.
 
Did you ask them to show you the logbook sign off for the servicing?

I would venture to say the MM states filling with nitrogen, but as with most things in GA looks like a "band aid" repair.

And yes, it will go flat again.
 
As long as it's not your airplane to pay MX bills on that's fine. The issue is that compressed air carries moisture which will form into ice crystals and tear up the seals in the strut. In this case they were already junk which is why the strut went flat when it got cold. You won't run into a safety of flight issue over it though, it's just pounds on the ground and eventually destroys the strut when flat.

As for the pressure regulator on their rig, those are cheap from any welding supply, never heard of rebuilding one, maybe they have a boost pump?.:dunno:
 
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Henning said:
As long as it's not your airplane to pay MX bills on that's fine. The issue is that compressed air carries moisture which will form into ice crystals and tear up the seals in the strut. In this case they were already junk which is why the strut went flat when it got cold. You won't run into a safety of flight issue over it though, it's just pounds on the ground and eventually destroys the strut when flat.

As for the pressure regulator on their rig, those are cheap from any welding supply, never heard of rebuilding one, maybe they have a boost pump?.:dunno:
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Thanks for the reassurance. I'm just not too well versed in maintenance and mechanical issues, so I was a bit worried.

Not sure about the regulator, but they said it was some sort of special, high-pressure regulator with very small tolerances. They said it had to be sent away for repairs, then sent somewhere else for FAA approval. Said it would be a minimum of 3 weeks until they got it back. I dunno. :dunno:
 
Well I would call BS on there theory that it was just cold and that is why it is flat. That has nothing to do with it. Many many other aircraft operate in the cold without flat struts. You can put shop air into it, but it is not recommended because it is usually dirty and has moisture in it. Chances are if it is flat it will go flat again, but its not really a safety issue.
 
many of us a strut pump that attaches to our air compressor output and boosts the pressure. I have one in my hangar as does almost everyone I know. I find it difficult to believe that your shop did not have either a nitrogen bottle or an air strut pump on hand. It's more likely that the monkeys you happened to talk to were clueless.
 
N2 or air ?

In practical usage, there is vary little difference. The seals will wear just as quick with either.

The Big OWT is air has water in it, but when you run it thru a compressor, the water will condense in the compressor tank. or be taken out by filters in the system.

when you consider the size of the air charge/N2, you will realize how much water that amount of air can hold.

Cold struts will show less chrome, N2 shrinks too. air is no different.
 
What was the % of water vapor in the air, in Jan, in central MO? I would say between zero and slightly above zero. So, when compressed, the water vapor will actually be lower as a percent than ambient pressure, most of it having been collected in the recovery tank of the compressor. Next, for the water vapor to 'freeze' it must first go through the liquid phase, under pressure, in an enclosed space where the % of water vapor in the first place is measured in a few moles.

Chance of it changing state from vapor to water to icing and getting lodged in a seal ~ 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000005%

Air is fine in the strut. Your strut seals on the nose gear could possibly need refreshing. Beech sells a kit with the seals, packing and maybe the strut bushing too, I haven't checked recently. Something to plan for.
 
Since adding air and oil is on the magic list of 31 things a non A&P owner can perform I have wondered what is needed to do this. One of the posters mentioned an accessory that will boost the pressure of your home grown variety air compressor. Can you elaborate please?
 
I have the same pump but beware: when you hook the air up to it don't be standing over the handle or it'll knock your teeth out :hairraise:
 
Tom-D said:
N2 or air ?

In practical usage, there is vary little difference. The seals will wear just as quick with either.

The Big OWT is air has water in it, but when you run it thru a compressor, the water will condense in the compressor tank. or be taken out by filters in the system.

when you consider the size of the air charge/N2, you will realize how much water that amount of air can hold.

Cold struts will show less chrome, N2 shrinks too. air is no different.
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YHGTBSM... Seriously? LOL....
 
If you want to boost pressure, a Haskell pump is the way to go, we use them to jack SCUBA tanks to 4500-5000 psi.
 
Thanks for all of your replies... Most of that stuff's getting into stuff I don't know much about. My concerns were just from hear-say that said O2 was bad for aircraft operations. Obviously, it sounds like it's perfectly fine.

As for the current situation with the plane, I showed up to fly this morning, and ended up spending a couple hours doing the line-service's job defrosting it. Then, while it was being towed up to the start-cart, the strut seal completely gave out and collapsed. So they're overnighting a part and rebuilding the strut, and making it so it can be used with standard 130PSI-ish shop air. Regardless, it's just yet another major road-bump in the road to getting my multi-engine add-on, but that's a whole different saga.

Again, thanks.
 
HighFlyingA380 said:
My concerns were just from hear-say that said O2 was bad for aircraft operations.

So they're overnighting a part and rebuilding the strut, and making it so it can be used with standard 130PSI-ish shop air.
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You didn't put O2 in it, you put air in it which has about 17% O2, mostly N2 and some trace elements(including a small amount of water vapor).

It should be interesting to see what they rebuild it with so that it will inflate with 130-ish PSI air. Since it's holding up the same weight and will have the same volume as before. I'm guessing you'll still need a high pressure source of air, say about 300PSI like all struts on all aircraft in the GA fleet.
 
docmirror said:
You didn't put O2 in it, you put air in it which has about 17% O2, mostly N2 and some trace elements(including a small amount of water vapor).

It should be interesting to see what they rebuild it with so that it will inflate with 130-ish PSI air. Since it's holding up the same weight and will have the same volume as before. I'm guessing you'll still need a high pressure source of air, say about 300PSI like all struts on all aircraft in the GA fleet.
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My bad, that's right; Didn't really think through what ambient air really was.

Yeah, I'm not sure what they're doing. And honestly, the way they've been treating me (and other flight students), I don't really care, as long as it's safe for me to fly for a couple weeks. I'm leaving shortly, and won't have to deal with them again.
 
Henning said:
YHGTBSM... Seriously? LOL....
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Have you ever drained a compressor tank and seen how much water comes out?

You don't have water traps on your system?

How much water do you see being expelled from your air tools?

It shouldn't be coming out of the hose, but It went some where?
 
A few points from a metallurgist who designed landing gear for a Fortune 500 aerospace OEM:

-The interior of the oleo shock strut lacks the corrosion preventatives that are present elsewhere. For instance, certain portions of steel components are left completely bare rather than cadmium plated.

-The dissimilar material combinations have a wider tolerance of EMF than what are used outside of the gas tight seals. With an expectation of zero electrolyte presence you can put things in intimate contact that you would never tolerate in the presence of (any) water.

-Shock strut oil is flammable. In the event of some sort of pressure containment failure do you REALLY want ANY oxygen screaming out of the hole with it?

-We always put 99.999+% Nitrogen in our struts in assembly. We got inquiries all the time to use air compressors, we always said no.

-I have seen an environmental seal failure on a shock strut lead to corrosion that then caused catastrophic failure. ANY moisture in the strut is rolling dice.

Now I am sure lots of folks use compressed air and get away with it. Heck, lots of folks pencil whip annuals, too. One day, hard to tell how long from now, those decisions might catch up to them. I am sure I sound like a stick in the mud here but there are engineering realities to be considered.
 
HighFlyingA380 said:
...So they're overnighting a part and rebuilding the strut, and making it so it can be used with standard 130PSI-ish shop air...
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That's an interesting statement there. Look in the POH and I think it'll call for 250 psi in the nose and 300 psi in the mains. No magic o-ring on Earth is gonna change those numbers.
 
Tom-D said:
Have you ever drained a compressor tank and seen how much water comes out?

You don't have water traps on your system?

How much water do you see being expelled from your air tools?

It shouldn't be coming out of the hose, but It went some where?
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Yeah, I also know to get dry air requires refrigeration on the output. I've dealt with a lot of technical dry air, it's expensive to produce and I have never seen an Aircraft shop have the gear. I watch air tools spit water all the time. Traps alone don't do the job because they don't create a pressure drop.
 
docmirror said:
Next, for the water vapor to 'freeze' it must first go through the liquid phase, under pressure, in an enclosed space where the % of water vapor in the first place is measured in a few moles.
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Interesting concept but false. Water vapor can go directly to the solid state. Review the water phase diagram.

The more interesting problems with high pressure air vs nitrogen relate to corrosion at higher partial pressures and flammability.

In my line of work we have gotten spontaneous combustion at pressures below 1,000 psi with air systems. Of course our little aircraft aren't near that pressure but when folks start talking about 300 psi normal pressure then the compressed pressure (hard landing) gets up to where we might not want to be.

The next bit of problem comes with oxidation as pressure increases. Materials which are fine at atmospheric pressure can be a problem at higher pressures. Add any moisture and the oxidation potential can accelerate.

All that said it's hard to get too excited about air in a PA-28 struts. Probably not a big deal. Bigger aircraft and higher pressures, maybe think about it a bit more.
 
Henning said:
Yeah, I also know to get dry air requires refrigeration on the output. I've dealt with a lot of technical dry air, it's expensive to produce and I have never seen an Aircraft shop have the gear. I watch air tools spit water all the time. Traps alone don't do the job because they don't create a pressure drop.
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My compressor ran most all day yesterday, not a drop of water came into the blast cabinet, and all it has in the system is the tank and 1 water trap. You'd notice it quick if you got water in the beads.

Water traps are not 100% but close enough for struts. using compressed air for struts isn't the big problem that most pilots make it.
 
Clark1961 said:
Interesting concept but false. Water vapor can go directly to the solid state. Review the water phase diagram.
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Yes, it can. But the conditions are - well, beyond extreme and not to be found in a compressed strut of a GA plane. Thanks for delving into the hyper-physics of phase change. Interesting for the scientists, and useful in some corner cases, but not applicable to a GA strut.

Furthermore, even if by some miracle of physics you could get it to perform a deposition, the amount of water vapor present would be very small.
 
docmirror said:
Yes, it can. But the conditions are - well, beyond extreme and not to be found in a compressed strut of a GA plane. Thanks for delving into the hyper-physics of phase change. Interesting for the scientists, and useful in some corner cases, but not applicable to a GA strut.

Furthermore, even if by some miracle of physics you could get it to perform a deposition, the amount of water vapor present would be very small.
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Last time I checked, snow does not require extreme conditions. There's no "hyper-physics" (whatever that is) involved.

So what's the pressure in an extended strut? Do you realize that struts are extended while in flight?
 
The frost you get on the airplane when it sits out overnight has gone through a liquid phase?
 
Clark1961 said:
Last time I checked, snow does not require extreme conditions. There's no "hyper-physics" (whatever that is) involved.

So what's the pressure in an extended strut? Do you realize that struts are extended while in flight?
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Between 50-200PSI. I have several planes with phuematic struts. It's great, you think it can snow in a compressed sealed strut! LOL. :lol:
 
It's not just the moisture that might still be in the compressed air, and in any case I have seen moisture shooting out of an air hose when the compressor is busy and demands are high. The tank gets hot and the moisture doesn't all condense out of the air. As it leaves the nozzle its pressure drops, therefore its temperature, and the moisture is seen as a fog or fine spray. Not something I'd want in any strut I owned. Water is often a problem while spraying paint, too, without a decent air dryer. It leaves tiny craters in the finish.

Air also contains oxygen, 21% by volume, and it's the oxygen that causes corrosion, also known as oxidation. Oxygen will react directly with iron to cause rust, and it can mix with the components of oil to form acids. In compressed air, there's lots of oxygen present.

We used to use nitrogen in the little airplanes I maintained. We use it in the big ones now where I work. Of course, the pressures are around 1000 PSI and those struts are big and a tiny strut pump would be an expensive way to inflate a strut, in terms of labor. We also have to use it in the tires of transport-category aircraft, to minimize the fire risk when a tire overheats and blows up.

Dan
 
Tom-D said:
Have you ever drained a compressor tank and seen how much water comes out?

You don't have water traps on your system?

How much water do you see being expelled from your air tools?

It shouldn't be coming out of the hose, but It went some where?
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Tom,
While you are correct that SOME of the water in the ambient air will condense out in the compressor tank, a significant amount of moisture still comes out of the tank with the air. It can be removed for the most part but that either requires desiccant in a filter can, something similar to a room dehumidifier, or both.

I do know that I had problems in the winter with ice crystal defeating and/or damaging the seals in the pneumatic actuators that hold my hangar door closed until I added a desiccant filter. OTOH, I don't have any reason to expect the residual water content from a regular shop compressor would be enough to make any difference in a strut.
 
There's not much air in the typical GA strut. To put it in perspective, when using the compounding hand pump shown earlier you don't even need a compressor - you could hook it up to a pressurized tire and it would work.

In airliners flying at forty thousand feet with huge struts and tires that stand chest high it's an issue but not in a Duchess.
 
Silvaire said:
In airliners flying at forty thousand feet with huge struts and tires that stand chest high it's an issue but not in a Duchess.
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You might be right about that. But ask yourself, do dissimilar metal couples know what model aircraft they are in? Does stress-corrosion cracking check to see if it is a Duchess or a 737 before propogating?

What is the cutoff? There are GA piston twins with service ceilings in the FL300 neighborhood.

My point is that unless you KNOW what the design engineering team considered then you should not GUESS whether or not it is OK to cut a corner.
 
dell30rb said:
Thats if the wheel is off the ground when you fill it.
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Methinks that's gonna be the case with the strut "fully extended" whether or not the wheel is touching the ground.:D
 
Morne said:
You might be right about that. But ask yourself, do dissimilar metal couples know what model aircraft they are in?
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All I can say is that regular shop air gets put in GA struts all the time, has been for decades - more than half a century at least and I've never once seen an NTSB report or even a Maintenance Alert citing the practice as a contributing factor to any on-going problem concerning dissimilar metals or ice crystals or anything else.

If using nitrogen makes you feel safer then, by all means, use it exclusively but don't try to make out like using air is going to cause a catastrophe. Furthermore, show me a Cessna, Piper, Mooney or Beechcraft maintenance manual that specifies nitrogen must be used in struts or tires.

For us it's a non-issue.
 
You'll never see any reports or alerts because it's not a safety issue, just a potential $$$ issue. Nobody, especially not those who profit from selling you the parts is going to disabuse you from damaging them, same as all the mechanics that tell you never run LOP, it means they'll have to send their kids to a cheap school because they won't be doing cylinders on your engine.
 
Henning said:
...same as all the mechanics that tell you never run LOP, it means they'll have to send their kids to a cheap school because they won't be doing cylinders on your engine.
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Are you're seriously suggesting that mechanics who advise in accordance with the manufacturer's recommendations are involved in a secret, deceptive business practice to get more money from you?

That's just utter, bat***** crazy nonsense.
 
Struts do not wear due to ice and water, the seals wear due to dirt getting into the strut because pilots do not care for them as they should.

I refuse to pump up a strut that is leaking hydraulic fluid. It's a waste of my time and your money.

And yes I do use N2 in struts.

The reason the large airliner type of aircraft use N2 in their tires is because they have fuse plugs in their rims/wheels, and when the brake over heats the wheel the fuse plug melts and the escaping N2 cools the brake and helps putting out the fire by starving it of O2.
 
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