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RetiredBA/BY
13th Jan 2016, 15:36
Anyone care to comment on the use of nitrogen in aircraft tyres.
I was always led to believe that as an inert gas it was that n2 would not feed a fire in the event of a brake fire or the fusible plugs melting and releasing gas into the fire.
Comment ?

Plane Speaker
13th Jan 2016, 15:48
Correct. A tyre failure introduced a significant volume of compressed gas in a very short space of time. Thus an inert gas is preferable to air with c.21% O2 content. F1 teams use N2 to inflate their tyres. I got some from Costco at Watford when they replaced the tyres on my car.

Krystal n chips
13th Jan 2016, 16:04
You might also want to consider the fact nitrogen doesn't contain water / moisture.....which is a key contributor as a source of corrosion.

grounded27
13th Jan 2016, 16:21
I have never heard of any calculation for nitrogen to be used to combat a burning tire, it is most obviously useless for that purpose and secondly wheel well fires are rare and the first thing you are going to do if you detect one is drop the gear. An inert gas is needed as it is non reactive with petrol products. The tire needs a method of thermal protection, if a plug melted and compressed air started to discharge it would fuel the fire. No need to think further into the subject.

hunterboy
13th Jan 2016, 17:11
[QUOTE]No need to think further into the subject./QUOTE]
I'd say that depends on the examination board setting the Question...:O

grounded27
13th Jan 2016, 17:35
This is not theory, it is all required procedure. Never heard of anyone making it up on to the red carpet over the issue, not rocket science. From my aspect the biggest fear is a greenhorn attempting to service a tire with oxygen, rarely completes the task without first starting a fire.

tdracer
13th Jan 2016, 18:02
Grounded27, the concern is mainly a wheel/brake fire during something like an RTO - as the fuse plugs melt you don't want to dump 200 psi oxygen containing air into the fire (think blowtorch). Dumping 200 psi N2 won't extinguish the fire, but it won't feed it either.
So the prime reason for use of nitrogen is to avoid feeding a wheel/brake fire. Of secondary importance is that it's dry (although 'dry' compressed air is also readily available) so you don't have the adverse affects of water in the tire (both corrosion, and dramatic changes it tire pressure due to phase change of the water from liquid to vapor or back).
That lack of moisture is the primary reason for use of N2 for automotive tires. Also, lack of O2 in the tire reduces oxidation damage to the rubber (obviously only for the inside of the tire) which can be of concern on vehicles that don't get used much and the tires can stay on-rim for 10 years or more.

grounded27
13th Jan 2016, 18:22
tdracer, forgive me I may not have spelled it out as clearly as you had but I exerted the same primary reason you had for the use of an inert gas in aircraft tires. At high pressures air can become volatile especially with the addition of heat bringing flammable gasses into the mix from the rubber of a tire that reacts with the compressed air. In the past I had heard of allowable use of compressed air for a leg or two if nitrogen was not available to get it home to a station where nitrogen was available. The long term effects of moisture and oxygen on rubber are common knowledge yet are not the reason for the use of nitrogen in aircraft tyres, only an additional benefit.

TURIN
13th Jan 2016, 20:18
biggest fear is a greenhorn attempting to service a tire with oxygen, rarely completes the task without first starting a fire.

Left hand threads on the O2 hose unions help. I've still seen it done though.

In the past I had heard of allowable use of compressed air for a leg or two if nitrogen was not available to get it home to a station where nitrogen was available.

I think it is still allowable. AMM ref required though.

Someone once told me that N2 molecules are bigger than the gaps between the rubber molecules therefore they hold the pressure better. Not sure if that is just an urban myth.

tdracer
13th Jan 2016, 20:31
N2 expands less that air with increased altitude????? That's definitely a myth - both 'air' (~79% N2/21% O2) and pure nitrogen conform nicely to the ideal gas law - which basically says all gases react the same to changes in temperature and pressure.


Someone once told me that N2 molecules are bigger than the gaps between the rubber molecules therefore they hold the pressure better. Not sure if that is just an urban myth. My gut feeling is that it's a myth (the molecular weight of N2 is lower than O2, so everything else being equal the N2 molecule would be slightly smaller than O2) but there could be some subtle way the molecule reacts with the rubber that makes it effectively larger. At any rate the difference would be in the mud

Winnerhofer
13th Jan 2016, 20:31
Helium for Tyres - Forum - F1technical.net (http://www.f1technical.net/forum/viewtopic.php?f=6&t=7717)

Chu Chu
13th Jan 2016, 22:59
I have my doubts about compressed air feeding an external fire. It isn't going to create any kind of lasting overpressure, so it won't cause a significantly higher concentration of oxygen molecules as a general matter.

A jet of compressed air could certainly mix air into an oxygen-deprived area of hot gas and cause a flashover. But a jet of nitrogen would entrain air and cause general mixing, and might be almost as effective at causing flashover.

Compressed air could scatter burning fuel, especially if it's liquid. So could nitrogen.

I'm not sure about fire risk *inside* the tire. Compressed air isn't anywhere near the combustion threat of oxygen -- for example there's no strict prohibition on using oil on compressed air equipment there way there is with oxygen. I think that's because there are more oxygen molecules than with air at atmospheric pressure, but also a correspondingly higher number of nitrogen molecules to absorb heat and generally get in the way. On the other hand, if there's any internal fire risk at all from compressed air, it's infinitely higher than that from nitrogen.

megan
14th Jan 2016, 00:04
Answer here

Federal Aviation Administration (http://nitrogentiremachine.com/federal_aviation_administration.htm)

galaxy flyer
14th Jan 2016, 02:28
grounded27,

The C-5 carried 1200# of LN2 for fire suppression purposes--in the dry bays, pylons, fuel tank ullage. So, yes, it can be fire supressant.

GF

_Phoenix
14th Jan 2016, 02:49
Millions of cars have already nitrogen in tires. The only real advantage is the absence of O2, the rim doesn t rust or oxidate it holds pressure longer period of time, then it's safer (less risk of underinflation, blowout) better fuel consumtion, tire usure and life span. Same results though, if inject engine oil time to time.
Yeah, it's a multi million bussiness selling air or 78% of it, whatever.

ACMS
14th Jan 2016, 03:59
Good info from the RACQ


RACQ - Nitrogen In Tyres - Using Nitrogen For Car Tyres (http://www.racq.com.au/cars-and-driving/cars/owning-and-maintaining-a-car/car-maintenance/nitrogen-for-tyres)

riff_raff
14th Jan 2016, 06:42
Aircraft tires are inflated with nitrogen for a couple reasons.

First, dry nitrogen gas is used instead of compressed air because compressed air could contain some moisture. At touchdown, the tires can generate enough heat to cause any moisture within the air volume to turn to steam, which would create excessive pressures in the tire.

Second, jet aircraft tires require very high inflation pressures. Much higher than commercial air compressors operate at. So cylinders of industrial dry nitrogen gas compressed to 1500psig or more are used to inflate the tires.

Jet II
14th Jan 2016, 23:12
From Mr Boeing's Maintenance Manual. Chap 12-15-03


WARNING:

A TIRE MUST INITIALLY BE INFLATED WITH NITROGEN. IF TIRE WERE INFLATED WITH AIR, VOLATILE GASES GENERATED BY AN OVERHEATED TIRE COULD COMBINE EXPLOSIVELY WITH OXYGEN IN THE AIR INSIDE THE TIRE. A TIRE EXPLOSION CAN CAUSE DAMAGE TO EQUIPMENT AND/OR INJURIES TO PERSONS.

underfire
15th Jan 2016, 07:40
We use Helium when it gets warmer and need greater lift.

grounded27
16th Jan 2016, 14:57
chu chu

I have my doubts about compressed air feeding an external fire. It isn't going to create any kind of lasting overpressure, so it won't cause a significantly higher concentration of oxygen molecules as a general matter.
Have you ever lit a fire using a wad of paper or kindling?

Chu Chu
16th Jan 2016, 18:27
Sure. A gentle, sustained flow of air works wonders. A sudden blast will either cause a brief flare-up or blow the flame out altogether.

Smilin_Ed
16th Jan 2016, 21:35
Back in the dark ages 40 years ago when I was flying off carriers, tire pressures were usually 400 psi for carrier operations and 300 psi for field operations. Those high pressures exerted a lot of stress on the tire especially during carrier landings. Navy pilots were taught that inflating tires with an inert gas precluded oxygen under pressure reacting with the rubber and weakening it.

bvcu
17th Jan 2016, 19:08
Think the accident that started the change was a DC8 in Saudi i think which had a wheel well fire and the tyres exploded leading to loss of the aircraft , sure someone else will have the relevant details

riff_raff
18th Jan 2016, 01:02
FAR part 25.733(e) (https://www.gpo.gov/fdsys/pkg/CFR-2002-title14-vol1/pdf/CFR-2002-title14-vol1-sec25-733.pdf):
"(e) For an airplane with a maximum certificated takeoff weight of more than 75,000 pounds, tires mounted on braked wheels must be inflated with dry nitrogen or other gases shown to be inert so that the gas mixture in the tire does not contain oxygen in excess of 5 percent by volume, unless it can be shown that the tire liner material will not produce a volatile gas when heated or that means are provided to prevent tire temperatures from reaching unsafe levels."

From Michelin Aircraft Tire Service Manual (http://www.airmichelin.com/CSM_PDF/careandservice.pdf):
"NOTE: Michelin recommends that all tires, regardless of position or aircraft rating, be inflated with dry nitrogen of at least 97% purity. Some OEM (airframers) recommend 99.5% purity. When adding 97% pure nitrogen to an “empty” tire, the nitrogen concentration will end up about 95% when the pressure reaches about 125 psi. When the operating pressure is less than 125 psi and the desired concentration is ≥ 95% nitrogen by volume, use a double inflation process if the source nitrogen is 97% purity."

Consider the Space Shuttle tires (http://www.airmichelin.com/generalcontent149.php):
"Like most aircraft tires, the Space Shuttle tires are filled with nitrogen because of its stability at different altitudes and temperatures. Due to the extremely heavy loads, these bias ply tires are inflated to 340 psi (main gear) and 300 psi (nose gear). The main landing gear shuttle tires are only used one time, and the nose landing gear tires are used for two landings."

Here what they looked like after one landing:

http://media-cdn.tripadvisor.com/media/photo-s/05/2d/4d/41/california-science-center.jpg

Ozlander1
24th Jan 2016, 23:20
Left hand threads on the O2 hose unions help. I've still seen it done though.

Left hand threads are used on inflammable gases, i.e. propane, acetylene.

TURIN
28th Jan 2016, 09:19
Left hand threads are used on inflammable gases, i.e. propane, acetylene.

I didn't know that. Good job we don't have those hanging around on the ramp.
I don't think an oxygen/flammable gas combo would go down very well in the crew O2 system.

hoss183
28th Jan 2016, 11:00
The reason N2 is used in aircraft (and F1 and general cars) is the following:

1. Pure nitrogen is dry and does not absorb moisture. This helps with corrosion, but the main reason was that with air filled tyres and moisture collect at the bottom, froze in flight and created an imbalance when landing.

2. N2 molecules are larger than than O2, and so pressure loss is slower.

3. Its inert and wont contribute to a fire.

megan
29th Jan 2016, 01:39
the main reason was that with air filled tyres and moisture collect at the bottom, froze in flight and created an imbalance when landingNot the case at all hoss. The real reasons are in this document.

Federal Aviation Administration (http://nitrogentiremachine.com/federal_aviation_administration.htm)

timboeing707
29th Jan 2016, 03:44
I seriously recommend Nitrogen in Car tires, better fuel economy and less road noise

Here in NZ its cheap, and to get them checked and topped up when necessary is FREE!!!

Best Regards

peekay4
29th Jan 2016, 04:29
Nitrogen on passenger cars, for the most part, is just a scam:

Why nitrogen in your tires is a waste of money - WHEELS.ca (http://www.wheels.ca/news/why-nitrogen-in-your-tires-is-a-waste-of-money/)
Perrin Post: Don't fall for nitrogen - Travel - Travel Tips | NBC News (http://www.nbcnews.com/id/19353980/ns/travel-travel_tips/t/dont-fall-nitrogen)
Should You Fill Your Car's Tires With Nitrogen? (http://www.edmunds.com/car-care/should-you-fill-your-cars-tires-with-nitrogen.html)
Should You Put Nitrogen In Your Car's Tires? » AutoGuide.com News (http://www.autoguide.com/auto-news/2013/12/should-you-put-nitrogen-in-your-car-tires.html)
Filling tyres with nitrogen | AA (http://www.theaa.com/motoring_advice/safety/filling-tyres-with-nitrogen.html)
Car Talk Scam Detector | Nitrogen Tire Inflation (http://www.cartalk.com/content/scam-detector-7)

Krystal n chips
29th Jan 2016, 04:56
1. Pure nitrogen is dry and does not absorb moisture. This helps with corrosion, but the main reason was that with air filled tyres and moisture collect at the bottom, froze in flight and created an imbalance when landing.

You started so well, but......whilst we are never too old to learn, and despite my lifetime involvement as an engineer, I have never heard, or read, anything about wheels becoming inbalanced due to, erm, frozen water in the wheel hub.... 9/10 for creative thinking....1/10 for factual content. Do you, by any chance, have any documented evidence concerning aircraft wheel and tyre assemblies to support this please ?....because, having perused some excellent material from Mr Goodrich / Dunlop et al, I cannot find, or recall, any reference to this phenomenon.

2. N2 molecules are larger than than O2, and so pressure loss is slower.

Oh ?

3. Its inert and wont contribute to a fire

At last ! success !.....:D

Tarq57
29th Jan 2016, 06:32
I was told by an automotive tyre guy that pressure loss with nitrogen is slightly less.

The molecule is larger than the oxygen molecule.

Rubber does a pretty good job at containing air/gas. At a molecular level, some gas very slowly leaks through the material. It might take several weeks for a pressure loss to become evident - maybe months - but it will gradually deflate.

The higher the inflation pressure, the greater/faster the loss.

Makes sense. However, at the end of the discussion, I opted for regular air. I don't mind recharging the tyres every few months when/if required.

barit1
30th Jan 2016, 17:41
ASN Aircraft accident Boeing 727-264 XA-MEM Las Mesas (http://aviation-safety.net/database/record.php?id=19860331-1)

Chris Scott
1st Feb 2016, 10:20
Evidently there are a lot of young aviators contributing to this thread. And I wonder if the operative that serviced that B727's tyre with air in 1986 had ever read the following accident report. It was still fresh in aviation circles when I started flying:

https://en.wikipedia.org/wiki/Swissair_Flight_306

And it has figured here (on AH&N) in 2010:

http://www.pprune.org/aviation-history-nostalgia/416029-swissair-caravelle-disaster-zurich-1963-a.html#post5710327

boofhead
4th Feb 2016, 18:01
In Alaska we use nitrogen for all the reasons given here plus it is not as affected by the cold. If the tyre is inflated with compressed air and then flown to the Slope for example, the tyre pressure drops as the temperature drops and we have had tyres roll off the rims when taxiing. With nitrogen the pressure is more stable and since the change we have had no problems with tyres in cold conditions.

tdracer
4th Feb 2016, 18:12
In Alaska we use nitrogen for all the reasons given here plus it is not as affected by the cold. If the tyre is inflated with compressed air and then flown to the Slope for example, the tyre pressure drops as the temperature drops and we have had tyres roll off the rims when taxiing. With nitrogen the pressure is more stable and since the change we have had no problems with tyres in cold conditions.

Nitrogen and dry air will respond exactly the same to changes in temperature - it's called the ideal gas law. The potential problem with compressed air is if it isn't dry and there is significant water vapor present. The change in state of vapor to liquid (or visa-verse) is the potential issue.

Bottled compressed air is normally dry (running it through some sort of dryer is part of the process) - as is bottled nitrogen. It's the air out of the shop compressor that may be a problem.

evansb
4th Feb 2016, 18:22
Indeed. I recall refilling a tyre at an OAT of -32C with shop compressed air. The tyre valve promptly froze with ice in the open position, releasing a large quantity of air, resulting in an even more under-inflated tyre.. Nitrogen has benefits for cold weather ops.

joema
4th Feb 2016, 22:38
"My gut feeling is that it's a myth (the molecular weight of N2 is lower than O2, so everything else being equal the N2 molecule would be slightly smaller than O2) but there could be some subtle way the molecule reacts with the rubber that makes it effectively larger. At any rate the difference would be in the mud"

As you'd expect, diffusion rate of O2 and N2 through various membranes has been studied for over a century, and is well understood.

Diffusion rate is defined by Fick's Law, and is dependent on the frictional coefficient of the molecule, not just its physical size. The frictional coefficient is based on (a) Stokes Radius (not physical diameter) of the molecule, and (b) The viscosity coefficient of the gas at the given pressure and temperature. I'm guessing that N2 has a greater frictional coefficient than O2, due to either greater viscosity and/or larger Stokes Radius than O2, and that probably explains the lower diffusion rate.

The equations to calculate these are complex, but it's not a myth. If any physics major is reading this, they could probably give more details.

http://en.wikipedia.org/wiki/Fick%27s_law_of_diffusion
http://en.wikipedia.org/wiki/Stokes_radius

Chu Chu
5th Feb 2016, 00:59
Last time there was a thread about this I found a U.S. Department of Transportation (I think) study on nitrogen on tires. My recollection is that it found that the nitrogen concentration in air-inflated tires increased slightly over time. But I don't think the study concluded whether this was due to oxygen diffusing from the tires more rapidly, or due to it's being "consumed" in oxidizing the tire walls.

In any event, the change in concentration was quite slight, meaning that if the oxygen was diffusing faster, it wasn't by much. And when you consider that air is only 20% oxygen to start with, a slight decrease in the diffusion rate for that 20% isn't going to make much difference in tire pressure.

joema
5th Feb 2016, 12:50
"if the oxygen was diffusing faster, it wasn't by much. And when you consider that air is only 20% oxygen to start with, a slight decrease in the diffusion rate for that 20% isn't going to make much difference in tire pressure."

According to a 2009 NHTSA study, the inflation pressure loss for tires inflated with nitrogen was approximately two-thirds of the loss rate of tires inflated with air. They tested both static and in-use automotive tires.

"The Effects of Varying the Levels of Nitrogen in the Inflation Gas of Tires on Laboratory Test Performance" (4.6MB .pdf): http://tinyurl.com/jvcgfxo

riff_raff
6th Feb 2016, 03:08
Filling your tires with nitrogen is totally passe. Everyone knows all the cool kids now use argon. Imagine the embarrassment of being seen flying in an aircraft that had tires filled with a low molar mass inert gas like nitrogen? Aircraft tire inflation gas diffusion losses are totally lame.

I've even heard recent rumors of a couple more progressive members of the underground aircraft tire culture experimenting with using xenon and krypton gas for inflation. What makes it especially cool is that they produce their own gas supply by distilling the earth's air, so it's totally carbon neutral and socially responsible! :cool:

abgd
6th Feb 2016, 05:18
Barit1 has it - look up his post.

The issue is that if you have air at 10-20 atmospheres then the partial pressure of oxygen will be equivalent of 2-4 atmospheres. Things tend to burn very well in high pressure oxygen, and will start to ignite at a lower temperature. Plainly a tyre filled with oxygen is the equivalent of a small bomb. The question then being, what causes it to go off?

I'll confess I'm not sure of the answer to that, but I guess you could get a static spark / spontaneous combustion of hot volatile compounds..?