Nitrogen in tyres
 

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 ?

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.

You might also want to consider the fact nitrogen doesn't contain water / moisture.....which is a key contributor as a source of corrosion.

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.

[QUOTE]No need to think further into the subject./QUOTE]
I'd say that depends on the examination board setting the Question...:O

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.

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.

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.

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

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.

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.


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

Helium
 

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.

Answer here

Federal Aviation Administration

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

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.

Good info from the RACQ


RACQ - Nitrogen In Tyres - Using Nitrogen For Car Tyres

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.

From Mr Boeing's Maintenance Manual. Chap 12-15-03

We use Helium when it gets warmer and need greater lift.

chu chu


Have you ever lit a fire using a wad of paper or kindling?