Mythbusters...
- Thread starter sawood12
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robwright
New member
I love Mythbusters and admit to having seen every episode - MORE THAN ONCE!!!! Surprising given the amount of time I actually get access to the tele. It's my dream job along with Top Gear presenter. As for the nitrogen business aircraft applications yes for obvious reasons. As for everyday road use I have to agree it's for the birds that one.
poprock
New member
Up here in the Scotland South club region we had a talk from some Michelin guys last year and they were pretty clear on filling tyres with nitrogen being a waste of time and money. They admitted that there are advantages, but explained that those advantages are so miniscule as to be irrelevant.
sawood12
New member
ORIGINAL: tscaptain
You only had to ask...........[] A wheel fire, a possibility especially after a high speed rejected take off, does not need oxygen in air filled tyres to help it along! Aircraft tyres are inflated to pressures up to 200psi [&:]
You're right, but the risk is not for those circumstances - it is due to the fuel tanks being ruptured and the close proximity of the wheels, i.e. nestled with the wings fuel tanks therefore surrounded on all sides bar one by fuel. If there was a fuel leak at high altitude you want to reduce the risk of it catching fire at altitude, or if there was a crash and maybe belly landing which may rupture the fuel tank.
Well, at a pound a corner, and having seen the more stable temperatures and less frequent top-ups needed for myself, I'll keep using it thanks.
What on earth is the Lexus article going on about? Who goes around sucking tyre valves? [&:]
What on earth is the Lexus article going on about? Who goes around sucking tyre valves? [&:]
A fuel leak will be into the air so a bit of air in the tyres won't make a lot of difference. A crash and you've got bigger problems on your hands and similarly a belly landing.[&:]
If the fuel tanks are ruptured it will only be a problem if the brakes are hot or, a la Concorde, it feeds the engines. I always thought N2 is used in a/c tyres for two main reasons 1) It's inert in the case of fire and 2) Next to no water content - which at -56C in the cruise is an issue! There is another issue which is heatsoak from the brakes. Aircraft brakes can generate some impressive amounts of heat which will transfer through to the tyres over time. Commercial aircraft have brake cooling charts to show how much time is needed between landing and taking off again depending on how hard the brakes were used. Significant amounts of water vapour in the tyre would affect the pressure and at close to 200psi you don't need any more![
]See this
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgNPRM.nsf/0/8c17569ad3ded4128625694a005bb65d!OpenDocument
sawood12
New member
The scenario I was thinking about is if the fire started after a belly landing then burst the tyres and as you've said at 200psi there is quire alot of air in the 16 tyres of the main undercarriage of a B747. The wheel brakes are not likely to be an ignition source as they are only hot once they've slowed the aircraft after landing. Statistically the majority of people who die in aircraft accidents actually survive the impact of the crash and die in the fire that usually follows as they are trapped in their seats with broken arms and shins because the seat pitch is so small (i'm only 5'9" tall and cannot get into the crash position!).
Also if a fire was to start in the the undercarriage bay broke out at 35000 ft then it would be suppressed due to the lack of oxygen up at that altitude. But if the tyres were to following that and compressed air was used then you've just injected a lot of oxygen to a smouldering fire in a contained area at low air pressure thereby creating alot of fuel vapour. Basically you've a recipe for an explosion that would split the aircraft in two (just like the B747 centre fuel tank explosion coming back in the mid '90's that was caused by arcing electrics in the fuel tank). If the tyres are filled with N2 then you will probably extinguish the fire out rather than feed it, so I think this is the context of using N2 in in aircraft tyres.
The issue about moisture is a red herring. Nitrogen is generally dryer because it has been dried through the process of separating the nitrogen from air. There is no reason why air couldn't be dried thereby taking the moisture issue away - and it is cheaper to dry compressed air than to use dry N2 so that is not a significant reason not to use air in aircraft tyres. When diving you breath compressed air that has been dried which is why after a 30minute dive you're mouth is as dry as the proverbial sandal!
Also if a fire was to start in the the undercarriage bay broke out at 35000 ft then it would be suppressed due to the lack of oxygen up at that altitude. But if the tyres were to following that and compressed air was used then you've just injected a lot of oxygen to a smouldering fire in a contained area at low air pressure thereby creating alot of fuel vapour. Basically you've a recipe for an explosion that would split the aircraft in two (just like the B747 centre fuel tank explosion coming back in the mid '90's that was caused by arcing electrics in the fuel tank). If the tyres are filled with N2 then you will probably extinguish the fire out rather than feed it, so I think this is the context of using N2 in in aircraft tyres.
The issue about moisture is a red herring. Nitrogen is generally dryer because it has been dried through the process of separating the nitrogen from air. There is no reason why air couldn't be dried thereby taking the moisture issue away - and it is cheaper to dry compressed air than to use dry N2 so that is not a significant reason not to use air in aircraft tyres. When diving you breath compressed air that has been dried which is why after a 30minute dive you're mouth is as dry as the proverbial sandal!
That's all fair comment Scott but the main reason for N2 in aircraft tyres is still to minimise the risk of the tyre going bang and causing subsequent damage to the surrounding pipework (especially if the gear is retracted). As the FAA doc states
" This notice proposes to require that an inert gas, such as nitrogen, be used in lieu of air, for inflation of tires on certain transport category airplanes. This action is prompted by at least three cases in which the oxygen in air-filled tires combined with volatile gases given off by a severely overheated tire and exploded upon reaching autoignition temperature"
There are the other benefits which you point out but the chances ofkeeping the wheel/ tyre assembly intact are greatly enhanced by filling them with N2
" This notice proposes to require that an inert gas, such as nitrogen, be used in lieu of air, for inflation of tires on certain transport category airplanes. This action is prompted by at least three cases in which the oxygen in air-filled tires combined with volatile gases given off by a severely overheated tire and exploded upon reaching autoignition temperature"
There are the other benefits which you point out but the chances ofkeeping the wheel/ tyre assembly intact are greatly enhanced by filling them with N2
Neil Haughey
New member
Yes I can remember getting my Hankooks filled with Nitrogen. Well after bleeding the tyres 3 times during a track day and then having to pump em back up again it did indeed prove to be a waste of time.
Yep, but much hotter than a car tyre will ever get!
"Aircraft tires also include heat fuses, designed to melt at a certain temperature. Tires often overheat if maximum braking is applied during a rejected takeoff or an emergency landing. The fuses provide a safer failure mode that prevents tire explosions by deflating in a controlled manner, thus minimizing damage to aircraft and objects in the surrounding environment. The main purpose of requiring that an inert gas, such as nitrogen, be used instead of air, for inflation of tires on certain transport category airplanes is prompted by at least three cases in which the oxygen in air-filled tires combined with volatile gases given off by a severely overheated tire and exploded upon reaching autoignition temperature. The use of an inert gas for tire inflation will eliminate the possibility of a tire explosion."
The fuse plugs will melt around 180C.
"Aircraft tires also include heat fuses, designed to melt at a certain temperature. Tires often overheat if maximum braking is applied during a rejected takeoff or an emergency landing. The fuses provide a safer failure mode that prevents tire explosions by deflating in a controlled manner, thus minimizing damage to aircraft and objects in the surrounding environment. The main purpose of requiring that an inert gas, such as nitrogen, be used instead of air, for inflation of tires on certain transport category airplanes is prompted by at least three cases in which the oxygen in air-filled tires combined with volatile gases given off by a severely overheated tire and exploded upon reaching autoignition temperature. The use of an inert gas for tire inflation will eliminate the possibility of a tire explosion."
The fuse plugs will melt around 180C.
sawood12
New member
Yes, I guess I agree don't I? - i.e. that inert gas is used primarily due to fire risk in the event of the gases escaping from the tyre or in causing the tyre to burst in the first place due to Oxygen being a pretty lethal gas when it comes to fire. I.e. it is not because of the usual reasons cited by tyre fitting companies in that N2 is more thermally stable gas than air or any of the other reasons usually cited for the benefit of N2 being used to inflate tyres i.e. more thermally stable (it isn't) and contains less moisture, or that the tyre holds pressure better (I've never had a problem with pressure loss unless I have a puncture).
In anycase I don't think the whole volatile gas thing is an issue on road cars. An exploding tyre on an aircraft is only usually ever caused by a heavy landing or FOD on the runway. They only occasion where so much heat gets into the tyre where it could cause this issue is in the case of a thrust reverser problem that prevents the use of thrust reversers (very very rare event indeed). In this case the wheel brakes are used on their own and if the aircraft is landing heavy the brake temps can get so high that the carbon disks themselves catch fire which in turn can cause the tyre to catch fire, In this case pressure and temperature fuses built into the tyre valve will allow the controlled deflation of the tyre thus preventing any risk of the tyre bursting or exploding as a result of heat of a tyre or brake fire. Clearly it is the case that this venting gas must not be a risk to any fire that may be happening around the tyre i.e. inert gas is used primarily due to the risk of fire. Clearly not an issue on a road car.
Under normal landing conditions the brake temps never get anywhere near the temps needed to cause a problem - these things are vented and usually reverse thrust takes the strain. Aircraft Carbon brakes are so expensive that they often maximise the use of reverse thrust to reduce brake wear.
In anycase I don't think the whole volatile gas thing is an issue on road cars. An exploding tyre on an aircraft is only usually ever caused by a heavy landing or FOD on the runway. They only occasion where so much heat gets into the tyre where it could cause this issue is in the case of a thrust reverser problem that prevents the use of thrust reversers (very very rare event indeed). In this case the wheel brakes are used on their own and if the aircraft is landing heavy the brake temps can get so high that the carbon disks themselves catch fire which in turn can cause the tyre to catch fire, In this case pressure and temperature fuses built into the tyre valve will allow the controlled deflation of the tyre thus preventing any risk of the tyre bursting or exploding as a result of heat of a tyre or brake fire. Clearly it is the case that this venting gas must not be a risk to any fire that may be happening around the tyre i.e. inert gas is used primarily due to the risk of fire. Clearly not an issue on a road car.
Under normal landing conditions the brake temps never get anywhere near the temps needed to cause a problem - these things are vented and usually reverse thrust takes the strain. Aircraft Carbon brakes are so expensive that they often maximise the use of reverse thrust to reduce brake wear.
Don't work at RR by any chance Scott?
You will also know that a major problem with a/c brakes is heatsoak - i.e the time it takes for the brake heat to find it way into the wheels/tyres. A long taxy to the runway with lots of braking can build up a lot of heat - then you take off and tuck the wheels up nice and snugly out of the airflow and the heat finds it's way into the tyres and.......!
32 years of shorthaul flying taught me how to look after brakes temps![]
You will also know that a major problem with a/c brakes is heatsoak - i.e the time it takes for the brake heat to find it way into the wheels/tyres. A long taxy to the runway with lots of braking can build up a lot of heat - then you take off and tuck the wheels up nice and snugly out of the airflow and the heat finds it's way into the tyres and.......!
32 years of shorthaul flying taught me how to look after brakes temps![
]
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