I noticed that the tires on the 737s are stored outside-in other words they are not pulled all the way up in to the fuselage, like most other passenger jets. I have often wondered how how come the tires do not burst once the plane reaches higher altitudes where the pressure is much lower. I am guessing the tires are strong enough to handle the difference in air pressure, right? I figured that in other planes the compartments that hold the landing gear was pressurized, but maybe not since the 737s tires are exposed.Richard

Tyres on all aircraft do not sit behind a pressure bulkhead (The undercarriage bays are not pressurised, only sealed against weather). They are all subject to the drop in outside pressure. They are as you say strong enough to withstand the pressure differential.Graham

Richard,Aircraft tires are serviced with Dry Nitrogen instead of Air like your car tires due to the fact that Nitrogen doesn't expand they way Air does when heated and cooled. Servicing a tire with normal Air would most certainly cause a blow out of the tire by either causing to much pressure when heated by the brakes, or having such low pressure due the extreme cold temperatures at altitude. Normally, after landing, tire pressure may increase maybe 10-15 psi on large transport aricraft tires, however with normal air in the tire, the pressure would easily double and cause a failure. Hope that helpsRegardsPaul Gollnick :-cool Technical Operations/Customer Operational SupportPrecision Manuals Development Groupwww.precisionmanuals.comhttp://www.precisionmanuals.com/images/forum/devteam.jpg

What pressure are most of the Air Transport tires maintained at. Someone once told me around 115 PSI. Does that sound about right?

That answers my question-I figured there had to be something different about them or they would burst...thanks again!Richard

Bob,Varies from aircraft to Aircraft. Range is anywhere from 100-200 psi. Our 757's are set at 165 for the mains and 160 for the nose tires just for example. RegardsPaul Gollnick :-cool Technical Operations/Customer Operational SupportPrecision Manuals Development Groupwww.precisionmanuals.comhttp://www.precisionmanuals.com/images/forum/devteam.jpg

I'm gonna stick my neck out and disagree with this. Considering nitrogen is already close to 80% of the air we breathe, the removal of oxygen (and other components) doesn't significantly change the properties of the gas.I'd hypothesize that using pure nitrogen is more from a fire standpoint than anything else. If the tires were to heat excessively, and an oxidizer were present internally, the rubber could more easily catch fire, possibly creating more of an explosion or fire hazard, rather than just a raw pressure increase.From a purely technical standpoint, a standard mix of air has about 5% greater heat capacity than a pure nitrogen environment. This means that air would be able to absorb more heat energy before increasing in temperature. Less temp increase would lead to less pressure rise (assuming the gas is in ideal conditions, which it nearly is).Also, if you figure, max tire pressure of a DC-10-10 (found on Airliners.net) is 195 psig with Nitrogen. Figure that the tire's filled to 180 psig. That's about a 12.25 atmosphere pressure differential already present in the tire just sitting on the ground filled properly. On climbout to altitude, going to say 250 mb (around 35,000 feet), you only lose 3/4 of an atmosphere of pressure externally to the tire, basically making the deltaP 13 atm, not even above the max rated ground pressure of the tire. The tire basically doesn't hardly feel the altitude increase.The heat of landing definitely shoots the temp up of the tire, and associated pressure, but it's obviously designed for that horrendous punishment (with an appropriate amount of wear, of course).I just don't see a reason other than possible fire that pure nitrogen would be used over air (not trying to say N2 is bad to use at all).~Nate

>>>I'm gonna stick my neck out and disagree with this.:)

I'm also a chemical engineer (well... 1 semester away from having said degree anyway). Now, I didn't run off to check in my thermo book on this, and maybe I just proved that I need to brush up on my gas dynamics, but, if we're just talkin about gases getting hot with pressure spikes, I'd hope my educated guess is sufficient :-D~Nate

Hi Nate,No head chopping here. The great misbelief is the pressure theory when in fact it is the flamability issue. The likely scenario for tire combustion when filled with air is at high altitude when the oxygen in the air can easily act as an oxidizer using the rubber of the tire as fuel. This is what I was taught and that the probability of tire explosion on landing is less than at altitude when filled with air.Cheers, JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

>I'm also a chemical engineer (well... 1 semester away from>having said degree anyway). Now, I didn't run off to check in>my thermo book on this, and maybe I just proved that I need to>brush up on my gas dynamics, but, if we're just talkin about>gases getting hot with pressure spikes, I'd hope my educated>guess is sufficient :-D>>~NateI believe Nate is correct from a temperature/expansion point of view, but one of the reasons tires are filled with nitrogen is that it is both noncorrosive and does not support combustion, so that when you do put on max braking, have a brake fire, and have the tires melt, you do get a dispersal of gas that actually helps stop the fire, rather than fuel it.

Which is what I was trying to say with my last sentence in my first post :-D Poor grammar, I know... I could use another writing class... hmm, neh :)~Nate

Tires are not really going to expand at higher altitudes! The difference in pressure is too small. If a tire contains 200psi of pressure and the sea level pressure is 15psi, and at high altitude 5, the difference in pressure is only 10psi. Airplane tires are designed for that.After all, when standing on the ground the pressure difference is already 185psi.PS: as far as i know, there are no airplanes with pressurized gear bays. That would also make opening and closing of gear doors too complex.Cheers,Ian Udingahttp://www.precisionmanuals.com/images/forum/800driver.jpg

Darn.......no wonder all these people are putting themselves into crates these days, instead of the old fashioned "hiding in the gear bay". Its unfair.....the gear bay should be pressurised, just incase someone decides to stow away there.If I die in the gear bay on my next trip.....I'm sueing.:) :)

Hi,The use of nitrogen on aircraft is as you say from a combustion point of views. Landing gear oleos are filled with nitrogen to prevent a dieseling effect. This is the act of applying pressure to a gas mixed with oil, resulting in an explosion, this is the way diesel engines work ( no spark plugs in there ). Therefore we don't want an explosion on landing. We use dry nitrogen to prevent the icing up of delicate components such as needle valves at high altitudes that are within the punematic/hydraulic systems. Some electronic devices on the aircraft such as radio boxes are charged with a minimal charge of 5psi. This prevents arcing and sparking within the equipement again at higher altitudes.The pressure differential between cruise and sea altitude will have a negligable effect of an aircrafts tires. I personally work as technical instructor in Saudi Arabia training young Engineers to maintain aircraft. Tornado mainwheels sit at about 260PSI this depends largely on the load carried.Hope this helps adds to the thread nicelyGraham