I am pondering if anyone in here has actually topped off a plane tire with nitrogen, or r/r a plane tire, or seen the inside of a car/plane wheel..
Yes, I surely have. Extensively. I don't put anything but nitrogen in aircraft tires. When home, I put compressed air in my car tires, either from my own compressor, or one found at a gas station. I don't keep nitrogen at home.
Aircraft are a different matter. Aside from a general recommendation and sometimes requirement for nitrogen in aircraft tires, it's also good practice.
AC 41-131B, the FAA's publication on standard maintenance practices (and a sort of bible for any aviation maintenance technician and mechanic), states in chapter 9, section 1:
NOTE: The use of nitrogen to inflate tires is recommended. Do not use oxygen to inflate tires. Deflate tires prior to removing them from the aircraft or when built-up tire assemblies are being shipped.
It's also a certification standard ("
dry nitrogen or other gases shown to be inert") and requirement for large aircraft, as found in 14 CFR (code of federal regulations--the "FAR's) 25.733(e):
(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.
Advisory Circular AC 20-97B, Aircraft Tire Maintenance and Operational Practices, establishes the following, regarding nitrogen in aircraft tires:
(3) Initial Inflation for Tubeless Tires. Inflate the tire assembly in a safety cage using dry nitrogen to ensure, that the tire does not contain more than 5 percent oxygen by volume (Those aircraft required to comply with AD 87-08-09 shall inflate the tire wheel assembly in accordance with the procedure specified in that AD). The nitrogen source regulator should be pre-set to a value that is no more than the maximum load capacity of the tire. The tire growth will produce a drop in inflation pressure after initial inflation. Pressure stabilization normally occurs within 12 hours. After an initial 12-hour minimum stabilization period at the rated inflation pressure, the tire should maintain the inflation pressure within 5 percent of the initial pressure for a period of 24 hours. The leak source should be discovered and corrected if pressure losses exceed 5 percent. Operators should use this procedure when applicable or comply with the aircraft manufacture’s maintenance manuals, or CMM, as applicable to the aircraft and the assembly. Although AD 87-08-09 applies to a specific aircraft, the procedures set forth in the AD are acceptable procedures that should be used for similar tire assemblies.
(3) Service Pressure. Service pressure is the inflation pressure needed to support the maximum operating load for a wheel position. Service pressure is measured with the assembly under load. When pressure testing a loaded assembly, inflate and maintain mounted tires with nitrogen. The gauge pressure should indicate a range between 100 percent and 105 percent of the specified service pressure, provided that the rated pressure of the tire and the wheel’s TSO qualification pressure is not exceeded.
Advisory Circular AC 25-22, Certification of Transport Airplane Mechanical Systems, under Section 25.733, provides the reason that the regulation 25.733 requires nitrogen or other inert gas:
b. Intent of Rule. This rule specifies type certification requirements for both design and performance of tires used on transport category airplanes. The tire must be of proper fit and have approved speed and load ratings for a particular airplane application. The maximum static ground reaction for the condition specified must not exceed the maximum static load rating of each tire. Retractable gear system tires must have adequate clearance from surrounding structure and systems. The tire inflation medium is to be an inert gas to avoid explosions. Tires installed on landing gear axles with multi-wheels (main wheel tires only), must have a 7% load margin included in their rating. Tire performance standards are contained in Technical Standard Order (TSO) TSO-C62. A TSO approval is not approval to install tires on the airplane. The airframe manufacturer/user must conduct the applicable airplane certification tests and receive FAA approval for installation.
The same advisory circular goes on to explain:
(6) Amendment 25-78 (March 29, 1993) added paragraph § 25.733(e) to require that for airplanes with a maximum certified takeoff weight of more than 75,000 pounds, the tires mounted on braked wheels be inflated with dry nitrogen or other inert gases so that the tire does not contain oxygen in excess of 5 percent by volume to prevent tire explosions. The 75,000 pounds weight limit was based on a review of the service difficulty reports indicating that tire explosions, as opposed to tire bursts, tend to occur on the larger, heavier airplanes. The 5 percent by volume limit for oxygen content was based on a series of laboratory tests indicating that an abrupt auto-ignition could occur for oxygen concentrations of 10 percent or more.
(a) There had been several cases where tire explosions had occurred in transport category operations. A tire explosion differs from a tire burst, which can occur when an overheated or over inflated tire fails and releases the high pressure air contained therein. Protection against tire burst is required under § 25.729(f). A tire explosion is the result of a chemical reaction occurring when gases released from overheated tire material mix with oxygen in the inflation air and ignite. In 1987, the FAA issued an airworthiness directive (AD 87-08-09) requiring use of nitrogen for tire inflation to ensure that the tires on braked wheels of airplanes do not contain more than 5 percent oxygen. Amendment 25-78 was intended to accomplish the same purpose for new airplanes.
Finally, the 800 lb. gorilla: Airworthiness Directive AD 87-08-09, called out above in the advisory circular. It applies specifically to certain Airbus, Boeing, McDonnell Doublas, and Lockheed products, but is also applied or referenced by other manufacturers and other applications, for the language contained therein, and specifically the intent. That is, the AD can't be applied to aircraft not called-out in the AD, as an AD, but it's an oft-referenced directive, and the basis of further certification and operating ammendments that affected regulations, and aircraft beyond the scope of the AD. I'm citing the AD itself here, as it was specifically referenced in the portion of the advisory circular quoted above. Suffice it to say that the application is substantially more far-reaching than the direct application of the AD itself:
87-08-09 AIRBUS INDUSTRIE, BOEING, BRITISH AEROSPACE, LOCKHEED, AND MCDONNELL DOUGLAS: Amendment 39-5613. Applies to Airbus Industries Models A300 and A310; Boeing Models 707, 720, 727, 737, 747, 757, and 767; British Aerospace Models BAe 146 and BAC 1-11; Lockheed Model L-1011; and McDonnell Douglas Models DC-8, DC-9 (includes MD-80 series), and DC-10; certificated in any category.
To eliminate the possibility of a chemical reaction between atmospheric oxygen and volatile gases from the tire inner liner producing a tire explosion, accomplish the following, unless already accomplished:
A. Within 180 days after the effective date of this AD, to ensure that all aircraft tires mounted on braked wheels do not contain more than 5 percent oxygen by volume, accomplish paragraph 1. or 2., below. Either of these procedures is acceptable, or they may be used together:
1. Install a placard, either in each wheel well or on or near each landing gear strut incorporating braked wheels, and in a location so as to be easily seen and readable by a person performing routine tire servicing. This placard shall state "INFLATE TIRES WITH NITROGEN ONLY." The words "SERVICE" or "FILL" may be substituted for the word "INFLATE".
2. Incorporate into the FAA-approved maintenance program procedures that include the following items:
a. On braked wheels, install only tires that have been inflated with dry nitrogen or other gases shown to be inert such that the gas mixture does not exceed 5 percent oxygen by volume.
b. Tires on braked wheels may be serviced with air at remote locations where dry nitrogen is not available, provided that:
i. the oxygen content does not exceed 5 percent by volume; or
ii. within the next 15 hours time-in-service, the tire must be purged of air and inflated with dry nitrogen so that the oxygen does not exceed 5 percent by volume.
B. An alternate means of compliance or adjustment of the compliance time, which provides an acceptable level of safety, may be used when approved by the Manager, Seattle Aircraft Certification Office, FAA, Northwest Mountain Region (Airbus Industrie, Boeing, and British Aerospace models); or the Manager, Los Angeles Aircraft Certification Office, FAA, Northwest Mountain Region (Lockheed and McDonnell Douglas models).
C. Special flight permits may be issued in accordance with FAR 21.197 and 21.199 to operate airplanes to a base for the accomplishment of the modification required by this AD.
This Amendment becomes effective June 1, 1987.
In addition to these requirements, many manufacturers will issue service instructions and documentation calling out nitrogen as a specific requirement for tire inflation and maintenance.
As an aside, for those who think underinflation isn't a significant factor, the AC 20-97B states:
NOTE:
Accurately maintaining the correct inflation pressure is the single- most effective task in the preventive maintenance regimen for safe tire operations!
So...to get back to the original point of the thread, which asked "
Can anyone shed any light on the reason for using Nitrogen instead of compressed air for inflation?," the answer is yes. Aircraft tires use nitrogen for inerting purposes, because it's a more stable gas, because it doesn't support oxidation of aircraft wheel assemblies or contain as much moisture (use of dry nitrogen is required, remember), and doesn't support tire combustion at elevated temperatures during gassing of tire structure materials under heat and pressure.
My Honda dealer disagrees. They have the word all the way from the engineers in Japan.
Do you mean to say that the Honda dealer (and their engineers in Japan) recommend against the use of nitrogen?
I've taken tire courses from several tire manufacturers, who all recommended the use of nitrogen where it's available. Generally the use of nitrogen in car tires isn't common (except high performance tires and certain industrial applications), largely because of availability and cost.
The use of nitrogen is especially a good idea for those using alloy wheel assemblies, but also steel assemblies, to help prevent corrosion in the presence of moisture.