Aviation From My Rocking Chair: Aircraft Oxygen Systems (Breath In, Breath Out)

Howdy again, everyone. Being basically a low and slow type guy, you may have noticed a lack of articles from me about systems related to fast, and/or high altitude aircraft. So, from time to time I will try to cover subjects that relate more to these high performance birds. This month, let’s discuss Oxygen systems and safety.

As you may remember from High School science class, oxygen constitutes about 21% of the gases in our atmosphere. This percentage remains constant with altitude. Unfortunately, as altitude increases, air density decreases. In order to survive, we humans need supplemental oxygen. The first thing required to provide this life support is a container to put the gases in. This is provided by a pressure cylinder of which there are 3 types, each with different Hydrostatic Test and Service Life requirements.

The types are:

(1) DOT 3AA 1800 (3AA)- Standard industrial steel cylinder, usually installed in older aircraft. Hydrostatic tests must be done every 5 years, and service life is indefinite as long as service life and inspection criteria are met. Weight is the prime disadvantage of these cylinders.

(2) 3HT- High-Tensile, thin wall, steel cylinders. These are lighter than 3AA. Hydrostatic test every 3 years, and service life of 24 years after which the bottle must be destroyed.

(3) Composite- Typically aluminum lined cylinders with a Kevlar™ over-wrap. Hydro test each 3 yrs and must be destroyed after 15 yr. Service life.

All cylinder types have identification marks stamped around the neck. The first letters Must be DOT, which indicates Dept. of Transportation approval. Without this, you may not be able to have the cylinder filled commercially. The 4 numbers after DOT indicate rated bottle pressure (typically 2015 or 2216 psi). After the rated pressure will be 2 numbers followed by an inverted capital A and then 2 more numbers. These indicate the date of manufacture. The first numbers are the month and the last 2 are the year. Hydrostatic test dates will also be stamped on the cylinder, and it is illegal to charge a bottle which is past its due date. Note: Hydrostatic dates for new cylinders are based on date of manufacture, not on date of purchase or installation.

There are four types of commercially available oxygen, Aviators Breathing Oxygen (ABO), Medical, Welding, and Research. Be certain to only use ABO. While Medical and ABO are both pure oxygen, only ABO has a control on moisture content and is tested for it. ABO is very dry with a dew point of about -85deg(f). Moisture must ge kept out of aircraft O2 systems because of possible damage to valves and regulators. Depending where they are mounted, O2 cylinders may be exposed to sub-zero temperatures in flight. Then as system pressure is dropped from 1800psi to 70psi across a pressure reducer, the expanding gas will further reduce the temperature to -80 or -100deg(f). Any moisture in the system is instantly ice, and this ice can badly damage regulator and valves as well as plug lines. In order to keep moisture out of aircraft systems, always be certain to cap lines and ports during maintenance, and leave components in their sealed containers until use. Also be certain to leave some pressure in the O2 cylinder to exclude moisture entry and rust.

As y’all know by now, I can’t finish this article without going over some cautions and safety tips to keep in mind when working with oxygen. In itself, oxygen is NOT flammable. However, it is such a powerful oxi-dizer that virtually anything can act as a fuel for a fire, including metal shavings, wood chips, sawdust, or dirt. That said, the most dangerous mix of all is Oxygen and Hydrocarbons of Any kind. This is known as a Hypergolic mix and violent fire can erupt without any other ignition source. (continued on next page)

To prevent injuries, all tools, rags, work areas, and your hands must be cleaned completely before beginning maintenance on an oxygen system. Use only cleaners specifically intended for oxygen components. When a lubricant is required for ‘O’ rings or fittings, the only one approved for use by Scott is Dupont Krytox™ , which must still be used sparingly.

As a pilot, you will probably not be servicing your own aircraft O2 system, but, don’t allow anyone to do the job without the following:

(1) Using a clean service cart, free from contaminants--- especially oil or grease. (2) Making sure all service hoses, fittings, and protective covers are being used to guard against dirt and dust.

(3) Being sure the service person purges delivery lines before hooking up to the aircraft. (4) Monitoring the fill process to be sure the O2 bottle is filled S-L-O-W-L-Y. Excess heat buildup due to fast compression can elevate the gas temperature hundreds of degrees nearly instantly, possibly causing extreme damage to the aircraft and people around it. Besides, if you are lucky enough to avoid disaster, it will be under-filled when it cools.

In closing, I have been around long enough to have made nearly every bonehead mistake that can be made around aircraft. So far, the Lord has seen fit to frighten me just enough to stay alive. I cannot recommend this technique!!

Please work safe Rick Vaux TC4130