Aircraft Oxygen Systems
Some safety precautions
By Joe Escobar
Oxygen — the life-sustaining gas we need in order to survive. It makes up around 21 percent of the air in the atmosphere. Although this amount is sufficient at or near sea level, at higher altitude we require supplemental oxygen due to the decreased density of the air. At higher altitudes in non-pressurized aircraft, pilots require it to avoid hypoxia, a physiological condition where the brain gets sluggish due to the decreased oxygen supply. Pressurized aircraft rely on oxygen systems as a supplement when pressurization problems occur. Although it is a lifesaving gas, oxygen can be a hazard to life when improper handling or servicing practices are used. This article will cover some of the issues to be aware of when working with oxygen systems.
During maintenance, all open ports and fittings should be capped to prevent moisture and contamination from being introduced.
The two most common types of cylinders used in aircraft gaseous oxygen systems are steel cylinders and composite cylinders. Steel cylinders can be divided into two categories: 3AA and 3HT cylinders.
DOT 3AA 1800 (3AA) cylinders are standard industrial size steel cylinders found mostly in older airplanes. They are being replaced with 3HT cylinders.
3HT cylinders are high-tensile steel cylinders. They have a thinner wall than 3AA cylinders, thereby affording a weight savings.
Even lighter than either of the steel cylinder types are composite cylinders. They are typically aluminum-lined cylinders with a Kevlar™ overwrap. Many airframe manufacturers are now either installing composite cylinders in their aircraft or offering them as an option due to the weight savings.
For the purpose of identification, letters and numbers are found around the neck of cylinders. The beginning letters are ’DOT’, which indicates Department of Transportation approval. This MUST be stamped on the cylinder or it may not be able to be commercially filled.
After DOT, there will be four numbers that identify the rated pressure of the cylinder — 2015 and 2216 are common. Following the rated pressure identification will be two numbers, followed by a letter that looks like an inverted capital ’A,’ then two more numbers. This is the cylinder’s date of manufacture. The first two numbers are the month and the last two are the year of manufacture. Although many oxygen servicing vales have filters incorporated in them, they should not be solely relied upon to control contaminants from entering the system.
Oxygen cylinders are scheduled for hydrostatic testing on a regular basis. This testing is required by the DOT to ensure the continued safety of the cylinder. During a hydrostatic test, the cylinder is visually inspected for stress cracks and corrosion and is pressurized to 1.67 (5/3) times its rated pressure to ensure its continued ability for safe operation. The time frame for required hydrostatic inspection varies from cylinder to cylinder as follows:
3AA steel cylinders are required to be tested every five years and have an indefinite service life provided they meet service life requirements and inspection criteria.
3HT steel cylinders must be tested every three years and have a service life of 24 years, after which they must be destroyed.
Composite cylinders must be tested every three years and have a 15-year service life, after which they must be destroyed.
It is illegal to charge a cylinder if it is past its hydrostatic due date. Hydrostatic due dates are based on the most recent test date stamped on the cylinder. An important item to note is that the hydrostatic due date for new cylinders is based on the date of manufacture, not on the date of purchase or installation. Even though buying a new cylinder, it may have already lost a significant amount of time from the manufacturing date just sitting on a shelf.
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