Hydrostatic testing of aviation oxygen cylinders

What is hydrostatic testing?

Hydrostatic testing involves filling a cylinder with water and pressurizing it beyond its normal operating pressure to check for leaks, structural integrity, and permanent expansion. The test measures how much the cylinder expands under pressure and whether it returns to its original shape. If the cylinder shows excessive permanent expansion, it fails the test and must be removed from service.

This method is preferred because water is incompressible and safer than using gases for pressure testing. It allows technicians to detect weaknesses or damage that could lead to catastrophic failure during flight.

Regulatory framework

In the United States, the Department of Transportation regulates the testing of aviation oxygen cylinders under 49 CFR Part 180 Subpart C. The frequency and method of testing depend on the cylinder’s material and specification:

• DOT-3AA and DOT-3AL (steel and aluminum cylinders): Must be hydrostatically tested every 5 years.
• DOT-3HT (thin-walled steel cylinders): Require testing every 3 years and have a 24-year service life, at the end of which the cylinders must be removed from service.
• Composite cylinders (e.g., Kevlar-wrapped): Require testing every 5 years and have a 15-year service life, at the end of which the cylinders must be removed from service.

Pass the test

Practically speaking, if you have a cylinder that contains oxygen but is past its hydrostatic test date, you can continue to use that cylinder until the oxygen is consumed (the oxygen in the cylinder never goes “bad”). However, before that cylinder is refilled it must be retested and the new hydrostatic test date stamped onto it. Keep in mind that Kevlar and steel cylinders do have a life limit from the date of manufacture–not installation–and must be removed from service on or before that expiration date.

Aluminum oxygen cylinders can typically be hydrostatic tested locally by any facility that services fire extinguishers. Kevlar and steel cylinders typically require more specialized equipment, and align with the overhaul intervals of the regulators, so it's best to send these cylinders to an aviation oxygen system maintenance, repair, and overhaul facility for a hydrostatic test and regulator overhaul. After testing, the DOT-authorized tester will stamp out the old hydro test date and re-mark the cylinder with the new test date.

Oxygen cylinders operate under high pressure—often exceeding 1,800 pounds per square inch—and are subject to temperature fluctuations, vibration, and handling stress. Over time, these factors can degrade the cylinder’s structural integrity. Hydrostatic testing helps prevent failures that could result in oxygen leaks, fire hazards, or an explosion that leads to injury or loss of cabin pressure.

Best practices for operators

Aircraft operators and maintenance personnel should:

• Maintain accurate records of cylinder test dates and specifications.
• Use only certified facilities for testing and requalification.
• Inspect cylinders regularly for signs of damage, corrosion, or wear.
• Replace cylinders that fail testing or exceed their service life.