Definition
A U.S. Department of Transportation specification for high-pressure seamless steel cylinders used to store compressed gases such as oxygen, nitrogen, and other gases found on aircraft. Cylinders built to this specification must meet defined material, manufacturing, testing, and periodic re-inspection standards, and are stamped with the DOT 3AA marking along with their service pressure rating.
Plain English
A government rulebook for building strong steel gas bottles. If a cylinder is stamped 'DOT 3AA,' it was made to that rulebook and is approved to safely hold compressed gas at a stated pressure.
Context Anchor
Seen stamped on aircraft oxygen cylinders and other high-pressure gas bottles during inspection, servicing, or maintenance records review.
Derivation
DOT stands for Department of Transportation, the U.S. agency that regulates the transport of hazardous materials. '3AA' is simply the catalog number of one specific cylinder specification within DOT's published rules. Knowing the letters come from the regulator helps explain why the marking matters legally, not just technically.
Why Pilots Care
Only cylinders meeting an approved specification like DOT 3AA may be filled and used for compressed gases on aircraft. A cylinder past its required hydrostatic test date, or one without proper markings, must not be returned to service.
Intuition Check
DOT 3AA is not an oxygen type or a fluid grade. It is a cylinder construction and approval marking.
Example Sentence 1
Before refilling the aircraft's oxygen bottle, the technician verified the DOT 3AA stamp and confirmed the hydrostatic test was still in date.
Example Sentence 2
Aircraft oxygen systems require DOT 3AA cylinders to ensure they can safely hold the necessary pressure.