Cylinder Overpack Letter of Interpretation 24-0042

July 7, 2025

Posted in

Aviation

Problem

On June 4^th, 2024 Tym’s LLC formally requested clarification for a Letter of Interpretation (LoI) issued by the US Department of Transportation, Pipeline and Hazardous Materials Safety Administration (PHMSA) regarding the use of Overpack for compressed gas cylinders.

On January 3, 2025 the agency issued a revised Letter of Interpretation, based on our concerns.

However, there were still questions, and while informal communications persisted between Tym’s and PHMSA, we asked for another Letter of Interpretation to benefit the aviation and compressed gas industry, which has struggled with the topic for decades.

Letter of Interpretation 24-0042 Issued

The latest LoI, dated June 26, 2025, clearly outlines the fact that specification cylinders must be marked and labeled when offered for transport and must also conform to the rules of an Overpack as outlined in 49CFR 173.25(a)(4). That is, all specification cylinders not listed in 49 CFR 173.301(a)(9) must have marking and labeling on the cylinder. Additionally, having marking and labeling only on the outside box is not acceptable.

The Letter also states that the cylinder specifications in 49 CFR 173.301(a)(9) (e.g. 3HT), may have the marking and labeling on the outer box and need not have it on the cylinder inside. These types of cylinders do not qualify for shipment as an Overpack.

Letter of Interpretation 24-0042 can be downloaded directly from the DOT PHMSA website here.

Tym’s Solutions

Regulatory topics like these are confusing, complicated, and may seem trivial but have major consequences during audits and enforcement actions. It is important to formally document interpretations that pertain to your operation to shield you from unfair enforcement and penalties.

Tym’s is uniquely qualified to answer your complex regulatory questions and, more importantly, ensure the answers are backed up with documentation from the regulatory agency when needed.