Every piece of aircraft equipment, irrespective of its size, must receive approval from authorities, affirming its compliance with specific minimum performance standards. At the zenith of this certification hierarchy lies the Type Certificate (TC), conferred upon aircraft manufacturers once authorities confirm adherence to requisite performance criteria.

A step below the TC lies the Supplementary Type Certificate (STC), typically granted for significant modifications or additions to specific aircraft types. These certificates are often customized for individual aircraft types and lack versatility across different models.

Enter the Technical Standard Order (TSO), serving as a vital bridge for manufacturers producing items applicable across various aircraft. The TSO system establishes a unified performance standard regardless of the equipment's ultimate application. With numerous TSOs covering a broad spectrum of products, ranging from simple to highly intricate, this system holds particular importance within ULD operations.

In the realm of ULDs, two TSOs currently hold relevance: TSO C90d and TSO C172, with TSO C203 awaiting final issuance by the FAA. Detailed contents of any TSO are readily accessible on the FAA website.

For many TSOs, especially those pertaining to ULDs, the FAA often relies on external standards, primarily those established by SAE International. These standards, developed through collaboration among industry experts, form the backbone of TSO requirements, with additional specifications mandated by regulatory authorities, including marking guidelines for TSO-approved items.

It's essential to note that while an item may possess TSO approval, its loading or installation onto an aircraft remains subject to the aircraft's operational instructions. In the context of ULDs, weight and balance manuals typically reference TSO C90 when outlining applicable ULD requirements.

The international scope of the TSO system is noteworthy. Apart from FAA-issued TSOs, EASA in Europe releases European Technical Standard Orders (ETSOs), while China issues its own China Technical Standard Orders (CTSOs). Fortunately, FAA and EASA TSOs for ULDs closely align and enjoy global acceptance, with bilateral agreements facilitating validation of TSOs across different jurisdictions.

TSOs undergo periodic updates to reflect evolving industry standards. For instance, the latest iteration, TSO C90d, supersedes the original TSO C90 issued over three decades ago. Notably, prior TSO-approved designs remain valid, with new TSO requirements applying solely to newly submitted designs.

The requirements for marking TSO-approved items are rigorously defined by regulatory authorities to ensure clarity and compliance. These mandates encompass details such as manufacturer information, article identification, TSO number, weight specifications, directional indicators, and expiration dates.