HTAWS are currently mandated under SPA.HOFO.160(c), which requires HTAWS to ‘meet the requirements for class A equipment as specified in an acceptable standard’.
The only defined standards for H-TAWS are TSO-C194 and ETSO-C194, which both refer to Radio Technical Commission for Aeronautics (RTCA) document DO-309. Any H-TAWS meeting these standards, or coming with improved features, is acceptable.
UK CAP 1519 specifications are considered to introduce improvements to the existing standards. They are designed to reduce false warnings. They also introduce Helicopter Flight Envelope Warnings (H-FEWs) in addition to basic HTAWS functions. A CAP 1519 compliant HTAWS is therefore acceptable.
The HTAWS rulemaking task (RMT.708) may change the requirements for HTAWS in the future.