To be safe, an aircraft must be airworthy. The converse is not true.
As we are talking about legacy aircraft, we must consider the Military Aircraft Release and Release to Service documents. I’ve therefore quoted some older policy documents while assuming the current equivalents are available to anyone who is interested. The newer GARP process differs, primarily in that there is no MAR, but the fundamentals remain the same. That is, the Safety Case is based on a build standard, which must be maintained for the SC to be valid. The SC is the primary evidence supporting airworthiness. Lacking this audit trail, the MAR and RTS cannot be issued. (The Mull failings in a nutshell!).
The MAR was based on the build standard presented at MAR trials, which was not necessarily the “as flown” standard “
and is, inter alia, the statement that an acceptable Safety Case has been prepared for the aircraft”. (AP 3456, CA Instructions etc).
The RTS includes the MAR as Part 1, but reflects the “as flown” standard by, for example, including Service Deviations.
The RTS is the Master Airworthiness Reference, and it can be seen, by definition, it also requires a maintained build standard and a valid Safety Case. The Secretary of State mandates all this but, demonstrably, this has not been funded policy since 1991. Hence, the “systemic failures” noted in many reports, most recently by Mr Haddon-Cave.
Also, from AP3456 and CA Instructions.....
"Airworthiness is defined as the ability of an aircraft, or other airborne equipment or system, to operate without significant hazard to aircrew, ground crew, passengers (where relevant) or to the general public over which such airborne systems are flown. Airworthiness is not only concerned with engineering aspects, but also with the way an aircraft is flown and how its systems are operated".