The way the FAA grants airworthiness certificates to new aircraft is fundamentally different than the way NASA certifies new spacecraft. There are some good reasons for the difference.
There is an old saying that the airworthiness regs have been paid for in blood and a review of aviation history would bear that out. Those regs are generally simple, direct, and not open to much interpretation. Airworthiness regs are generally considered to be performance based. That is to say, a new aircraft must demonstrate that it can perform to the requirement. Certifying a new aircraft design takes lots and lots of flight hours. Many of the demonstrations are hazardous. For one dramatic example consider the requirement that a fully loaded jet airliner accelerating to takeoff speed must demonstrate stopping performance. If the takeoff is aborted at the last possible instant, the brakes can stop the plane before the end of the runway. This demonstration almost always leads to the wheels glowing cherry red from the heat of the brakes; it is not uncommon to set the tires on fire from the heat. Another example is the capability to take off with an engine shutdown: a fully loaded aircraft accelerating to takeoff speed is required to switch off an engine just at the most critical time and still be able to take off, climb out safely, and return to the airport for a safe landing. A multi-million dollar jet liner must be put at risk to demonstrate simple safety requirements such as these. And there are dozens of similar requirements that a new design aircraft has to demonstrate in actual practice. Not in a computer simulation, nor in a piece-part test, nor even with engineering calculation (although some of these are also done); demonstration in actual flight is mandatory. This can take months (sometimes years!) and frequently thousands of flight hours to achieve.
Launch vehicles and spacecraft are much more expensive than aircraft. And, with few exceptions, they are not reusable – the reason for that is another topic for another day. So launching dozens of test flights to demonstrate different safety requirements is not an economically viable option. Generally the number of test flights is few; one, two, maybe three, sometimes none. Meeting certification requirements is a matter of engineering analysis, computer simulation, piece-part testing, and standards on parts and design.
Thus the requirements and process for certifying space vehicles is fundamentally different from that of aircraft. Not that there are no aircraft standards for parts or design processes, but the airworthiness of a new jetliner is demonstrated, not just analyzed, before the first passenger steps on board.
There are some exceptions in the space vehicle world; the NASA Launch Services Program can certify new launch vehicles to be used to propel expensive and unique scientific satellites to orbit based on the past performance of launch vehicles. In the LSP requirements documents, a launch vehicle that has successfully launched several times (the magic number is 11) is subject to much less scrutiny than a brand new launch vehicle which is untried. So Deltas and Atlass and Pegususs (Pegusi?) are partly certified on the basis of past performance.
But that is not how NASA has ever certified a spacecraft for human conveyance – except Soyuz.
When the Space Shuttle was launched for the first time, it was “certified” to the standards of the day; thousands of hours of piece-part and subsystem tests; millions of hours of computer simulation, billions of hours of engineering calculation and analysis. So Young and Crippen made the boldest test flight ever on STS-1 and we found out . . . that some of that analysis and calculation was wrong. NASA and its contractors were good; but in fundamental ways, we were just lucky.
At the end of the cold war, with détente, the space policy makers in Washington decreed that NASA would cooperate with the Russian space organizations to build the new space station. First part of that would be a series of demonstration flights between NASA and Russian. This including flying Russians on the Space Shuttle and Americans on the Russian Soyuz spacecraft. Norm Thagard would be first.
This threw the agency into a tizzy. There was no way that the Russians were going to disclose all their technical specifications for all the sub tier parts; after all much of the technology was applicable to military uses. And there was no way that the Russians had complied with US standards – most of which were either promulgated by the US Military (the dreaded “Mil Spec”) or by American professional and commercial groups (e.g., ASME, ANSI, etc., where the A stands for American).
Yet, the Soyuz had been flying for a long time, a huge number of flights by spacecraft standards, and it had demonstrated a safety record at least as good as the Space Shuttle. And even more, the Russians were ready to fly on our shuttle based on the American assertion that it was “safe.”
So a new standard was promulgated – not officially published, but actually used. A standard that basically said, if a spacecraft is demonstrated to be adequately safe, then it is certifiable. Sort of like the simple, direct, and performance based standards of the FAA airworthiness regs.
Now there was a lot of nervousness, and many questions asked (not all answered), paper was signed, and Norm flew. It was not easy, but it happened.
You can read all about it here:
So as new human certification ratings are proposed, they rely heavily on new standards and specifications, requirements for analysis, engineering calculation, computer simulation, piece-part testing and just a little bit on flight demonstration. Of course, the Shuttle and the Soyuz don’t comply with those standards; they were built in different times with more primitive standards. But they demonstrate a level of reliability or safety that is apparently acceptable.
If someone were to build their own spacecraft and/or launch vehicle; fly it successfully many times, demonstrate its capabilities in actual flight; then I suspect the new human rating requirements would be tossed aside in favor of demonstrated actual flight performance.
And that is as it should be.
But we are a long way to space travel being at the maturity, economy, and routine of air travel.