Hi Sean,
You've asked a good question there, and got some great answers. If I may add a bit... As usual, there's a lot of 'it depends', and I'm not clear what type of museum and whose aircraft we are talking about, but that's never stopped me before.
Sean Curtiss wrote:
One doubt I have when restoring an aircraft is how far you must go to be faithful to the factories original blueprints.
As has been touched on, the original blueprints are an indication of how it should have been, rather than necessarily being how it actually was. Generally a museum won't be aiming for restoring an aircraft to 'factory drawings' but to as
original as possible, where factory drawings are a great guide, but not the objective themselves. Paul's comment on the T33 are a great illustration of this.
Sean Curtiss wrote:
In the end what matters is what the public sees of the outside of an aircraft or does the inside also counts?
Depends on the museum's objective. For the highest standard, and assuming we aren't talking an active or airworthy aircraft, the 'plane is regarded as an artefact, which can be used as a research tool to show how it really was in use. Therefore original parts and paint should be kept as far as possible, any damaged or degraded parts stabilised rather than replaced (as far as possible) any parts replaced with as original parts as are obtainable, but these should be marked as replacement, not original to that airframe, and any parts replaced with dummies (such as colour copies) likewise marked as such. This is 'conservation' rather than 'restoration'. The superficial view (e.g. a look at the cockpit) should ideally look as 'original' as possible, but documentation of discrete tags should show the status of those parts - which is why documentation is important.
Sean Curtiss wrote:
Some flying aircraft have modified cockpits, i.e., some have new avionics otherwise they wouldn’t be safe or have FAA approval to fly.
When dealing with active aircraft, obviously there have to be compromised for certification and safety, and items like new wiring and cables, to modern rather than period standards are obvious. However replication of the original 'look' is moving ahead all the time, with the recent wiring in Ron Fagan's (?) P-40 restoration for instance. An airworthy aircraft isn't able to achieve the highest standard of originality for that airframe, but it fulfils another important role of demonstration. Which is why we need, and have both fliers and top quality static restorations. However, there's no such thing as 'just like the original' or that contradiction in terms 'better than original'. It's either original, or not. Any replication will fail some tests of originality, and could mislead future students of how that original aircraft was
really made.
Sean Curtiss wrote:
For a static plane, replacing some types of gauges for others affects the fidelity based on the original blueprint? I may think that for the lack of options, some restorers put similar gauges in place. For example, if I wanted to assemble an instrument panel and didn’t find a particular gauge, could I substitute it with a similar one with the same function, but with different markings? Would that still validate my project? I know that this may sound a bit too vague, but…
If it's your project, it's validity is up to you. (The only issue I can think of is if you misrepresent something for sale, knowing it's not actually original.) You set and achieve the standard that matters to you. If it's a group museum, then it's a good idea to have an agreed standards and objectives document or statement, so everyone agrees and aims for the same thing - for instance, lookalike semi-scale aircraft are great, provided they aren't mis-represented as original.
For national museums, or state owned aircraft, national museum standards should be set and achieved and documented.
A great book on this area is Robert Mikesh's 'Restoring Museum Aircraft'. Recommended.
Just some thoughts, don't know if it helps much!