I remember your @tag from the old days - should have said "Welcome Back" instead.
OK, you seem to understand my babble, so let's go a bit more in-depth with fatigue-science. It's a good video (of course it's good - it has aeroplanes in it!) - but the maneouvres, although spectacular looking, weren't particularly load-intensive on the airframe. At a guess, I'd say of the order of 1-2G load only. You see that sort of loading during display flying. The prototype Boeing 707 was rolled 360 degrees during a filmed flight test. The test pilot knew what he was doing and intended to demonstrate the control responsiveness of the 707 flight systems. (Well, that's what he said in the interview....he was probably just having fun too.) The entire roll was conducted smoothly with 1G load all the way around, and minimum side load. Looked GREAT on film and probably caused Boeing management a few heart flutters!
Now, I refer to G-loading as a convenient point of reference. G, of course is an acceleration. What causes damage is the actual force applied to the airframe - Force (F) = mass (M) x acceleration (A). The more mass being carried, means more force being applied. So our Skyraider(1), 10,000kg carrying 5,000kg under-wing is experiencing more load than Skyraider(2) carrying 1,000kg (again made-up numbers for the example). G as an acceleration is normally 9.81m/s/s, but I'll use 10m/s/s to simplify the calculation.
Skyraider(1): F = (10,000 + 5,000)kg x 10 m/s/s = 150,000 Newtons (unit of force = kg.m/s/s)
Skyraider(2): F = (10,000 + 1,000)kg x 10 m/s/s = 110,000 N
- both during normal 1G flight.
Now in a 6G pull-out from a dive - so 6 x 10 m/s/s acceleration:
Sky(1): (15,000) x 10 x 6 = 900,000 N
Sky(2): (11,000) x 10 x 6 = 600,000 N
- Skyraider(1) is experiencing 50% more airframe load than Skyraider(2).
We tend to use G-load as an easier pilot work-load reference point - simply because most modern aircraft have a G-meter in the instrument panel. A pilot experienced in operating a certain type of aircraft should be familiar with the maneuvre G-load based on aircraft weight or load (eg. 2,000 kg fuel and load = 6G limit; 4,000 kg = 4G limit; 6,000 kg = 2.5G limit, etc).
More fatigue terminology:
- zero timing.
What this means is to bring the fatigue life of the aircraft back to "almost" as new off the production line stage. Several ways to do this:
- replace main components (as Greg indicated with the Planes of Fame display aircraft), if you replace the main spar of the aircraft, you put in a component with zero effective fatigue flight hours on it;
- inspect and reanalyse - if you have a $400,000,000 FA-18 or F-111, etc, you will probably have a comprehensive fatigue analysis report which gives you "expected" fatigue lives (in terms of flight hours or landing cycles). It should also highlight the critical items for inspection at which period of maintenance. It you do a regular appropriate-level inspection on these items and find no fatigue-induced cracks (or any other damage), you can effectively "zero-time" that component - i.e. declare it safe, and start the flight-time/landing cycle clock on that item. For our 65+ y.o. Skyraider - probably no fatigue analysis report - so regular inspection is the key.
Now to your question - yes, x-ray techniques are expensive and time consuming, so you may only schedule that in for the major overhaul (e.g. once every 5 years, rather than annually). You need to expect there to be microscopic cracks - just can't avoid it due to the manufacturing process with metals. The microscopic crack should become visible well before it becomes dangerous. Hence the regular visual inspection - every pre-flight, and more closely inspected 6-monthly or every 100 hours. Magnetic particle inspection, and/or crack-revealing paint (this stuff we use in offshore oil and gas, not sure how regularly the aviation indistry makes use of it) - dust or paint it on, then use ultra-violet light to reveal crack lines. Some areas you can't inspect easily, so if it's a reasonably available part, just replace it every 200 hours. (Again, all the time periods, etc are made up numbers for comparative example.) ....and you don't need an electron microscope that's housed in a lab and need 32 technicians to operate, there are simpler field use kits for application. The tricky bit is exposing the aeroplane part for inspection.
Again, all the above stuff is what some engineers spend a life-time studying (not me!), this is meant as a very rough explanation in (hopefully) easy to understand terms.