I’m actually going to address this, being a pilot. I’ll keep to the US, because it’s the country I know best.
There are approximately 700,000 licensed pilots in the US right now. So… 20% is 140,000. That’s still a substantial number of pilots. Assuming similar numbers for mechanics, that’s a lot of those, too.
It’s safe to assume there will be substantially less leisure travel for awhile, so there will be less demand for the airlines. But that means they will have to maintain fewer airplanes. There will be fewer air traffic controllers, but there will also be substantially less traffic requiring their services. We won’t need to expand O’Hare airport after all, saving billions of dollars and much effort and material.
The big problem will be supplying fuel. There are going to be fights over fuel, and aircraft drink a LOT of fuel. That, I believe, will be the key - can you keep the darn things fueled? And yes, the military and government will take first dibs at the fuel.
If things get bad enough, we may lose ALL the big jet airlines.
That doesn’t mean we lose aviation. The two big problems with aviation historically has been understanding aerodynamics sufficiently to achieve a stable aircraft, and having a sufficiently powerful engine to power it. Again, we’re back to literacy. A group of POW’s in WWII derieved sufficient knowledge to build a working glider from reading just a single book in their prison library - see the Colditz Glider THAT’s the power of literacy. As long as the books exist aviation can be reconstituted even if we run out of fuel for internal combustion engines for a generation or two.
But it doesn’t have to be that way - even if we lose the BIG airlines that doesn’t mean we lose all flying. Technology may have to regress for awhile, but a less efficient engine can still generate enough power to enable powered flight. If we have to convert to burning methanol or corn oil so be it - that doesn’t eliminate the possibility of flight, or long distance trucking. It will be less efficient for awhile, that’s all. The techonology to run trucks on various fuels, including cooking oil, already exists and has been used by various militaries world-wide. Engines used for ground vehicles can be and have been adapted for use in airplanes. Rotax snowmobile engines and VW auto engines being two favorites among homebuilders.
Safety standards, however, may slide for awhile - quality control may suffer. Physical standards for pilots might be lowered as well, at least for awhile, so people who today would not be eligible for a commercial license might wind up flying passengers or cargo. Quality control on replacement parts might also suffer. The situation in North America might be more like that in, say, Africa, where standards are lower in many areas. Yet they still utilize aircraft in Africa. In Africa, aviation might, in fact, be LESS affected because they operate in a very different manner than in the US and “bush repairs” are a more common way of doing business. They’re also much closer to the big oil fields of the Middle East than we are, and might find it easier to obtain fuel.
Even so, we would be an enormous advantage over, say, the pilots of 1930. We have a much better understanding of the weather, for starters, and what the limits of airplanes are. Problems that routinely killed people before 1930 are now part of the basic syllabus of every student pilot. Arguably, a new-minted private pilot of 2004 is far more competant than some of the guys flying airmail for a living in 1930. Beyond that - North America has been mapped and measured. Those maps aren’t going to vanish overnight, nor will the major topography change. We’ve figured out how to fly on instruments in bad weather. We have several designs for dealing with ice on airplanes, and not all of them require super-high technology. Again, we won’t be starting over from square one, even with a worst-case scenario.
In summ - the industry would take an enormous hit - at the same time every other industry is. But there will be enough mechanics, pilots, and other support staff to continue the endeavor.
Now space travel might be put on hold for awhile - lose 80% of the folks involved we may simply not be able to put folks up into orbit or maintain all our current satellites. I don’t think that industry has a large enough base to sustain a hit like that. Losing satellites will have an enormous impact on both navigation and weather prediction. However, at least in aviation, older methods of navigation than GPS are still required learning, so for now it would be a matter of us dusting off the old E6B “whiz wheels” and course plotters. Another generation, though, and it might be a different story.