How does an aircraft reduce its speed for landing?

 Drag is what slows an airplane in the air. One thing that surprises many people is that when you reduce engine power, you don't actually slow down. Instead, when you pull the throttles back, the plane descends, but at basically the same speed. The real way to slow an airplane down is to increase angle of attack.

Essentially, pull the nose up, and the plane slows down. In an airplane, every control input always effects the others, so you must coordinate reducing power, increasing angle of attack, and adjusting trim.

The real job of wing flaps isn't to slow down the airplane, although they do slow you down in some cases by a whole lot. Instead, it's to keep the plane both flying and controllable once you do slow down. Every airplane's wing is designed with a certain goal in mind. In the case of a typical airliner, it's to fly really fast and high, with an eye towards fuel savings. Unfortunately, fast high-flying wings are really bad at flying low and slow. Since every successful flight ends low and slow, we need the flaps to essentially change the shape of the wing to one that will allow us to approach the runway at a safe speed. As a bonus, flaps allow us to point the nose of the airplane more downwards, giving a better view of the runway on approach.

Spoilers are almost cheating. What spoilers do is to stick up into the airflow, spoiling the lift generated by the wing, essentially making it quite inefficient. The spoilers will create some drag of their own, but really, they're making the plane descend quicker. Extending landing gear also provides more drag and helps slow the aircraft.

Once on the runway, airliners primarily use wheel brakes to decelerate after landing. Wing spoilers extend on the top of the wings which add aerodynamic drag, but they primarily kill the lift on the wings which places more weight on the wheels and improves braking. Thrust reversers are deployed on touchdown to both attenuate the thrust and to direct the thrust forward for deceleration purposes. Once the airspeed decelerates to about 60 knots, the reverse thrust is reduced to idle because that's when the possibility of the reverse exhaust flow could ingest foreign objects into the inlet of the engines.

Why has it been so hard for China to build aircraft jet engines?

 Empirical evidence proved it is more difficult to design and build aircraft engines than it is to build atomic bombs or send satellites to space. The jet engine is HARD.

It took many decades for US, British and French engine makers to perfect the technologies that they use today. China couldn't just acquire the plans and start making them. Creating turbine parts that can survive extreme heat has been a major engineering challenge. Meeting it has required fundamentally rethinking the material structure of the turbine blades, making metals do things that they do not normally do in nature. Turbine blades may be operating in temperatures far exceeding their melting point, and thus must be cooled to typically 85% of melting temperature to maintain integrity. Blades must be cast with intricate internal passages and surface hole patterns needed to channel and direct cooling air within and over their exterior surfaces.

The turbine blades in a modern jet are a single metal crystal. Even if you have a jet and can take it apart that tells you nothing about how to cast the turbine blades, and you can have a turbine blade that looks fine but self destructs after 200 hours. As an example, the load on a single turbine blade that's only 3 inches long can be as much as 35 TONS due to the centrifugal force on a turbine spinning at 20,000 rpm at temperatures over 1,100F. Only single crystal nickle-steel can sustain that kind of load for 2,000 hours without fail.

One of the biggest reasons why China is so lagging behind in jet engine is the Culture Revolution, which thousands of scientist/engineers were purged, thus created a talent gap which China took a generation to pick up. By the time China got its sanity back, it's already in the 1970s. Chinese engines have to compete with mature jets already in the market place, and US and UK engines didn't. This let them make profit while they were making junky engines. China doesn't have that luxury.

China has been making jet engines for decades, starting with WP-5 in 1956, which is a licensed copy of Soviet VK-1, which itself a knock-off from Rolls Royce Nene. China and UK signed a license deal in 1974 for the Rolls Royce Spey engine, but since UK committed to transfer technology, but not manufacturing know-how, by the time China overcame all the technical difficulties associated with manufacturing the jet engine, it was already mid-2003. China probably will surpass France by end of the 2020s, as China has multiple engine developments in the pipeline.

Which is the largest aircraft in the world, and is it possible to land that in all airports?

 Not even close.

The largest aircraft in the world is the Antonov AN-225 “Mriya”.

Photo from Reddit

This behemoth has a landing distance of 2,400 meters when empty. That is around 1.5 miles, or about 2/3 the length of Central Park in New York.

Now that that’s covered, the world’s shortest commercial runway is Juancho E Yrausquin Airport, Saba coming in at a whopping 1,312 feet long, or about 400 meters.

Photo from Charismatic Planet

The 225 would touch down and promptly fall into the ocean on the other side of the runway. It would probably be safer to ditch in the ocean beside the airport.

Without using the runway above, there are many, many regional and municipal airports where the 225 simply could not land. For instance, Skyline Airport in Idaho has a runway length of only 400 feet. It is so small, I couldn’t find a picture of it. Either way, the AN225 is 275’ long, over half the length of the airport’s runway. No matter which way you cut it, the 225 isn’t going to be landing there anytime soon.