Perhaps we have answered our own question from the title.

Well, there several issues but assuming we are in something like a normal jet airliner, then one of the main problems is the air, or lack of it as we get closer to space.

A plane flies because as it is propelled forward, the wings, which are shaped to make the air flow faster over the top of them than the bottom, generate lift. As the plane goes faster the wings create more lift and when the lift is greater than the weight of the plane, it will climb up into the air.

For our plane to continue to climb it needs more speed to increase the lift. If you throttle back on the speed a bit, the plane will settle in to level flight and if you decrease the speed the plane will start to fall as lift from the wings is not enough to overcome the weight of the plane.

As our plane climbs higher and higher in to the atmosphere, the air becomes less and less dense, so the plane has to fly faster to create more lift until eventually it reaches an altitude where the engines cease to function correctly because of the lack of oxygen or the air is too thin to create enough lift.

Now, This is a greatly simplified way of looking at this because as you approach the speed of sound or Mach 1, which also changes with altitude and if your plane has quite straight wings, the airflow over the wing can become unstable and it loses li ft. This unstable airflow can also shake the control surfaces, that’s the flaps on the wings that go up and down, so violently it could break them and you then lose control of the plane. That’s why supersonic or hypersonic planes have highly swept back and often delta shaped wings like Concorde and the space shuttle.

Just as we need air to breath, so the engines need oxygen to burn the fuel to create thrust to propel the plane forward.

Jet Engine however can work at higher altitudes than people. We humans have a limit of about 8000 meters or around 26,000 feet, above this is what climbers call the “death zone” where there is not oxygen for humans to survive for sustained periods.

The summit of Mount Everest is 29,000 feet high and the air density there is about 33% of that at sea level. This means that with each breath you take, you are getting only 33% of the oxygen.