text: Q:
  
  Is plane altitude limited by engine power and if so does air density cause this?
  
  I notice that, for example, human-powered flight operates at low altitudes. This might of course be due to safety but I wonder if in fact the delta in air pressure is greater at lower altitudes and this prevents low-powered aircraft from reaching higher altitudes?
  
  A:
  
  Human-powered flight is only barely possible because humans cannot generate large amounts of power. Human-powered planes therefore fly almost exclusively in what is called ground effect, where the plane is no more than about one wingspan off the ground. When flying this close to the ground, the plane is partly supported by a "bubble" of air that is caught between the underside of the wing and the ground, which reduces the amount of drag experienced by the plane and enhances the lifting power of its wings. This in turn reduces the power requirement to keep the plane in the air. 
  Regarding small planes with low-powered (piston) engines, they are limited in how high they can fly because as they climb they move into air which is less dense and which therefore furnishes less oxygen for the engine to burn with its fuel. This causes the power output of the engine to decrease with increasing altitude, and at some point the engine's power output is so diminished that the plane can climb no further. 
  In small planes, this maximum altitude point can be increased by putting a bigger engine in the plane, reducing the plane's weight, or putting a compressor on the engine's intake to furnish it with denser air at high altitudes so it can deliver more power. 
  In large planes with jet engines, those engines are specifically designed to deliver high power and best economy at high altitudes.
  
  