If you are referring to the highest altitude a military craft has gone, then it is Lockheed Martin U-2. It set the record for 70,000 feet.
Zero, if the jet plane is on the ground, otherwise it depends at what altitude the plane happens to be flying. Cruising altitude for many passenger airliners is around five to seven miles. Military aircraft often fly at much higher altitudes.
Ventilation is important on a plane to compensate for air pressure as the plane is in a high altitudes and also to maintain suitable temperature and oxygen levels,
Well a simple answer would be that if a plane is not pressurized, on high altitudes or just very high in the sky, the plane will expand like a balloon and if it expands to much, big problems happen and I don't really know how it is pressurized but the air inside the plane is usually very different than the air outside the plane, at high altitudes.
When a plane loses pressure by depressurization, air escapes through an opening in the plane. Depressurization can cause passengers to have trouble breathing at certain altitudes.
Depends which plane
A military plane is usually used whenever there is a military operation.
Because they have winglets which cause them to release farts. These farts eventually cause the plane to explode.
The air is thicker at lower altitudes, we know that sound travels much better when it can move through solid mass. As you climb, the air starts to get thinner. It becomes much more difficult to do anything at high altitudes, including keeping a plane aloft, keeping warm, and of course projecting sound.
Better fuel economy. The air is thinner at higher altitudes.
Efficiency is reduced because the propeller exerts less force at high density altitudes than at low density altitudes. Less efficient slow plane
Antonov An-225
The Military