What heights do civilian and military aircrafts usually fly at?

Civilian aircrafts usually fly between 9,000 metres and 13,000 metres above sea level (30,000 feet to 42,000 feet- the aircraft industry still use feet and inches). This height is known as a ‘sweet spot’. As you increase in height, the air resistance gets lower but so does the abundance of oxygen. Between this range, there is a relatively low air resistance, yet still enough oxygen to fuel the engines. Of course, the exact height of planes changes depending on weight, fuel or cost. When planes are ascending or descending they follow special flight paths cleared by air traffic control.

 

Fun fact: the highest ever commercial airliner actually flew at 60,000 feet. Anyone know the name of the aircraft? (no cheating)

 

Military aircrafts can fly at many different heights with a minimum of 200 metres (500 feet) to the highest military engine flying at 27,000 metres (90,000 feet). However, generally military jets fly at around 15,000 metres above sea level (50,000 feet).

 

Also, I wanted to mention the SR71 Blackbird which flew at 26000 metres (85,000 feet) and had a clever variable cone on the front used to slow the speed of the air entering from supersonic speed to subsonic speed before entering the engine. The air passes through a conical shock wave that forms on the apex of the cone then passes through a strong normal shock wave and exits at subsonic speed.

References:

• https://www.traveller.com.au/why-planes-fly-at-35000-feet-the-reason-for-high-altitude-flights-guxhc9
• https://www.physlink.com/education/askexperts/ae610.cfm