Choosing the right altitude

A light aircraft as the Cirrus SR22 comes in two versions. One has a normally aspirated piston engine with a service ceiling of 17,000ft. The other is turbo normalized, which means that the same power is available at any altitude. The turbo models can fly as high as 25,000ft.

Oxygen required

Without a pressurized cabin pilot and passengers need to breath pure oxygen above 12,000ft. Up to 18,000ft that can be done using canulas that deliver the oxygen through the nose. Above that a face mask is required. Supposedly using a canula for longer periods of time is comfortable and one will amost forget its presence while a face mask can become quite a hassle.

Where everyone else is flying

In Germany controlled airspace begins at 10,000ft. Local traffic around small airports and airfields moves below 5,000ft. There is also, especially during summer, a lot of gliders in the air. The activity is more on the weekend than during a regular week when everybody is at work. Gliders typically stay below the clouds but the clouds can be quite high during summer.

So flying above 10,000ft in controlled airspace seems like a good idea. Up there everyone else will be in contact with ATC and carries equipment to signal their own position (transponder, etc.). Further there is equipment available that detects other aircraft in the vicinity and displays them on a screen in the cockpit.