What type of airplanes, what altitudes and what type of operations?
Light piston or turbin twin: non commercial use, you risk your own neck.
Part 25 certified airplane: aircraft limitations such as max altitude for take off and landing, OAT limitations, and Engine starting limitations. Minimum runway lenghts. The highest approved altitude I have seen approved for take off and landing 14000 feet.
Part 25 certified airplane with commercial use (airline type). Same as above with additional limitions variable on each operator.
When you get up as high as La poz, you have to start thinking about your personal well-being. Hypoxia will set in !