14 CFR §23.2320

1. (a)The applicant must design the airplane to—
   1. (1)Allow clear communication between the flightcrew and passengers;
   2. (2)Protect the pilot and flight controls from propellers; and
   3. (3)Protect the occupants from serious injury due to damage to windshields, windows, and canopies.
2. (b)For level 4 airplanes, each windshield and its supporting structure directly in front of the pilot must withstand, without penetration, the impact equivalent to a two-pound bird when the velocity of the airplane is equal to the airplane's maximum approach flap speed.
3. (c)The airplane must provide each occupant with air at a breathable pressure, free of hazardous concentrations of gases, vapors, and smoke during normal operations and likely failures.
4. (d)If a pressurization system is installed in the airplane, it must be designed to protect against—
   1. (1)Decompression to an unsafe level; and
   2. (2)Excessive differential pressure.
5. (e)If an oxygen system is installed in the airplane, it must—
   1. (1)Effectively provide oxygen to each user to prevent the effects of hypoxia; and
   2. (2)Be free from hazards in itself, in its method of operation, and its effect upon other components.