14 CFR § 23.2320

The applicant must design the airplane to—

Allow clear communication between the flightcrew and passengers;

Protect the pilot and flight controls from propellers; and

Protect the occupants from serious injury due to damage to windshields, windows, and canopies.

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.

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.

If a pressurization system is installed in the airplane, it must be designed to protect against—

Decompression to an unsafe level; and

Excessive differential pressure.

If an oxygen system is installed in the airplane, it must—

Effectively provide oxygen to each user to prevent the effects of hypoxia; and

Be free from hazards in itself, in its method of operation, and its effect upon other components.