At what PSI does normal pressurization operate below 18,000 feet MSL?

Normal pressurization in aircraft typically operates within a certain range to ensure passenger comfort and safety during flight, especially below an altitude of 18,000 feet MSL (Mean Sea Level). The correct answer indicates that the pressurization system maintains a differential pressure of 3.6 PSI, which is a common standard used for cabin pressurization in commercial aviation.

This level of pressure supports the physiological needs of passengers and crew, as it simulates a lower altitude environment that is easier for the body to acclimatize to, helping to prevent hypoxia and other altitude-related issues during flight. Maintaining the right cabin pressure is vital for safety and comfort, and 3.6 PSI is effectively the balance that has been optimized for flights operating at altitudes below this threshold.

The other options, while they represent various pressure levels, do not align with the standard operating pressure for normal pressurization below 18,000 feet MSL established in aviation practice. Therefore, the choice of 3.6 PSI reflects accepted industry standards and operational protocols.