Final answer:
The barometric pressure to set on the altimeter during cruise flight in the standard pressure region is 29.92 inches of mercury (in Hg) or 101,325 Pascals (Pa), matching the standard atmospheric pressure at sea level.
Step-by-step explanation:
During cruise flight while in the standard pressure region, the barometric pressure that shall be set on the altimeter is 101,325 Pascals (Pa) or 29.92 inches of mercury (in Hg). This value corresponds to the standard atmospheric pressure at sea level. It's vital for aircraft to utilize this standard setting in the altimeter to ensure uniformity in altitude reporting above the transition altitude. This helps in the vertical separation of aircrafts in different flight levels.
The process of maintaining this altimeter setting is important for flight safety and navigation. As an airplane moves to higher altitudes, the atmospheric pressure decreases, which is reflected in a change in the height of a mercury column in a barometer due to the hydrostatic pressure exerted by the column above. For convenience in aviation, the use of mercury in a barometer is preferred, as it is denser and requires a much shorter column than water, making it easier to read and manage.