What is "if it is necessary to set the altimeter from 29.15 to 29.85?

Setting the altimeter from 29.15 to 29.85 may be necessary if there is a change in atmospheric pressure or altitude. The altimeter is a crucial instrument in aviation that measures the pressure of the surrounding air and displays altitude above sea level.

When there is a change in pressure, it can affect the accuracy of the altimeter readings. Pilots must adjust the altimeter setting to ensure accurate altitude readings while flying. In this case, changing the altimeter setting from 29.15 to 29.85 would account for the change in pressure and provide more accurate altitude information.

It is important for pilots to regularly update the altimeter setting during flight to account for changes in atmospheric pressure and ensure the safety of the aircraft and passengers. Ignoring this important task could lead to incorrect altitude readings and potential safety hazards.