Before an aircraft can be weighed and reliable readings obtained, it must be in a level flight attitude. One method that can be used to check for a level condition is to use a spirit level, sometimes thought of as a carpenter’s level, by placing it on or against a specified place on the aircraft. Spirit levels consist of a vial full of liquid, except for a small air bubble. When the air bubble is centered between the two black lines, a level condition is indicated.
In Figure 4-16, a spirit level is being used on a Mooney M20 to check for a flight level attitude. By looking in the Type Certificate Data Sheet, it is determined that the leveling means is two screws on the left side of the airplane fuselage, in line with the trailing edge of the wing.