Which tool can be used to level an aircraft?

A spirit level or plumb bob is specifically designed to indicate whether a surface is level or plumb (vertically straight). In aviation, ensuring that an aircraft is level is crucial for various maintenance procedures and during inspections. A spirit level utilizes a fluid-filled vial with an air bubble; when the bubble is centered within the marked lines, the surface is level. Similarly, a plumb bob helps in establishing a vertical reference line, which is essential for certain checks.

While a digital scale measures weight, an altimeter determines altitude, and a weight measurement device assesses mass, none of these tools directly assess the leveling of an aircraft. Therefore, the use of a spirit level or plumb bob is the appropriate choice for achieving a level position when working on or inspecting an aircraft.