Answer taken from CAAIPS PT 10
For some purposes, such as rigging or weighing, an aircraft must be leveled laterally and longitudinally and a number of different methods may be employed.
1 Spirit Level
Many aircraft are levelled by use of a spirit level, which is placed at jigged positions on the airframe structure. On light aircraft the longitudinally level position is generally obtained by placing the spirit level on two pegs or on the heads of two partially withdrawn screws on the side of the fuselage and adjusting the jacks (or the shock absorber extension or tyre pressures, if the aircraft is resting on its wheels) until the spirit level is centred. The laterally level position is obtained by placing the spirit level on the centre-section spar boom (or other nominated position) and again adjusting the jacks or tyre pressures until the level is centred.
With some large aircraft a spirit level may be used in conjunction with special fittings, which are secured by locations in the centre fuselage or in one of the wheel bays; these fittings must be removed before flight and should have warning streamers attached. If adjustments have been necessary to level an aircraft laterally, the longitudinal level should be re-checked.
NOTE: In cases where tyre pressures are adjusted to level the aircraft, care must be taken not to over-inflate or to completely deflate a tyre.
2 Plumb Bob
On many aircraft a plumb bob is used in conjunction with a levelling plate. The plumb bob is suspended from a fixed position in the cabin roof or upper part of a wheel bay and hangs over a levelling plate, which may be a permanent fixture or a separate fitting accurately located on the cabin floor or lower part of the wheel bay.
The levelling plate is marked with a zero position and scales indicating the adjustments required about the lateral and longitudinal axes to centre the plumb bob. 3 Engineers Transit
The most accurate