## The moment arm ( H-arm).

The moment arm

A manufacturer or operator may use sometimes the Moment Arm or H-arm(both are correct), and it is the horizontal distance between the datum line in the nose of the aircraft and the centre of gravity of the aircraft or an object in or outside of the aircraft.

See the diagram below.

A moment
As we have seen earlier a moment/force is the product of weight and moment arm. Moments in respect of the datum line in an aircraft are calculated by multiplying the weight by the moment arm (H-arm).

Aircraft manufacturers are using kilograms and pounds to express weight.

Moment arms are expressed as millimeters or inches.

Therefore moments/forces dimensions are expressed as mm.kg or in.lbs and are therefore dependent on the preferred options of the operator.