The ICA system which is now being considered by the Civil Aviation Administrations in the UK and the USA is based on a new height unit, the ‘millica’. Below an arbitrary fixed level (29000 ft) this unit is equal to 0.3 m; above that level it increases gradually, the rate of increase corresponding to the standard deviation of aircraft height from assigned cruising level. At 10 000 m the unit is equal to 0.325 m and 15 000 m to 0.477 m. In this way the ratio between the separation of cruising levels and the standard deviation of the departure from assigned level remains constant; that is to say, the safety margin is constant. At present, cruising levels up to 29 000 ft are separated by 1000 ft and above this level by 2000 ft, so that there are five cruising levels between 29000 and 39000 ft; in the new system there would be nine.

If δh is the height difference, δp the barometric pressure difference and p the mean atmospheric density in this interval, it is clear that δp = pδh and δh(1/p)δp. From this it follows that if δh is constant between adjacent flight levels the δp decreases with altitude and the vertical separation safety factor also decreases.

If, as has sometimes been suggested, δp remains constant between adjacent flight levels, then δh will increase with altitude and the safety factor will increase also.