A Caution message in AFT Impulse.

Full message is:

Head loss, head rise, hydraulic gradeline, and energy gradeline output does not have meaning for zero g systems.

If the user sets the gravity to zero presumably for a space application then the concept of head and gradeline lose meaning. This can be set in the Environmental Properties Panel. For example see Equation 1 in Network Solution Methodology and see that a zero g will leads to a meaningless HGL (hydraulic gradeline).

Users should ignore all head loss, head rise and gradelines displayed in the output and focus on pressure results.