23.1549—Powerplant and auxiliary power unit instruments.
For each required powerplant and auxiliary power unit instrument, as appropriate to the type of instruments—
(a)
Each maximum and, if applicable, minimum safe operating limit must be marked with a red radial or a red line;
(b)
Each normal operating range must be marked with a green arc or green line, not extending beyond the maximum and minimum safe limits;
(d)
Each engine, auxiliary power unit, or propeller range that is restricted because of excessive vibration stresses must be marked with red arcs or red lines.